Sync to upstream/release/512 (#330)

- Improve refinement support for unions, in particular it's now possible to implement tagged unions as a union of tables where individual branches use a string literal type for one of the fields.
- Fix `string.split` type information
- Optimize `select(_, ...)` to run in constant time (~2.7x faster on VariadicSelect benchmark)
- Improve debug line information for multi-line assignments
- Improve compilation of table literals when table keys are constant expressions/variables
- Use forward GC barrier for `setmetatable` which slightly accelerates GC progress

Analysis/include/Luau/AstQuery.h

@@ -42,6 +42,21 @@ struct ExprOrLocal
     {
         return expr ? expr->location : (local ? local->location : std::optional<Location>{});
     }
+    std::optional<AstName> getName()
+    {
+        if (expr)
+        {
+            if (AstName name = getIdentifier(expr); name.value)
+            {
+                return name;
+            }
+        }
+        else if (local)
+        {
+            return local->name;
+        }
+        return std::nullopt;
+    }
 
 private:
     AstExpr* expr = nullptr;

Analysis/include/Luau/Module.h

@@ -13,6 +13,8 @@
 #include <unordered_map>
 #include <optional>
 
+LUAU_FASTFLAG(LuauPrepopulateUnionOptionsBeforeAllocation)
+
 namespace Luau
 {
 
@@ -58,6 +60,12 @@ struct TypeArena
     template<typename T>
     TypeId addType(T tv)
     {
+        if (FFlag::LuauPrepopulateUnionOptionsBeforeAllocation)
+        {
+            if constexpr (std::is_same_v<T, UnionTypeVar>)
+                LUAU_ASSERT(tv.options.size() >= 2);
+        }
+
         return addTV(TypeVar(std::move(tv)));
     }
 

Analysis/include/Luau/TypeInfer.h

@@ -135,7 +135,8 @@ struct TypeChecker
     void checkBlock(const ScopePtr& scope, const AstStatBlock& statement);
     void checkBlockTypeAliases(const ScopePtr& scope, std::vector<AstStat*>& sorted);
 
-    ExprResult<TypeId> checkExpr(const ScopePtr& scope, const AstExpr& expr, std::optional<TypeId> expectedType = std::nullopt);
+    ExprResult<TypeId> checkExpr(
+        const ScopePtr& scope, const AstExpr& expr, std::optional<TypeId> expectedType = std::nullopt, bool forceSingleton = false);
     ExprResult<TypeId> checkExpr(const ScopePtr& scope, const AstExprLocal& expr);
     ExprResult<TypeId> checkExpr(const ScopePtr& scope, const AstExprGlobal& expr);
     ExprResult<TypeId> checkExpr(const ScopePtr& scope, const AstExprVarargs& expr);
@@ -160,14 +161,12 @@ struct TypeChecker
     // Returns the type of the lvalue.
     TypeId checkLValue(const ScopePtr& scope, const AstExpr& expr);
 
-    // Returns both the type of the lvalue and its binding (if the caller wants to mutate the binding).
-    // Note: the binding may be null.
-    // TODO: remove second return value with FFlagLuauUpdateFunctionNameBinding
-    std::pair<TypeId, TypeId*> checkLValueBinding(const ScopePtr& scope, const AstExpr& expr);
-    std::pair<TypeId, TypeId*> checkLValueBinding(const ScopePtr& scope, const AstExprLocal& expr);
-    std::pair<TypeId, TypeId*> checkLValueBinding(const ScopePtr& scope, const AstExprGlobal& expr);
-    std::pair<TypeId, TypeId*> checkLValueBinding(const ScopePtr& scope, const AstExprIndexName& expr);
-    std::pair<TypeId, TypeId*> checkLValueBinding(const ScopePtr& scope, const AstExprIndexExpr& expr);
+    // Returns the type of the lvalue.
+    TypeId checkLValueBinding(const ScopePtr& scope, const AstExpr& expr);
+    TypeId checkLValueBinding(const ScopePtr& scope, const AstExprLocal& expr);
+    TypeId checkLValueBinding(const ScopePtr& scope, const AstExprGlobal& expr);
+    TypeId checkLValueBinding(const ScopePtr& scope, const AstExprIndexName& expr);
+    TypeId checkLValueBinding(const ScopePtr& scope, const AstExprIndexExpr& expr);
 
     TypeId checkFunctionName(const ScopePtr& scope, AstExpr& funName, TypeLevel level);
     std::pair<TypeId, ScopePtr> checkFunctionSignature(const ScopePtr& scope, int subLevel, const AstExprFunction& expr,
@@ -322,8 +321,6 @@ private:
         return addTV(TypeVar(tv));
     }
 
-    TypeId addType(const UnionTypeVar& utv);
-
     TypeId addTV(TypeVar&& tv);
 
     TypePackId addTypePack(TypePackVar&& tp);
@@ -349,6 +346,8 @@ public:
     ErrorVec resolve(const PredicateVec& predicates, const ScopePtr& scope, bool sense);
 
 private:
+    void refineLValue(const LValue& lvalue, RefinementMap& refis, const ScopePtr& scope, TypeIdPredicate predicate);
+
     std::optional<TypeId> resolveLValue(const ScopePtr& scope, const LValue& lvalue);
     std::optional<TypeId> DEPRECATED_resolveLValue(const ScopePtr& scope, const LValue& lvalue);
     std::optional<TypeId> resolveLValue(const RefinementMap& refis, const ScopePtr& scope, const LValue& lvalue);

Analysis/include/Luau/TypeVar.h

@@ -111,16 +111,16 @@ struct PrimitiveTypeVar
 
 // Singleton types https://github.com/Roblox/luau/blob/master/rfcs/syntax-singleton-types.md
 // Types for true and false
-struct BoolSingleton
+struct BooleanSingleton
 {
     bool value;
 
-    bool operator==(const BoolSingleton& rhs) const
+    bool operator==(const BooleanSingleton& rhs) const
     {
         return value == rhs.value;
     }
 
-    bool operator!=(const BoolSingleton& rhs) const
+    bool operator!=(const BooleanSingleton& rhs) const
     {
         return !(*this == rhs);
     }
@@ -145,7 +145,7 @@ struct StringSingleton
 // No type for float singletons, partly because === isn't any equalivalence on floats
 // (NaN != NaN).
 
-using SingletonVariant = Luau::Variant<BoolSingleton, StringSingleton>;
+using SingletonVariant = Luau::Variant<BooleanSingleton, StringSingleton>;
 
 struct SingletonTypeVar
 {

Analysis/include/Luau/Unifier.h

@@ -85,6 +85,13 @@ public:
 
     Unifier makeChildUnifier();
 
+    // A utility function that appends the given error to the unifier's error log.
+    // This allows setting a breakpoint wherever the unifier reports an error.
+    void reportError(TypeError error)
+    {
+        errors.push_back(error);
+    }
+
 private:
     bool isNonstrictMode() const;
 

Analysis/src/Autocomplete.cpp

@@ -14,9 +14,9 @@
 
 LUAU_FASTFLAG(LuauUseCommittingTxnLog)
 LUAU_FASTFLAGVARIABLE(LuauAutocompleteAvoidMutation, false);
-LUAU_FASTFLAGVARIABLE(LuauAutocompleteFirstArg, false);
 LUAU_FASTFLAGVARIABLE(LuauCompleteBrokenStringParams, false);
 LUAU_FASTFLAGVARIABLE(LuauMissingFollowACMetatables, false);
+LUAU_FASTFLAGVARIABLE(PreferToCallFunctionsForIntersects, false);
 
 static const std::unordered_set<std::string> kStatementStartingKeywords = {
     "while", "if", "local", "repeat", "function", "do", "for", "return", "break", "continue", "type", "export"};
@@ -194,8 +194,6 @@ static ParenthesesRecommendation getParenRecommendation(TypeId id, const std::ve
 
 static std::optional<TypeId> findExpectedTypeAt(const Module& module, AstNode* node, Position position)
 {
-    LUAU_ASSERT(FFlag::LuauAutocompleteFirstArg);
-
     auto expr = node->asExpr();
     if (!expr)
         return std::nullopt;
@@ -266,43 +264,63 @@ static TypeCorrectKind checkTypeCorrectKind(const Module& module, TypeArena* typ
         }
     };
 
-    TypeId expectedType;
+    auto typeAtPosition = findExpectedTypeAt(module, node, position);
 
-    if (FFlag::LuauAutocompleteFirstArg)
+    if (!typeAtPosition)
+        return TypeCorrectKind::None;
+
+    TypeId expectedType = follow(*typeAtPosition);
+
+    if (FFlag::PreferToCallFunctionsForIntersects)
     {
-        auto typeAtPosition = findExpectedTypeAt(module, node, position);
+        auto checkFunctionType = [&canUnify, &expectedType](const FunctionTypeVar* ftv) {
+            auto [retHead, retTail] = flatten(ftv->retType);
 
-        if (!typeAtPosition)
-            return TypeCorrectKind::None;
+            if (!retHead.empty() && canUnify(retHead.front(), expectedType))
+                return true;
 
-        expectedType = follow(*typeAtPosition);
+            // We might only have a variadic tail pack, check if the element is compatible
+            if (retTail)
+            {
+                if (const VariadicTypePack* vtp = get<VariadicTypePack>(follow(*retTail)); vtp && canUnify(vtp->ty, expectedType))
+                    return true;
+            }
+
+            return false;
+        };
+
+        // We also want to suggest functions that return compatible result
+        if (const FunctionTypeVar* ftv = get<FunctionTypeVar>(ty); ftv && checkFunctionType(ftv))
+        {
+            return TypeCorrectKind::CorrectFunctionResult;
+        }
+        else if (const IntersectionTypeVar* itv = get<IntersectionTypeVar>(ty))
+        {
+            for (TypeId id : itv->parts)
+            {
+                if (const FunctionTypeVar* ftv = get<FunctionTypeVar>(id); ftv && checkFunctionType(ftv))
+                {
+                    return TypeCorrectKind::CorrectFunctionResult;
+                }
+            }
+        }
     }
     else
     {
-        auto expr = node->asExpr();
-        if (!expr)
-            return TypeCorrectKind::None;
-
-        auto it = module.astExpectedTypes.find(expr);
-        if (!it)
-            return TypeCorrectKind::None;
-
-        expectedType = follow(*it);
-    }
-
-    // We also want to suggest functions that return compatible result
-    if (const FunctionTypeVar* ftv = get<FunctionTypeVar>(ty))
-    {
-        auto [retHead, retTail] = flatten(ftv->retType);
-
-        if (!retHead.empty() && canUnify(retHead.front(), expectedType))
-            return TypeCorrectKind::CorrectFunctionResult;
-
-        // We might only have a variadic tail pack, check if the element is compatible
-        if (retTail)
+        // We also want to suggest functions that return compatible result
+        if (const FunctionTypeVar* ftv = get<FunctionTypeVar>(ty))
         {
-            if (const VariadicTypePack* vtp = get<VariadicTypePack>(follow(*retTail)); vtp && canUnify(vtp->ty, expectedType))
+            auto [retHead, retTail] = flatten(ftv->retType);
+
+            if (!retHead.empty() && canUnify(retHead.front(), expectedType))
                 return TypeCorrectKind::CorrectFunctionResult;
+
+            // We might only have a variadic tail pack, check if the element is compatible
+            if (retTail)
+            {
+                if (const VariadicTypePack* vtp = get<VariadicTypePack>(follow(*retTail)); vtp && canUnify(vtp->ty, expectedType))
+                    return TypeCorrectKind::CorrectFunctionResult;
+            }
         }
     }
 
@@ -741,29 +759,12 @@ std::optional<const T*> returnFirstNonnullOptionOfType(const UnionTypeVar* utv)
 
 static std::optional<bool> functionIsExpectedAt(const Module& module, AstNode* node, Position position)
 {
-    TypeId expectedType;
+    auto typeAtPosition = findExpectedTypeAt(module, node, position);
 
-    if (FFlag::LuauAutocompleteFirstArg)
-    {
-        auto typeAtPosition = findExpectedTypeAt(module, node, position);
+    if (!typeAtPosition)
+        return std::nullopt;
 
-        if (!typeAtPosition)
-            return std::nullopt;
-
-        expectedType = follow(*typeAtPosition);
-    }
-    else
-    {
-        auto expr = node->asExpr();
-        if (!expr)
-            return std::nullopt;
-
-        auto it = module.astExpectedTypes.find(expr);
-        if (!it)
-            return std::nullopt;
-
-        expectedType = follow(*it);
-    }
+    TypeId expectedType = follow(*typeAtPosition);
 
     if (get<FunctionTypeVar>(expectedType))
         return true;

Analysis/src/Frontend.cpp

@@ -18,7 +18,6 @@
 LUAU_FASTFLAG(LuauInferInNoCheckMode)
 LUAU_FASTFLAGVARIABLE(LuauTypeCheckTwice, false)
 LUAU_FASTFLAGVARIABLE(LuauKnowsTheDataModel3, false)
-LUAU_FASTFLAGVARIABLE(LuauPersistDefinitionFileTypes, false)
 
 namespace Luau
 {
@@ -102,8 +101,7 @@ LoadDefinitionFileResult loadDefinitionFile(TypeChecker& typeChecker, ScopePtr t
         generateDocumentationSymbols(globalTy, documentationSymbol);
         targetScope->bindings[typeChecker.globalNames.names->getOrAdd(name.c_str())] = {globalTy, Location(), false, {}, documentationSymbol};
 
-        if (FFlag::LuauPersistDefinitionFileTypes)
-            persist(globalTy);
+        persist(globalTy);
     }
 
     for (const auto& [name, ty] : checkedModule->getModuleScope()->exportedTypeBindings)
@@ -113,8 +111,7 @@ LoadDefinitionFileResult loadDefinitionFile(TypeChecker& typeChecker, ScopePtr t
         generateDocumentationSymbols(globalTy.type, documentationSymbol);
         targetScope->exportedTypeBindings[name] = globalTy;
 
-        if (FFlag::LuauPersistDefinitionFileTypes)
-            persist(globalTy.type);
+        persist(globalTy.type);
     }
 
     return LoadDefinitionFileResult{true, parseResult, checkedModule};

Analysis/src/Module.cpp

@@ -16,6 +16,8 @@ LUAU_FASTFLAGVARIABLE(DebugLuauTrackOwningArena, false)
 LUAU_FASTINTVARIABLE(LuauTypeCloneRecursionLimit, 300)
 LUAU_FASTFLAG(LuauTypeAliasDefaults)
 
+LUAU_FASTFLAGVARIABLE(LuauPrepopulateUnionOptionsBeforeAllocation, false)
+
 namespace Luau
 {
 
@@ -377,14 +379,28 @@ void TypeCloner::operator()(const AnyTypeVar& t)
 
 void TypeCloner::operator()(const UnionTypeVar& t)
 {
-    TypeId result = dest.addType(UnionTypeVar{});
-    seenTypes[typeId] = result;
+    if (FFlag::LuauPrepopulateUnionOptionsBeforeAllocation)
+    {
+        std::vector<TypeId> options;
+        options.reserve(t.options.size());
 
-    UnionTypeVar* option = getMutable<UnionTypeVar>(result);
-    LUAU_ASSERT(option != nullptr);
+        for (TypeId ty : t.options)
+            options.push_back(clone(ty, dest, seenTypes, seenTypePacks, cloneState));
 
-    for (TypeId ty : t.options)
-        option->options.push_back(clone(ty, dest, seenTypes, seenTypePacks, cloneState));
+        TypeId result = dest.addType(UnionTypeVar{std::move(options)});
+        seenTypes[typeId] = result;
+    }
+    else
+    {
+        TypeId result = dest.addType(UnionTypeVar{});
+        seenTypes[typeId] = result;
+
+        UnionTypeVar* option = getMutable<UnionTypeVar>(result);
+        LUAU_ASSERT(option != nullptr);
+
+        for (TypeId ty : t.options)
+            option->options.push_back(clone(ty, dest, seenTypes, seenTypePacks, cloneState));
+    }
 }
 
 void TypeCloner::operator()(const IntersectionTypeVar& t)

Analysis/src/ToString.cpp

@@ -10,7 +10,6 @@
 #include <algorithm>
 #include <stdexcept>
 
-LUAU_FASTFLAG(LuauOccursCheckOkWithRecursiveFunctions)
 LUAU_FASTFLAG(LuauTypeAliasDefaults)
 
 /*
@@ -374,7 +373,7 @@ struct TypeVarStringifier
 
     void operator()(TypeId, const SingletonTypeVar& stv)
     {
-        if (const BoolSingleton* bs = Luau::get<BoolSingleton>(&stv))
+        if (const BooleanSingleton* bs = Luau::get<BooleanSingleton>(&stv))
             state.emit(bs->value ? "true" : "false");
         else if (const StringSingleton* ss = Luau::get<StringSingleton>(&stv))
         {
@@ -617,9 +616,7 @@ struct TypeVarStringifier
 
             std::string saved = std::move(state.result.name);
 
-            bool needParens = FFlag::LuauOccursCheckOkWithRecursiveFunctions
-                                  ? !state.cycleNames.count(el) && (get<IntersectionTypeVar>(el) || get<FunctionTypeVar>(el))
-                                  : get<IntersectionTypeVar>(el) || get<FunctionTypeVar>(el);
+            bool needParens = !state.cycleNames.count(el) && (get<IntersectionTypeVar>(el) || get<FunctionTypeVar>(el));
 
             if (needParens)
                 state.emit("(");
@@ -675,9 +672,7 @@ struct TypeVarStringifier
 
             std::string saved = std::move(state.result.name);
 
-            bool needParens = FFlag::LuauOccursCheckOkWithRecursiveFunctions
-                                  ? !state.cycleNames.count(el) && (get<UnionTypeVar>(el) || get<FunctionTypeVar>(el))
-                                  : get<UnionTypeVar>(el) || get<FunctionTypeVar>(el);
+            bool needParens = !state.cycleNames.count(el) && (get<UnionTypeVar>(el) || get<FunctionTypeVar>(el));
 
             if (needParens)
                 state.emit("(");

Analysis/src/TypeAttach.cpp

@@ -97,7 +97,7 @@ public:
 
     AstType* operator()(const SingletonTypeVar& stv)
     {
-        if (const BoolSingleton* bs = get<BoolSingleton>(&stv))
+        if (const BooleanSingleton* bs = get<BooleanSingleton>(&stv))
             return allocator->alloc<AstTypeSingletonBool>(Location(), bs->value);
         else if (const StringSingleton* ss = get<StringSingleton>(&stv))
         {

Analysis/src/TypeInfer.cpp

@@ -26,8 +26,6 @@ LUAU_FASTFLAG(LuauKnowsTheDataModel3)
 LUAU_FASTFLAGVARIABLE(LuauEqConstraint, false)
 LUAU_FASTFLAGVARIABLE(LuauGroupExpectedType, false)
 LUAU_FASTFLAGVARIABLE(LuauWeakEqConstraint, false) // Eventually removed as false.
-LUAU_FASTFLAGVARIABLE(LuauCloneCorrectlyBeforeMutatingTableType, false)
-LUAU_FASTFLAGVARIABLE(LuauStoreMatchingOverloadFnType, false)
 LUAU_FASTFLAG(LuauUseCommittingTxnLog)
 LUAU_FASTFLAGVARIABLE(DebugLuauFreezeDuringUnification, false)
 LUAU_FASTFLAGVARIABLE(LuauRecursiveTypeParameterRestriction, false)
@@ -37,6 +35,7 @@ LUAU_FASTFLAGVARIABLE(LuauLengthOnCompositeType, false)
 LUAU_FASTFLAGVARIABLE(LuauQuantifyInPlace2, false)
 LUAU_FASTFLAGVARIABLE(LuauSealExports, false)
 LUAU_FASTFLAGVARIABLE(LuauSingletonTypes, false)
+LUAU_FASTFLAGVARIABLE(LuauDiscriminableUnions, false)
 LUAU_FASTFLAGVARIABLE(LuauTypeAliasDefaults, false)
 LUAU_FASTFLAGVARIABLE(LuauExpectedTypesOfProperties, false)
 LUAU_FASTFLAGVARIABLE(LuauErrorRecoveryType, false)
@@ -46,10 +45,8 @@ LUAU_FASTFLAGVARIABLE(LuauRefiLookupFromIndexExpr, false)
 LUAU_FASTFLAGVARIABLE(LuauPerModuleUnificationCache, false)
 LUAU_FASTFLAGVARIABLE(LuauProperTypeLevels, false)
 LUAU_FASTFLAGVARIABLE(LuauAscribeCorrectLevelToInferredProperitesOfFreeTables, false)
-LUAU_FASTFLAGVARIABLE(LuauFixRecursiveMetatableCall, false)
 LUAU_FASTFLAGVARIABLE(LuauBidirectionalAsExpr, false)
 LUAU_FASTFLAGVARIABLE(LuauUnsealedTableLiteral, false)
-LUAU_FASTFLAGVARIABLE(LuauUpdateFunctionNameBinding, false)
 
 namespace Luau
 {
@@ -1139,33 +1136,25 @@ void TypeChecker::check(const ScopePtr& scope, TypeId ty, const ScopePtr& funSco
     }
     else
     {
-        auto [leftType, leftTypeBinding] = checkLValueBinding(scope, *function.name);
+        TypeId leftType = checkLValueBinding(scope, *function.name);
 
         checkFunctionBody(funScope, ty, *function.func);
 
         unify(ty, leftType, function.location);
 
-        if (FFlag::LuauUpdateFunctionNameBinding)
-        {
-            LUAU_ASSERT(function.name->is<AstExprIndexName>() || function.name->is<AstExprError>());
+        LUAU_ASSERT(function.name->is<AstExprIndexName>() || function.name->is<AstExprError>());
 
-            if (auto exprIndexName = function.name->as<AstExprIndexName>())
+        if (auto exprIndexName = function.name->as<AstExprIndexName>())
+        {
+            if (auto typeIt = currentModule->astTypes.find(exprIndexName->expr))
             {
-                if (auto typeIt = currentModule->astTypes.find(exprIndexName->expr))
+                if (auto ttv = getMutableTableType(*typeIt))
                 {
-                    if (auto ttv = getMutableTableType(*typeIt))
-                    {
-                        if (auto it = ttv->props.find(exprIndexName->index.value); it != ttv->props.end())
-                            it->second.type = follow(quantify(funScope, leftType, function.name->location));
-                    }
+                    if (auto it = ttv->props.find(exprIndexName->index.value); it != ttv->props.end())
+                        it->second.type = follow(quantify(funScope, leftType, function.name->location));
                 }
             }
         }
-        else
-        {
-            if (leftTypeBinding)
-                *leftTypeBinding = follow(quantify(funScope, leftType, function.name->location));
-        }
     }
 }
 
@@ -1426,7 +1415,7 @@ void TypeChecker::check(const ScopePtr& scope, const AstStatDeclareFunction& glo
     currentModule->getModuleScope()->bindings[global.name] = Binding{fnType, global.location};
 }
 
-ExprResult<TypeId> TypeChecker::checkExpr(const ScopePtr& scope, const AstExpr& expr, std::optional<TypeId> expectedType)
+ExprResult<TypeId> TypeChecker::checkExpr(const ScopePtr& scope, const AstExpr& expr, std::optional<TypeId> expectedType, bool forceSingleton)
 {
     RecursionCounter _rc(&checkRecursionCount);
     if (FInt::LuauCheckRecursionLimit > 0 && checkRecursionCount >= FInt::LuauCheckRecursionLimit)
@@ -1443,14 +1432,14 @@ ExprResult<TypeId> TypeChecker::checkExpr(const ScopePtr& scope, const AstExpr&
         result = {nilType};
     else if (const AstExprConstantBool* bexpr = expr.as<AstExprConstantBool>())
     {
-        if (FFlag::LuauSingletonTypes && expectedType && maybeSingleton(*expectedType))
+        if (FFlag::LuauSingletonTypes && (forceSingleton || (expectedType && maybeSingleton(*expectedType))))
             result = {singletonType(bexpr->value)};
         else
             result = {booleanType};
     }
     else if (const AstExprConstantString* sexpr = expr.as<AstExprConstantString>())
     {
-        if (FFlag::LuauSingletonTypes && expectedType && maybeSingleton(*expectedType))
+        if (FFlag::LuauSingletonTypes && (forceSingleton || (expectedType && maybeSingleton(*expectedType))))
             result = {singletonType(std::string(sexpr->value.data, sexpr->value.size))};
         else
             result = {stringType};
@@ -1488,15 +1477,8 @@ ExprResult<TypeId> TypeChecker::checkExpr(const ScopePtr& scope, const AstExpr&
 
     result.type = follow(result.type);
 
-    if (FFlag::LuauStoreMatchingOverloadFnType)
-    {
-        if (!currentModule->astTypes.find(&expr))
-            currentModule->astTypes[&expr] = result.type;
-    }
-    else
-    {
+    if (!currentModule->astTypes.find(&expr))
         currentModule->astTypes[&expr] = result.type;
-    }
 
     if (expectedType)
         currentModule->astExpectedTypes[&expr] = *expectedType;
@@ -2242,7 +2224,6 @@ TypeId TypeChecker::checkRelationalOperation(
                 state.log.commit();
         }
 
-
         bool needsMetamethod = !isEquality;
 
         TypeId leftType = follow(lhsType);
@@ -2250,10 +2231,11 @@ TypeId TypeChecker::checkRelationalOperation(
         {
             reportErrors(state.errors);
 
-            const PrimitiveTypeVar* ptv = get<PrimitiveTypeVar>(leftType);
-            if (!isEquality && state.errors.empty() && (get<UnionTypeVar>(leftType) || (ptv && ptv->type == PrimitiveTypeVar::Boolean)))
+            if (!isEquality && state.errors.empty() && (get<UnionTypeVar>(leftType) || isBoolean(leftType)))
+            {
                 reportError(expr.location, GenericError{format("Type '%s' cannot be compared with relational operator %s", toString(leftType).c_str(),
                                                toString(expr.op).c_str())});
+            }
 
             return booleanType;
         }
@@ -2501,7 +2483,8 @@ ExprResult<TypeId> TypeChecker::checkExpr(const ScopePtr& scope, const AstExprBi
 
         ExprResult<TypeId> rhs = checkExpr(innerScope, *expr.right);
 
-        return {checkBinaryOperation(innerScope, expr, lhs.type, rhs.type), {AndPredicate{std::move(lhs.predicates), std::move(rhs.predicates)}}};
+        return {checkBinaryOperation(FFlag::LuauDiscriminableUnions ? scope : innerScope, expr, lhs.type, rhs.type),
+            {AndPredicate{std::move(lhs.predicates), std::move(rhs.predicates)}}};
     }
     else if (expr.op == AstExprBinary::Or)
     {
@@ -2513,7 +2496,7 @@ ExprResult<TypeId> TypeChecker::checkExpr(const ScopePtr& scope, const AstExprBi
         ExprResult<TypeId> rhs = checkExpr(innerScope, *expr.right);
 
         // Because of C++, I'm not sure if lhs.predicates was not moved out by the time we call checkBinaryOperation.
-        TypeId result = checkBinaryOperation(innerScope, expr, lhs.type, rhs.type, lhs.predicates);
+        TypeId result = checkBinaryOperation(FFlag::LuauDiscriminableUnions ? scope : innerScope, expr, lhs.type, rhs.type, lhs.predicates);
         return {result, {OrPredicate{std::move(lhs.predicates), std::move(rhs.predicates)}}};
     }
     else if (expr.op == AstExprBinary::CompareEq || expr.op == AstExprBinary::CompareNe)
@@ -2521,8 +2504,8 @@ ExprResult<TypeId> TypeChecker::checkExpr(const ScopePtr& scope, const AstExprBi
         if (auto predicate = tryGetTypeGuardPredicate(expr))
             return {booleanType, {std::move(*predicate)}};
 
-        ExprResult<TypeId> lhs = checkExpr(scope, *expr.left);
-        ExprResult<TypeId> rhs = checkExpr(scope, *expr.right);
+        ExprResult<TypeId> lhs = checkExpr(scope, *expr.left, std::nullopt, /*forceSingleton=*/FFlag::LuauDiscriminableUnions);
+        ExprResult<TypeId> rhs = checkExpr(scope, *expr.right, std::nullopt, /*forceSingleton=*/FFlag::LuauDiscriminableUnions);
 
         PredicateVec predicates;
 
@@ -2621,11 +2604,10 @@ ExprResult<TypeId> TypeChecker::checkExpr(const ScopePtr& scope, const AstExprIf
 
 TypeId TypeChecker::checkLValue(const ScopePtr& scope, const AstExpr& expr)
 {
-    auto [ty, binding] = checkLValueBinding(scope, expr);
-    return ty;
+    return checkLValueBinding(scope, expr);
 }
 
-std::pair<TypeId, TypeId*> TypeChecker::checkLValueBinding(const ScopePtr& scope, const AstExpr& expr)
+TypeId TypeChecker::checkLValueBinding(const ScopePtr& scope, const AstExpr& expr)
 {
     if (auto a = expr.as<AstExprLocal>())
         return checkLValueBinding(scope, *a);
@@ -2639,22 +2621,22 @@ std::pair<TypeId, TypeId*> TypeChecker::checkLValueBinding(const ScopePtr& scope
     {
         for (AstExpr* expr : a->expressions)
             checkExpr(scope, *expr);
-        return {errorRecoveryType(scope), nullptr};
+        return errorRecoveryType(scope);
     }
     else
         ice("Unexpected AST node in checkLValue", expr.location);
 }
 
-std::pair<TypeId, TypeId*> TypeChecker::checkLValueBinding(const ScopePtr& scope, const AstExprLocal& expr)
+TypeId TypeChecker::checkLValueBinding(const ScopePtr& scope, const AstExprLocal& expr)
 {
     if (std::optional<TypeId> ty = scope->lookup(expr.local))
-        return {*ty, nullptr};
+        return *ty;
 
     reportError(expr.location, UnknownSymbol{expr.local->name.value, UnknownSymbol::Binding});
-    return {errorRecoveryType(scope), nullptr};
+    return errorRecoveryType(scope);
 }
 
-std::pair<TypeId, TypeId*> TypeChecker::checkLValueBinding(const ScopePtr& scope, const AstExprGlobal& expr)
+TypeId TypeChecker::checkLValueBinding(const ScopePtr& scope, const AstExprGlobal& expr)
 {
     Name name = expr.name.value;
     ScopePtr moduleScope = currentModule->getModuleScope();
@@ -2662,7 +2644,7 @@ std::pair<TypeId, TypeId*> TypeChecker::checkLValueBinding(const ScopePtr& scope
     const auto it = moduleScope->bindings.find(expr.name);
 
     if (it != moduleScope->bindings.end())
-        return std::pair(it->second.typeId, &it->second.typeId);
+        return it->second.typeId;
 
     TypeId result = freshType(scope);
     Binding& binding = moduleScope->bindings[expr.name];
@@ -2673,15 +2655,15 @@ std::pair<TypeId, TypeId*> TypeChecker::checkLValueBinding(const ScopePtr& scope
     if (!isNonstrictMode())
         reportError(TypeError{expr.location, UnknownSymbol{name, UnknownSymbol::Binding}});
 
-    return std::pair(result, &binding.typeId);
+    return result;
 }
 
-std::pair<TypeId, TypeId*> TypeChecker::checkLValueBinding(const ScopePtr& scope, const AstExprIndexName& expr)
+TypeId TypeChecker::checkLValueBinding(const ScopePtr& scope, const AstExprIndexName& expr)
 {
     TypeId lhs = checkExpr(scope, *expr.expr).type;
 
     if (get<ErrorTypeVar>(lhs) || get<AnyTypeVar>(lhs))
-        return std::pair(lhs, nullptr);
+        return lhs;
 
     tablify(lhs);
 
@@ -2694,7 +2676,7 @@ std::pair<TypeId, TypeId*> TypeChecker::checkLValueBinding(const ScopePtr& scope
         const auto& it = lhsTable->props.find(name);
         if (it != lhsTable->props.end())
         {
-            return std::pair(it->second.type, &it->second.type);
+            return it->second.type;
         }
         else if (lhsTable->state == TableState::Unsealed || lhsTable->state == TableState::Free)
         {
@@ -2702,7 +2684,7 @@ std::pair<TypeId, TypeId*> TypeChecker::checkLValueBinding(const ScopePtr& scope
             Property& property = lhsTable->props[name];
             property.type = theType;
             property.location = expr.indexLocation;
-            return std::pair(theType, &property.type);
+            return theType;
         }
         else if (auto indexer = lhsTable->indexer)
         {
@@ -2720,17 +2702,17 @@ std::pair<TypeId, TypeId*> TypeChecker::checkLValueBinding(const ScopePtr& scope
             else if (FFlag::LuauUseCommittingTxnLog)
                 state.log.commit();
 
-            return std::pair(retType, nullptr);
+            return retType;
         }
         else if (lhsTable->state == TableState::Sealed)
         {
             reportError(TypeError{expr.location, CannotExtendTable{lhs, CannotExtendTable::Property, name}});
-            return std::pair(errorRecoveryType(scope), nullptr);
+            return errorRecoveryType(scope);
         }
         else
         {
             reportError(TypeError{expr.location, GenericError{"Internal error: generic tables are not lvalues"}});
-            return std::pair(errorRecoveryType(scope), nullptr);
+            return errorRecoveryType(scope);
         }
     }
     else if (const ClassTypeVar* lhsClass = get<ClassTypeVar>(lhs))
@@ -2739,29 +2721,29 @@ std::pair<TypeId, TypeId*> TypeChecker::checkLValueBinding(const ScopePtr& scope
         if (!prop)
         {
             reportError(TypeError{expr.location, UnknownProperty{lhs, name}});
-            return std::pair(errorRecoveryType(scope), nullptr);
+            return errorRecoveryType(scope);
         }
 
-        return std::pair(prop->type, nullptr);
+        return prop->type;
     }
     else if (get<IntersectionTypeVar>(lhs))
     {
         if (std::optional<TypeId> ty = getIndexTypeFromType(scope, lhs, name, expr.location, false))
-            return std::pair(*ty, nullptr);
+            return *ty;
 
         // If intersection has a table part, report that it cannot be extended just as a sealed table
         if (isTableIntersection(lhs))
         {
             reportError(TypeError{expr.location, CannotExtendTable{lhs, CannotExtendTable::Property, name}});
-            return std::pair(errorRecoveryType(scope), nullptr);
+            return errorRecoveryType(scope);
         }
     }
 
     reportError(TypeError{expr.location, NotATable{lhs}});
-    return std::pair(errorRecoveryType(scope), nullptr);
+    return errorRecoveryType(scope);
 }
 
-std::pair<TypeId, TypeId*> TypeChecker::checkLValueBinding(const ScopePtr& scope, const AstExprIndexExpr& expr)
+TypeId TypeChecker::checkLValueBinding(const ScopePtr& scope, const AstExprIndexExpr& expr)
 {
     TypeId exprType = checkExpr(scope, *expr.expr).type;
     tablify(exprType);
@@ -2771,7 +2753,7 @@ std::pair<TypeId, TypeId*> TypeChecker::checkLValueBinding(const ScopePtr& scope
     TypeId indexType = checkExpr(scope, *expr.index).type;
 
     if (get<AnyTypeVar>(exprType) || get<ErrorTypeVar>(exprType))
-        return std::pair(exprType, nullptr);
+        return exprType;
 
     AstExprConstantString* value = expr.index->as<AstExprConstantString>();
 
@@ -2783,9 +2765,9 @@ std::pair<TypeId, TypeId*> TypeChecker::checkLValueBinding(const ScopePtr& scope
             if (!prop)
             {
                 reportError(TypeError{expr.location, UnknownProperty{exprType, value->value.data}});
-                return std::pair(errorRecoveryType(scope), nullptr);
+                return errorRecoveryType(scope);
             }
-            return std::pair(prop->type, nullptr);
+            return prop->type;
         }
     }
 
@@ -2794,7 +2776,7 @@ std::pair<TypeId, TypeId*> TypeChecker::checkLValueBinding(const ScopePtr& scope
     if (!exprTable)
     {
         reportError(TypeError{expr.expr->location, NotATable{exprType}});
-        return std::pair(errorRecoveryType(scope), nullptr);
+        return errorRecoveryType(scope);
     }
 
     if (value)
@@ -2802,7 +2784,7 @@ std::pair<TypeId, TypeId*> TypeChecker::checkLValueBinding(const ScopePtr& scope
         const auto& it = exprTable->props.find(value->value.data);
         if (it != exprTable->props.end())
         {
-            return std::pair(it->second.type, &it->second.type);
+            return it->second.type;
         }
         else if (exprTable->state == TableState::Unsealed || exprTable->state == TableState::Free)
         {
@@ -2810,7 +2792,7 @@ std::pair<TypeId, TypeId*> TypeChecker::checkLValueBinding(const ScopePtr& scope
             Property& property = exprTable->props[value->value.data];
             property.type = resultType;
             property.location = expr.index->location;
-            return std::pair(resultType, &property.type);
+            return resultType;
         }
     }
 
@@ -2818,18 +2800,18 @@ std::pair<TypeId, TypeId*> TypeChecker::checkLValueBinding(const ScopePtr& scope
     {
         const TableIndexer& indexer = *exprTable->indexer;
         unify(indexType, indexer.indexType, expr.index->location);
-        return std::pair(indexer.indexResultType, nullptr);
+        return indexer.indexResultType;
     }
     else if (exprTable->state == TableState::Unsealed || exprTable->state == TableState::Free)
     {
         TypeId resultType = freshType(scope);
         exprTable->indexer = TableIndexer{anyIfNonstrict(indexType), anyIfNonstrict(resultType)};
-        return std::pair(resultType, nullptr);
+        return resultType;
     }
     else
     {
         TypeId resultType = freshType(scope);
-        return std::pair(resultType, nullptr);
+        return resultType;
     }
 }
 
@@ -3326,7 +3308,7 @@ void TypeChecker::checkArgumentList(
                     } // ok
                     else
                     {
-                        state.errors.push_back(TypeError{state.location, CountMismatch{minParams, paramIndex}});
+                        state.reportError(TypeError{state.location, CountMismatch{minParams, paramIndex}});
                         return;
                     }
                     ++paramIter;
@@ -3348,7 +3330,7 @@ void TypeChecker::checkArgumentList(
                     Location location = state.location;
                     if (!argLocations.empty())
                         location = {state.location.begin, argLocations.back().end};
-                    state.errors.push_back(TypeError{location, CountMismatch{minParams, std::distance(begin(argPack), end(argPack))}});
+                    state.reportError(TypeError{location, CountMismatch{minParams, std::distance(begin(argPack), end(argPack))}});
                     return;
                 }
                 TypePackId tail = state.log.follow(*paramIter.tail());
@@ -3405,7 +3387,7 @@ void TypeChecker::checkArgumentList(
                     if (!argLocations.empty())
                         location = {state.location.begin, argLocations.back().end};
                     // TODO: Better error message?
-                    state.errors.push_back(TypeError{location, CountMismatch{minParams, std::distance(begin(argPack), end(argPack))}});
+                    state.reportError(TypeError{location, CountMismatch{minParams, std::distance(begin(argPack), end(argPack))}});
                     return;
                 }
             }
@@ -3520,7 +3502,7 @@ void TypeChecker::checkArgumentList(
                     } // ok
                     else
                     {
-                        state.errors.push_back(TypeError{state.location, CountMismatch{minParams, paramIndex}});
+                        state.reportError(TypeError{state.location, CountMismatch{minParams, paramIndex}});
                         return;
                     }
                     ++paramIter;
@@ -3540,7 +3522,7 @@ void TypeChecker::checkArgumentList(
                     Location location = state.location;
                     if (!argLocations.empty())
                         location = {state.location.begin, argLocations.back().end};
-                    state.errors.push_back(TypeError{location, CountMismatch{minParams, std::distance(begin(argPack), end(argPack))}});
+                    state.reportError(TypeError{location, CountMismatch{minParams, std::distance(begin(argPack), end(argPack))}});
                     return;
                 }
                 TypePackId tail = *paramIter.tail();
@@ -3606,7 +3588,7 @@ void TypeChecker::checkArgumentList(
                     if (!argLocations.empty())
                         location = {state.location.begin, argLocations.back().end};
                     // TODO: Better error message?
-                    state.errors.push_back(TypeError{location, CountMismatch{minParams, std::distance(begin(argPack), end(argPack))}});
+                    state.reportError(TypeError{location, CountMismatch{minParams, std::distance(begin(argPack), end(argPack))}});
                     return;
                 }
             }
@@ -3825,22 +3807,11 @@ std::optional<ExprResult<TypePackId>> TypeChecker::checkCallOverload(const Scope
             metaArgLocations = *argLocations;
             metaArgLocations.insert(metaArgLocations.begin(), expr.func->location);
 
-            if (FFlag::LuauFixRecursiveMetatableCall)
-            {
-                fn = instantiate(scope, *ty, expr.func->location);
+            fn = instantiate(scope, *ty, expr.func->location);
 
-                argPack = metaCallArgPack;
-                args = metaCallArgs;
-                argLocations = &metaArgLocations;
-            }
-            else
-            {
-                TypeId fn = *ty;
-                fn = instantiate(scope, fn, expr.func->location);
-
-                return checkCallOverload(scope, expr, fn, retPack, metaCallArgPack, metaCallArgs, &metaArgLocations, argListResult,
-                    overloadsThatMatchArgCount, overloadsThatDont, errors);
-            }
+            argPack = metaCallArgPack;
+            args = metaCallArgs;
+            argLocations = &metaArgLocations;
         }
     }
 
@@ -3932,8 +3903,7 @@ std::optional<ExprResult<TypePackId>> TypeChecker::checkCallOverload(const Scope
             }
         }
 
-        if (FFlag::LuauStoreMatchingOverloadFnType)
-            currentModule->astOverloadResolvedTypes[&expr] = fn;
+        currentModule->astOverloadResolvedTypes[&expr] = fn;
 
         // We select this overload
         return {{retPack}};
@@ -4776,7 +4746,7 @@ TypeId TypeChecker::freshType(TypeLevel level)
 TypeId TypeChecker::singletonType(bool value)
 {
     // TODO: cache singleton types
-    return currentModule->internalTypes.addType(TypeVar(SingletonTypeVar(BoolSingleton{value})));
+    return currentModule->internalTypes.addType(TypeVar(SingletonTypeVar(BooleanSingleton{value})));
 }
 
 TypeId TypeChecker::singletonType(std::string value)
@@ -4813,13 +4783,6 @@ std::optional<TypeId> TypeChecker::filterMap(TypeId type, TypeIdPredicate predic
     return std::nullopt;
 }
 
-TypeId TypeChecker::addType(const UnionTypeVar& utv)
-{
-    LUAU_ASSERT(utv.options.size() > 1);
-
-    return addTV(TypeVar(utv));
-}
-
 TypeId TypeChecker::addTV(TypeVar&& tv)
 {
     return currentModule->internalTypes.addType(std::move(tv));
@@ -5347,54 +5310,35 @@ TypeId TypeChecker::instantiateTypeFun(const ScopePtr& scope, const TypeFun& tf,
 
     TypeId instantiated = *maybeInstantiated;
 
-    if (FFlag::LuauCloneCorrectlyBeforeMutatingTableType)
+    // TODO: CLI-46926 it's not a good idea to rename the type here
+    TypeId target = follow(instantiated);
+    bool needsClone = follow(tf.type) == target;
+    TableTypeVar* ttv = getMutableTableType(target);
+
+    if (ttv && needsClone)
     {
-        // TODO: CLI-46926 it's not a good idea to rename the type here
-        TypeId target = follow(instantiated);
-        bool needsClone = follow(tf.type) == target;
-        TableTypeVar* ttv = getMutableTableType(target);
-
-        if (ttv && needsClone)
+        // Substitution::clone is a shallow clone. If this is a metatable type, we
+        // want to mutate its table, so we need to explicitly clone that table as
+        // well. If we don't, we will mutate another module's type surface and cause
+        // a use-after-free.
+        if (get<MetatableTypeVar>(target))
         {
-            // Substitution::clone is a shallow clone. If this is a metatable type, we
-            // want to mutate its table, so we need to explicitly clone that table as
-            // well. If we don't, we will mutate another module's type surface and cause
-            // a use-after-free.
-            if (get<MetatableTypeVar>(target))
-            {
-                instantiated = applyTypeFunction.clone(tf.type);
-                MetatableTypeVar* mtv = getMutable<MetatableTypeVar>(instantiated);
-                mtv->table = applyTypeFunction.clone(mtv->table);
-                ttv = getMutable<TableTypeVar>(mtv->table);
-            }
-            if (get<TableTypeVar>(target))
-            {
-                instantiated = applyTypeFunction.clone(tf.type);
-                ttv = getMutable<TableTypeVar>(instantiated);
-            }
+            instantiated = applyTypeFunction.clone(tf.type);
+            MetatableTypeVar* mtv = getMutable<MetatableTypeVar>(instantiated);
+            mtv->table = applyTypeFunction.clone(mtv->table);
+            ttv = getMutable<TableTypeVar>(mtv->table);
         }
-
-        if (ttv)
+        if (get<TableTypeVar>(target))
         {
-            ttv->instantiatedTypeParams = typeParams;
-            ttv->instantiatedTypePackParams = typePackParams;
+            instantiated = applyTypeFunction.clone(tf.type);
+            ttv = getMutable<TableTypeVar>(instantiated);
         }
     }
-    else
-    {
-        if (TableTypeVar* ttv = getMutableTableType(instantiated))
-        {
-            if (follow(tf.type) == instantiated)
-            {
-                // This can happen if a type alias has generics that it does not use at all.
-                // ex type FooBar<T> = { a: number }
-                instantiated = applyTypeFunction.clone(tf.type);
-                ttv = getMutableTableType(instantiated);
-            }
 
-            ttv->instantiatedTypeParams = typeParams;
-            ttv->instantiatedTypePackParams = typePackParams;
-        }
+    if (ttv)
+    {
+        ttv->instantiatedTypeParams = typeParams;
+        ttv->instantiatedTypePackParams = typePackParams;
     }
 
     return instantiated;
@@ -5482,6 +5426,85 @@ GenericTypeDefinitions TypeChecker::createGenericTypes(const ScopePtr& scope, st
     return {generics, genericPacks};
 }
 
+void TypeChecker::refineLValue(const LValue& lvalue, RefinementMap& refis, const ScopePtr& scope, TypeIdPredicate predicate)
+{
+    LUAU_ASSERT(FFlag::LuauDiscriminableUnions);
+
+    const LValue* target = &lvalue;
+    std::optional<LValue> key; // If set, we know we took the base of the lvalue path and should be walking down each option of the base's type.
+
+    auto ty = resolveLValue(scope, *target);
+    if (!ty)
+        return; // Do nothing. An error was already reported.
+
+    // If the provided lvalue is a local or global, then that's without a doubt the target.
+    // However, if there is a base lvalue, then we'll want that to be the target iff the base is a union type.
+    if (auto base = baseof(lvalue))
+    {
+        std::optional<TypeId> baseTy = resolveLValue(scope, *base);
+        if (baseTy && get<UnionTypeVar>(follow(*baseTy)))
+        {
+            ty = baseTy;
+            target = base;
+            key = lvalue;
+        }
+    }
+
+    // If we do not have a key, it means we're not trying to discriminate anything, so it's a simple matter of just filtering for a subset.
+    if (!key)
+    {
+        if (std::optional<TypeId> result = filterMap(*ty, predicate))
+            addRefinement(refis, *target, *result);
+        else
+            addRefinement(refis, *target, errorRecoveryType(scope));
+
+        return;
+    }
+
+    // Otherwise, we'll want to walk each option of ty, get its index type, and filter that.
+    auto utv = get<UnionTypeVar>(follow(*ty));
+    LUAU_ASSERT(utv);
+
+    std::unordered_set<TypeId> viableTargetOptions;
+    std::unordered_set<TypeId> viableChildOptions; // There may be additional refinements that apply. We add those here too.
+
+    for (TypeId option : utv)
+    {
+        std::optional<TypeId> discriminantTy;
+        if (auto field = Luau::get<Field>(*key)) // need to fully qualify Luau::get because of ADL.
+            discriminantTy = getIndexTypeFromType(scope, option, field->key, Location(), false);
+        else
+            LUAU_ASSERT(!"Unhandled LValue alternative?");
+
+        if (!discriminantTy)
+            return; // Do nothing. An error was already reported, as per usual.
+
+        if (std::optional<TypeId> result = filterMap(*discriminantTy, predicate))
+        {
+            viableTargetOptions.insert(option);
+            viableChildOptions.insert(*result);
+        }
+    }
+
+    auto intoType = [this](const std::unordered_set<TypeId>& s) -> std::optional<TypeId> {
+        if (s.empty())
+            return std::nullopt;
+
+        // TODO: allocate UnionTypeVar and just normalize.
+        std::vector<TypeId> options(s.begin(), s.end());
+        if (options.size() == 1)
+            return options[0];
+
+        return addType(UnionTypeVar{std::move(options)});
+    };
+
+    if (std::optional<TypeId> viableTargetType = intoType(viableTargetOptions))
+        addRefinement(refis, *target, *viableTargetType);
+
+    if (std::optional<TypeId> viableChildType = intoType(viableChildOptions))
+        addRefinement(refis, lvalue, *viableChildType);
+}
+
 std::optional<TypeId> TypeChecker::resolveLValue(const ScopePtr& scope, const LValue& lvalue)
 {
     if (!FFlag::LuauLValueAsKey)
@@ -5645,18 +5668,29 @@ void TypeChecker::resolve(const TruthyPredicate& truthyP, ErrorVec& errVec, Refi
         return std::nullopt;
     };
 
-    std::optional<TypeId> ty = resolveLValue(refis, scope, truthyP.lvalue);
-    if (!ty)
-        return;
+    if (FFlag::LuauDiscriminableUnions)
+    {
+        std::optional<TypeId> ty = resolveLValue(refis, scope, truthyP.lvalue);
+        if (ty && fromOr)
+            return addRefinement(refis, truthyP.lvalue, *ty);
 
-    // This is a hack. :(
-    // Without this, the expression 'a or b' might refine 'b' to be falsy.
-    // I'm not yet sure how else to get this to do the right thing without this hack, so we'll do this for now in the meantime.
-    if (fromOr)
-        return addRefinement(refis, truthyP.lvalue, *ty);
+        refineLValue(truthyP.lvalue, refis, scope, predicate);
+    }
+    else
+    {
+        std::optional<TypeId> ty = resolveLValue(refis, scope, truthyP.lvalue);
+        if (!ty)
+            return;
 
-    if (std::optional<TypeId> result = filterMap(*ty, predicate))
-        addRefinement(refis, truthyP.lvalue, *result);
+        // This is a hack. :(
+        // Without this, the expression 'a or b' might refine 'b' to be falsy.
+        // I'm not yet sure how else to get this to do the right thing without this hack, so we'll do this for now in the meantime.
+        if (fromOr)
+            return addRefinement(refis, truthyP.lvalue, *ty);
+
+        if (std::optional<TypeId> result = filterMap(*ty, predicate))
+            addRefinement(refis, truthyP.lvalue, *result);
+    }
 }
 
 void TypeChecker::resolve(const AndPredicate& andP, ErrorVec& errVec, RefinementMap& refis, const ScopePtr& scope, bool sense)
@@ -5746,16 +5780,23 @@ void TypeChecker::resolve(const IsAPredicate& isaP, ErrorVec& errVec, Refinement
         return res;
     };
 
-    std::optional<TypeId> ty = resolveLValue(refis, scope, isaP.lvalue);
-    if (!ty)
-        return;
-
-    if (std::optional<TypeId> result = filterMap(*ty, predicate))
-        addRefinement(refis, isaP.lvalue, *result);
+    if (FFlag::LuauDiscriminableUnions)
+    {
+        refineLValue(isaP.lvalue, refis, scope, predicate);
+    }
     else
     {
-        addRefinement(refis, isaP.lvalue, errorRecoveryType(scope));
-        errVec.push_back(TypeError{isaP.location, TypeMismatch{isaP.ty, *ty}});
+        std::optional<TypeId> ty = resolveLValue(refis, scope, isaP.lvalue);
+        if (!ty)
+            return;
+
+        if (std::optional<TypeId> result = filterMap(*ty, predicate))
+            addRefinement(refis, isaP.lvalue, *result);
+        else
+        {
+            addRefinement(refis, isaP.lvalue, errorRecoveryType(scope));
+            errVec.push_back(TypeError{isaP.location, TypeMismatch{isaP.ty, *ty}});
+        }
     }
 }
 
@@ -5814,21 +5855,30 @@ void TypeChecker::resolve(const TypeGuardPredicate& typeguardP, ErrorVec& errVec
 
     if (auto it = primitives.find(typeguardP.kind); it != primitives.end())
     {
-        if (std::optional<TypeId> result = filterMap(*ty, it->second(sense)))
-            addRefinement(refis, typeguardP.lvalue, *result);
+        if (FFlag::LuauDiscriminableUnions)
+        {
+            refineLValue(typeguardP.lvalue, refis, scope, it->second(sense));
+            return;
+        }
         else
         {
-            addRefinement(refis, typeguardP.lvalue, errorRecoveryType(scope));
-            if (sense)
-                errVec.push_back(
-                    TypeError{typeguardP.location, GenericError{"Type '" + toString(*ty) + "' has no overlap with '" + typeguardP.kind + "'"}});
-        }
+            if (std::optional<TypeId> result = filterMap(*ty, it->second(sense)))
+                addRefinement(refis, typeguardP.lvalue, *result);
+            else
+            {
+                addRefinement(refis, typeguardP.lvalue, errorRecoveryType(scope));
+                if (sense)
+                    errVec.push_back(
+                        TypeError{typeguardP.location, GenericError{"Type '" + toString(*ty) + "' has no overlap with '" + typeguardP.kind + "'"}});
+            }
 
-        return;
+            return;
+        }
     }
 
     auto fail = [&](const TypeErrorData& err) {
-        errVec.push_back(TypeError{typeguardP.location, err});
+        if (!FFlag::LuauDiscriminableUnions)
+            errVec.push_back(TypeError{typeguardP.location, err});
         addRefinement(refis, typeguardP.lvalue, errorRecoveryType(scope));
     };
 
@@ -5853,55 +5903,87 @@ void TypeChecker::resolve(const TypeGuardPredicate& typeguardP, ErrorVec& errVec
 void TypeChecker::resolve(const EqPredicate& eqP, ErrorVec& errVec, RefinementMap& refis, const ScopePtr& scope, bool sense)
 {
     // This refinement will require success typing to do everything correctly. For now, we can get most of the way there.
-
     auto options = [](TypeId ty) -> std::vector<TypeId> {
         if (auto utv = get<UnionTypeVar>(follow(ty)))
             return std::vector<TypeId>(begin(utv), end(utv));
         return {ty};
     };
 
-    if (FFlag::LuauWeakEqConstraint)
+    if (FFlag::LuauDiscriminableUnions)
     {
-        if (!sense && isNil(eqP.type))
-            resolve(TruthyPredicate{std::move(eqP.lvalue), eqP.location}, errVec, refis, scope, true, /* fromOr= */ false);
-
-        return;
-    }
-
-    if (FFlag::LuauEqConstraint)
-    {
-        std::optional<TypeId> ty = resolveLValue(refis, scope, eqP.lvalue);
-        if (!ty)
-            return;
-
-        std::vector<TypeId> lhs = options(*ty);
         std::vector<TypeId> rhs = options(eqP.type);
 
-        if (sense && std::any_of(lhs.begin(), lhs.end(), isUndecidable))
-        {
-            addRefinement(refis, eqP.lvalue, eqP.type);
-            return;
-        }
-        else if (sense && std::any_of(rhs.begin(), rhs.end(), isUndecidable))
+        if (sense && std::any_of(rhs.begin(), rhs.end(), isUndecidable))
             return; // Optimization: the other side has unknown types, so there's probably an overlap. Refining is no-op here.
 
-        std::unordered_set<TypeId> set;
-        for (TypeId left : lhs)
-        {
-            for (TypeId right : rhs)
+        auto predicate = [&](TypeId option) -> std::optional<TypeId> {
+            if (sense && isUndecidable(option))
+                return FFlag::LuauWeakEqConstraint ? option : eqP.type;
+
+            if (!sense && isNil(eqP.type))
+                return (isUndecidable(option) || !isNil(option)) ? std::optional<TypeId>(option) : std::nullopt;
+
+            if (maybeSingleton(eqP.type))
             {
-                // When singleton types arrive, `isNil` here probably should be replaced with `isLiteral`.
-                if (canUnify(right, left, eqP.location).empty() == sense || (!sense && !isNil(left)))
-                    set.insert(left);
+                // Normally we'd write option <: eqP.type, but singletons are always the subtype, so we flip this.
+                if (!sense || canUnify(eqP.type, option, eqP.location).empty())
+                    return sense ? eqP.type : option;
+
+                // local variable works around an odd gcc 9.3 warning: <anonymous> may be used uninitialized
+                std::optional<TypeId> res = std::nullopt;
+                return res;
             }
+
+            return option;
+        };
+
+        refineLValue(eqP.lvalue, refis, scope, predicate);
+    }
+    else
+    {
+        if (FFlag::LuauWeakEqConstraint)
+        {
+            if (!sense && isNil(eqP.type))
+                resolve(TruthyPredicate{std::move(eqP.lvalue), eqP.location}, errVec, refis, scope, true, /* fromOr= */ false);
+
+            return;
         }
 
-        if (set.empty())
-            return;
+        if (FFlag::LuauEqConstraint)
+        {
+            std::optional<TypeId> ty = resolveLValue(refis, scope, eqP.lvalue);
+            if (!ty)
+                return;
 
-        std::vector<TypeId> viable(set.begin(), set.end());
-        TypeId result = viable.size() == 1 ? viable[0] : addType(UnionTypeVar{std::move(viable)});
-        addRefinement(refis, eqP.lvalue, result);
+            std::vector<TypeId> lhs = options(*ty);
+            std::vector<TypeId> rhs = options(eqP.type);
+
+            if (sense && std::any_of(lhs.begin(), lhs.end(), isUndecidable))
+            {
+                addRefinement(refis, eqP.lvalue, eqP.type);
+                return;
+            }
+            else if (sense && std::any_of(rhs.begin(), rhs.end(), isUndecidable))
+                return; // Optimization: the other side has unknown types, so there's probably an overlap. Refining is no-op here.
+
+            std::unordered_set<TypeId> set;
+            for (TypeId left : lhs)
+            {
+                for (TypeId right : rhs)
+                {
+                    // When singleton types arrive, `isNil` here probably should be replaced with `isLiteral`.
+                    if (canUnify(right, left, eqP.location).empty() == sense || (!sense && !isNil(left)))
+                        set.insert(left);
+                }
+            }
+
+            if (set.empty())
+                return;
+
+            std::vector<TypeId> viable(set.begin(), set.end());
+            TypeId result = viable.size() == 1 ? viable[0] : addType(UnionTypeVar{std::move(viable)});
+            addRefinement(refis, eqP.lvalue, result);
+        }
     }
 }
 

Analysis/src/TypeVar.cpp

@@ -18,14 +18,15 @@
 #include <unordered_map>
 #include <unordered_set>
 
+LUAU_FASTFLAG(DebugLuauFreezeArena)
+
 LUAU_FASTINTVARIABLE(LuauTypeMaximumStringifierLength, 500)
 LUAU_FASTINTVARIABLE(LuauTableTypeMaximumStringifierLength, 0)
 LUAU_FASTINT(LuauTypeInferRecursionLimit)
 LUAU_FASTFLAG(LuauLengthOnCompositeType)
 LUAU_FASTFLAGVARIABLE(LuauMetatableAreEqualRecursion, false)
-LUAU_FASTFLAGVARIABLE(LuauRefactorTagging, false)
+LUAU_FASTFLAGVARIABLE(LuauRefactorTypeVarQuestions, false)
 LUAU_FASTFLAG(LuauErrorRecoveryType)
-LUAU_FASTFLAG(DebugLuauFreezeArena)
 
 namespace Luau
 {
@@ -144,7 +145,20 @@ bool isNil(TypeId ty)
 
 bool isBoolean(TypeId ty)
 {
-    return isPrim(ty, PrimitiveTypeVar::Boolean);
+    if (FFlag::LuauRefactorTypeVarQuestions)
+    {
+        if (isPrim(ty, PrimitiveTypeVar::Boolean) || get<BooleanSingleton>(get<SingletonTypeVar>(follow(ty))))
+            return true;
+
+        if (auto utv = get<UnionTypeVar>(follow(ty)))
+            return std::all_of(begin(utv), end(utv), isBoolean);
+
+        return false;
+    }
+    else
+    {
+        return isPrim(ty, PrimitiveTypeVar::Boolean);
+    }
 }
 
 bool isNumber(TypeId ty)
@@ -154,7 +168,20 @@ bool isNumber(TypeId ty)
 
 bool isString(TypeId ty)
 {
-    return isPrim(ty, PrimitiveTypeVar::String);
+    if (FFlag::LuauRefactorTypeVarQuestions)
+    {
+        if (isPrim(ty, PrimitiveTypeVar::String) || get<StringSingleton>(get<SingletonTypeVar>(follow(ty))))
+            return true;
+
+        if (auto utv = get<UnionTypeVar>(follow(ty)))
+            return std::all_of(begin(utv), end(utv), isString);
+
+        return false;
+    }
+    else
+    {
+        return isPrim(ty, PrimitiveTypeVar::String);
+    }
 }
 
 bool isThread(TypeId ty)
@@ -167,37 +194,45 @@ bool isOptional(TypeId ty)
     if (isNil(ty))
         return true;
 
-    if (!get<UnionTypeVar>(follow(ty)))
-        return false;
-
-    std::unordered_set<TypeId> seen;
-    std::deque<TypeId> queue{ty};
-    while (!queue.empty())
+    if (FFlag::LuauRefactorTypeVarQuestions)
     {
-        TypeId current = follow(queue.front());
-        queue.pop_front();
+        auto utv = get<UnionTypeVar>(follow(ty));
+        if (!utv)
+            return false;
 
-        if (seen.count(current))
-            continue;
-
-        seen.insert(current);
-
-        if (isNil(current))
-            return true;
-
-        if (auto u = get<UnionTypeVar>(current))
+        return std::any_of(begin(utv), end(utv), isNil);
+    }
+    else
+    {
+        std::unordered_set<TypeId> seen;
+        std::deque<TypeId> queue{ty};
+        while (!queue.empty())
         {
-            for (TypeId option : u->options)
-            {
-                if (isNil(option))
-                    return true;
+            TypeId current = follow(queue.front());
+            queue.pop_front();
 
-                queue.push_back(option);
+            if (seen.count(current))
+                continue;
+
+            seen.insert(current);
+
+            if (isNil(current))
+                return true;
+
+            if (auto u = get<UnionTypeVar>(current))
+            {
+                for (TypeId option : u->options)
+                {
+                    if (isNil(option))
+                        return true;
+
+                    queue.push_back(option);
+                }
             }
         }
-    }
 
-    return false;
+        return false;
+    }
 }
 
 bool isTableIntersection(TypeId ty)
@@ -228,13 +263,27 @@ std::optional<TypeId> getMetatable(TypeId type)
         return mtType->metatable;
     else if (const ClassTypeVar* classType = get<ClassTypeVar>(type))
         return classType->metatable;
-    else if (const PrimitiveTypeVar* primitiveType = get<PrimitiveTypeVar>(type); primitiveType && primitiveType->metatable)
+    else if (FFlag::LuauRefactorTypeVarQuestions)
     {
-        LUAU_ASSERT(primitiveType->type == PrimitiveTypeVar::String);
-        return primitiveType->metatable;
+        if (isString(type))
+        {
+            auto ptv = get<PrimitiveTypeVar>(getSingletonTypes().stringType);
+            LUAU_ASSERT(ptv && ptv->metatable);
+            return ptv->metatable;
+        }
+        else
+            return std::nullopt;
     }
     else
-        return std::nullopt;
+    {
+        if (const PrimitiveTypeVar* primitiveType = get<PrimitiveTypeVar>(type); primitiveType && primitiveType->metatable)
+        {
+            LUAU_ASSERT(primitiveType->type == PrimitiveTypeVar::String);
+            return primitiveType->metatable;
+        }
+        else
+            return std::nullopt;
+    }
 }
 
 const TableTypeVar* getTableType(TypeId type)
@@ -696,7 +745,7 @@ TypeId SingletonTypes::makeStringMetatable()
         {"reverse", {stringToStringType}},
         {"sub", {makeFunction(*arena, stringType, {}, {}, {numberType, optionalNumber}, {}, {stringType})}},
         {"upper", {stringToStringType}},
-        {"split", {makeFunction(*arena, stringType, {}, {}, {stringType, optionalString}, {},
+        {"split", {makeFunction(*arena, stringType, {}, {}, {optionalString}, {},
                       {arena->addType(TableTypeVar{{}, TableIndexer{numberType, stringType}, TypeLevel{}})})}},
         {"pack", {arena->addType(FunctionTypeVar{
                      arena->addTypePack(TypePack{{stringType}, anyTypePack}),
@@ -1108,30 +1157,14 @@ static Tags* getTags(TypeId ty)
 
 void attachTag(TypeId ty, const std::string& tagName)
 {
-    if (!FFlag::LuauRefactorTagging)
-    {
-        if (auto ftv = getMutable<FunctionTypeVar>(ty))
-        {
-            ftv->tags.emplace_back(tagName);
-        }
-        else
-        {
-            LUAU_ASSERT(!"Got a non functional type");
-        }
-    }
+    if (auto tags = getTags(ty))
+        tags->push_back(tagName);
     else
-    {
-        if (auto tags = getTags(ty))
-            tags->push_back(tagName);
-        else
-            LUAU_ASSERT(!"This TypeId does not support tags");
-    }
+        LUAU_ASSERT(!"This TypeId does not support tags");
 }
 
 void attachTag(Property& prop, const std::string& tagName)
 {
-    LUAU_ASSERT(FFlag::LuauRefactorTagging);
-
     prop.tags.push_back(tagName);
 }
 
@@ -1140,7 +1173,6 @@ void attachTag(Property& prop, const std::string& tagName)
 // Unfortunately, there's already use cases that's hard to disentangle. For now, we expose it.
 bool hasTag(const Tags& tags, const std::string& tagName)
 {
-    LUAU_ASSERT(FFlag::LuauRefactorTagging);
     return std::find(tags.begin(), tags.end(), tagName) != tags.end();
 }
 

Analysis/src/Unifier.cpp

@@ -17,15 +17,11 @@ LUAU_FASTFLAGVARIABLE(LuauCommittingTxnLogFreeTpPromote, false)
 LUAU_FASTFLAG(LuauUseCommittingTxnLog)
 LUAU_FASTINTVARIABLE(LuauTypeInferIterationLimit, 2000);
 LUAU_FASTFLAGVARIABLE(LuauTableSubtypingVariance2, false);
-LUAU_FASTFLAGVARIABLE(LuauUnionHeuristic, false)
 LUAU_FASTFLAGVARIABLE(LuauTableUnificationEarlyTest, false)
-LUAU_FASTFLAGVARIABLE(LuauOccursCheckOkWithRecursiveFunctions, false)
 LUAU_FASTFLAG(LuauSingletonTypes)
 LUAU_FASTFLAG(LuauErrorRecoveryType);
 LUAU_FASTFLAG(LuauProperTypeLevels);
 LUAU_FASTFLAGVARIABLE(LuauUnifyPackTails, false)
-LUAU_FASTFLAGVARIABLE(LuauExtendedUnionMismatchError, false)
-LUAU_FASTFLAGVARIABLE(LuauExtendedFunctionMismatchError, false)
 
 namespace Luau
 {
@@ -229,8 +225,6 @@ static std::optional<TypeError> hasUnificationTooComplex(const ErrorVec& errors)
 // Used for tagged union matching heuristic, returns first singleton type field
 static std::optional<std::pair<Luau::Name, const SingletonTypeVar*>> getTableMatchTag(TypeId type)
 {
-    LUAU_ASSERT(FFlag::LuauExtendedUnionMismatchError);
-
     type = follow(type);
 
     if (auto ttv = get<TableTypeVar>(type))
@@ -291,7 +285,7 @@ void Unifier::tryUnify_(TypeId subTy, TypeId superTy, bool isFunctionCall, bool
 
     if (FInt::LuauTypeInferIterationLimit > 0 && FInt::LuauTypeInferIterationLimit < sharedState.counters.iterationCount)
     {
-        errors.push_back(TypeError{location, UnificationTooComplex{}});
+        reportError(TypeError{location, UnificationTooComplex{}});
         return;
     }
 
@@ -403,7 +397,7 @@ void Unifier::tryUnify_(TypeId subTy, TypeId superTy, bool isFunctionCall, bool
         if (subGeneric && !subGeneric->level.subsumes(superLevel))
         {
             // TODO: a more informative error message? CLI-39912
-            errors.push_back(TypeError{location, GenericError{"Generic subtype escaping scope"}});
+            reportError(TypeError{location, GenericError{"Generic subtype escaping scope"}});
             return;
         }
 
@@ -448,7 +442,7 @@ void Unifier::tryUnify_(TypeId subTy, TypeId superTy, bool isFunctionCall, bool
         if (superGeneric && !superGeneric->level.subsumes(subFree->level))
         {
             // TODO: a more informative error message? CLI-39912
-            errors.push_back(TypeError{location, GenericError{"Generic supertype escaping scope"}});
+            reportError(TypeError{location, GenericError{"Generic supertype escaping scope"}});
             return;
         }
 
@@ -561,13 +555,13 @@ void Unifier::tryUnify_(TypeId subTy, TypeId superTy, bool isFunctionCall, bool
         }
 
         if (unificationTooComplex)
-            errors.push_back(*unificationTooComplex);
+            reportError(*unificationTooComplex);
         else if (failed)
         {
             if (firstFailedOption)
-                errors.push_back(TypeError{location, TypeMismatch{superTy, subTy, "Not all union options are compatible.", *firstFailedOption}});
+                reportError(TypeError{location, TypeMismatch{superTy, subTy, "Not all union options are compatible.", *firstFailedOption}});
             else
-                errors.push_back(TypeError{location, TypeMismatch{superTy, subTy}});
+                reportError(TypeError{location, TypeMismatch{superTy, subTy}});
         }
     }
     else if (const UnionTypeVar* uv = FFlag::LuauUseCommittingTxnLog ? log.getMutable<UnionTypeVar>(superTy) : get<UnionTypeVar>(superTy))
@@ -582,50 +576,44 @@ void Unifier::tryUnify_(TypeId subTy, TypeId superTy, bool isFunctionCall, bool
         bool foundHeuristic = false;
         size_t startIndex = 0;
 
-        if (FFlag::LuauUnionHeuristic)
+        if (const std::string* subName = getName(subTy))
         {
-            if (const std::string* subName = getName(subTy))
+            for (size_t i = 0; i < uv->options.size(); ++i)
             {
-                for (size_t i = 0; i < uv->options.size(); ++i)
+                const std::string* optionName = getName(uv->options[i]);
+                if (optionName && *optionName == *subName)
                 {
-                    const std::string* optionName = getName(uv->options[i]);
-                    if (optionName && *optionName == *subName)
-                    {
-                        foundHeuristic = true;
-                        startIndex = i;
-                        break;
-                    }
+                    foundHeuristic = true;
+                    startIndex = i;
+                    break;
                 }
             }
+        }
 
-            if (FFlag::LuauExtendedUnionMismatchError)
+        if (auto subMatchTag = getTableMatchTag(subTy))
+        {
+            for (size_t i = 0; i < uv->options.size(); ++i)
             {
-                if (auto subMatchTag = getTableMatchTag(subTy))
+                auto optionMatchTag = getTableMatchTag(uv->options[i]);
+                if (optionMatchTag && optionMatchTag->first == subMatchTag->first && *optionMatchTag->second == *subMatchTag->second)
                 {
-                    for (size_t i = 0; i < uv->options.size(); ++i)
-                    {
-                        auto optionMatchTag = getTableMatchTag(uv->options[i]);
-                        if (optionMatchTag && optionMatchTag->first == subMatchTag->first && *optionMatchTag->second == *subMatchTag->second)
-                        {
-                            foundHeuristic = true;
-                            startIndex = i;
-                            break;
-                        }
-                    }
+                    foundHeuristic = true;
+                    startIndex = i;
+                    break;
                 }
             }
+        }
 
-            if (!foundHeuristic && cacheEnabled)
+        if (!foundHeuristic && cacheEnabled)
+        {
+            for (size_t i = 0; i < uv->options.size(); ++i)
             {
-                for (size_t i = 0; i < uv->options.size(); ++i)
-                {
-                    TypeId type = uv->options[i];
+                TypeId type = uv->options[i];
 
-                    if (cache.contains({type, subTy}) && (variance == Covariant || cache.contains({subTy, type})))
-                    {
-                        startIndex = i;
-                        break;
-                    }
+                if (cache.contains({type, subTy}) && (variance == Covariant || cache.contains({subTy, type})))
+                {
+                    startIndex = i;
+                    break;
                 }
             }
         }
@@ -650,7 +638,7 @@ void Unifier::tryUnify_(TypeId subTy, TypeId superTy, bool isFunctionCall, bool
             {
                 unificationTooComplex = e;
             }
-            else if (FFlag::LuauExtendedUnionMismatchError && !isNil(type))
+            else if (!isNil(type))
             {
                 failedOptionCount++;
 
@@ -664,15 +652,15 @@ void Unifier::tryUnify_(TypeId subTy, TypeId superTy, bool isFunctionCall, bool
 
         if (unificationTooComplex)
         {
-            errors.push_back(*unificationTooComplex);
+            reportError(*unificationTooComplex);
         }
         else if (!found)
         {
-            if (FFlag::LuauExtendedUnionMismatchError && (failedOptionCount == 1 || foundHeuristic) && failedOption)
-                errors.push_back(
+            if ((failedOptionCount == 1 || foundHeuristic) && failedOption)
+                reportError(
                     TypeError{location, TypeMismatch{superTy, subTy, "None of the union options are compatible. For example:", *failedOption}});
             else
-                errors.push_back(TypeError{location, TypeMismatch{superTy, subTy, "none of the union options are compatible"}});
+                reportError(TypeError{location, TypeMismatch{superTy, subTy, "none of the union options are compatible"}});
         }
     }
     else if (const IntersectionTypeVar* uv =
@@ -702,9 +690,9 @@ void Unifier::tryUnify_(TypeId subTy, TypeId superTy, bool isFunctionCall, bool
         }
 
         if (unificationTooComplex)
-            errors.push_back(*unificationTooComplex);
+            reportError(*unificationTooComplex);
         else if (firstFailedOption)
-            errors.push_back(TypeError{location, TypeMismatch{superTy, subTy, "Not all intersection parts are compatible.", *firstFailedOption}});
+            reportError(TypeError{location, TypeMismatch{superTy, subTy, "Not all intersection parts are compatible.", *firstFailedOption}});
     }
     else if (const IntersectionTypeVar* uv =
                  FFlag::LuauUseCommittingTxnLog ? log.getMutable<IntersectionTypeVar>(subTy) : get<IntersectionTypeVar>(subTy))
@@ -754,10 +742,10 @@ void Unifier::tryUnify_(TypeId subTy, TypeId superTy, bool isFunctionCall, bool
         }
 
         if (unificationTooComplex)
-            errors.push_back(*unificationTooComplex);
+            reportError(*unificationTooComplex);
         else if (!found)
         {
-            errors.push_back(TypeError{location, TypeMismatch{superTy, subTy, "none of the intersection parts are compatible"}});
+            reportError(TypeError{location, TypeMismatch{superTy, subTy, "none of the intersection parts are compatible"}});
         }
     }
     else if ((FFlag::LuauUseCommittingTxnLog && log.getMutable<PrimitiveTypeVar>(superTy) && log.getMutable<PrimitiveTypeVar>(subTy)) ||
@@ -801,7 +789,7 @@ void Unifier::tryUnify_(TypeId subTy, TypeId superTy, bool isFunctionCall, bool
         tryUnifyWithClass(subTy, superTy, /*reversed*/ true);
 
     else
-        errors.push_back(TypeError{location, TypeMismatch{superTy, subTy}});
+        reportError(TypeError{location, TypeMismatch{superTy, subTy}});
 
     if (FFlag::LuauUseCommittingTxnLog)
         log.popSeen(superTy, subTy);
@@ -1067,7 +1055,7 @@ void Unifier::tryUnify_(TypePackId subTp, TypePackId superTp, bool isFunctionCal
 
     if (FInt::LuauTypeInferIterationLimit > 0 && FInt::LuauTypeInferIterationLimit < sharedState.counters.iterationCount)
     {
-        errors.push_back(TypeError{location, UnificationTooComplex{}});
+        reportError(TypeError{location, UnificationTooComplex{}});
         return;
     }
 
@@ -1166,7 +1154,7 @@ void Unifier::tryUnify_(TypePackId subTp, TypePackId superTp, bool isFunctionCal
                 {
                     tryUnify_(*subIter, *superIter);
 
-                    if (FFlag::LuauExtendedFunctionMismatchError && !errors.empty() && !firstPackErrorPos)
+                    if (!errors.empty() && !firstPackErrorPos)
                         firstPackErrorPos = loopCount;
 
                     superIter.advance();
@@ -1251,7 +1239,7 @@ void Unifier::tryUnify_(TypePackId subTp, TypePackId superTp, bool isFunctionCal
                     size_t actualSize = size(subTp);
                     if (ctx == CountMismatch::Result)
                         std::swap(expectedSize, actualSize);
-                    errors.push_back(TypeError{location, CountMismatch{expectedSize, actualSize, ctx}});
+                    reportError(TypeError{location, CountMismatch{expectedSize, actualSize, ctx}});
 
                     while (superIter.good())
                     {
@@ -1272,7 +1260,7 @@ void Unifier::tryUnify_(TypePackId subTp, TypePackId superTp, bool isFunctionCal
         }
         else
         {
-            errors.push_back(TypeError{location, GenericError{"Failed to unify type packs"}});
+            reportError(TypeError{location, GenericError{"Failed to unify type packs"}});
         }
     }
     else
@@ -1372,7 +1360,7 @@ void Unifier::tryUnify_(TypePackId subTp, TypePackId superTp, bool isFunctionCal
                 {
                     tryUnify_(*subIter, *superIter);
 
-                    if (FFlag::LuauExtendedFunctionMismatchError && !errors.empty() && !firstPackErrorPos)
+                    if (!errors.empty() && !firstPackErrorPos)
                         firstPackErrorPos = loopCount;
 
                     superIter.advance();
@@ -1459,7 +1447,7 @@ void Unifier::tryUnify_(TypePackId subTp, TypePackId superTp, bool isFunctionCal
                     size_t actualSize = size(subTp);
                     if (ctx == CountMismatch::Result)
                         std::swap(expectedSize, actualSize);
-                    errors.push_back(TypeError{location, CountMismatch{expectedSize, actualSize, ctx}});
+                    reportError(TypeError{location, CountMismatch{expectedSize, actualSize, ctx}});
 
                     while (superIter.good())
                     {
@@ -1480,7 +1468,7 @@ void Unifier::tryUnify_(TypePackId subTp, TypePackId superTp, bool isFunctionCal
         }
         else
         {
-            errors.push_back(TypeError{location, GenericError{"Failed to unify type packs"}});
+            reportError(TypeError{location, GenericError{"Failed to unify type packs"}});
         }
     }
 }
@@ -1493,7 +1481,7 @@ void Unifier::tryUnifyPrimitives(TypeId subTy, TypeId superTy)
         ice("passed non primitive types to unifyPrimitives");
 
     if (superPrim->type != subPrim->type)
-        errors.push_back(TypeError{location, TypeMismatch{superTy, subTy}});
+        reportError(TypeError{location, TypeMismatch{superTy, subTy}});
 }
 
 void Unifier::tryUnifySingletons(TypeId subTy, TypeId superTy)
@@ -1508,13 +1496,13 @@ void Unifier::tryUnifySingletons(TypeId subTy, TypeId superTy)
     if (superSingleton && *superSingleton == *subSingleton)
         return;
 
-    if (superPrim && superPrim->type == PrimitiveTypeVar::Boolean && get<BoolSingleton>(subSingleton) && variance == Covariant)
+    if (superPrim && superPrim->type == PrimitiveTypeVar::Boolean && get<BooleanSingleton>(subSingleton) && variance == Covariant)
         return;
 
     if (superPrim && superPrim->type == PrimitiveTypeVar::String && get<StringSingleton>(subSingleton) && variance == Covariant)
         return;
 
-    errors.push_back(TypeError{location, TypeMismatch{superTy, subTy}});
+    reportError(TypeError{location, TypeMismatch{superTy, subTy}});
 }
 
 void Unifier::tryUnifyFunctions(TypeId subTy, TypeId superTy, bool isFunctionCall)
@@ -1536,10 +1524,7 @@ void Unifier::tryUnifyFunctions(TypeId subTy, TypeId superTy, bool isFunctionCal
     {
         numGenerics = std::min(superFunction->generics.size(), subFunction->generics.size());
 
-        if (FFlag::LuauExtendedFunctionMismatchError)
-            errors.push_back(TypeError{location, TypeMismatch{superTy, subTy, "different number of generic type parameters"}});
-        else
-            errors.push_back(TypeError{location, TypeMismatch{superTy, subTy}});
+        reportError(TypeError{location, TypeMismatch{superTy, subTy, "different number of generic type parameters"}});
     }
 
     size_t numGenericPacks = superFunction->genericPacks.size();
@@ -1547,10 +1532,7 @@ void Unifier::tryUnifyFunctions(TypeId subTy, TypeId superTy, bool isFunctionCal
     {
         numGenericPacks = std::min(superFunction->genericPacks.size(), subFunction->genericPacks.size());
 
-        if (FFlag::LuauExtendedFunctionMismatchError)
-            errors.push_back(TypeError{location, TypeMismatch{superTy, subTy, "different number of generic type pack parameters"}});
-        else
-            errors.push_back(TypeError{location, TypeMismatch{superTy, subTy}});
+        reportError(TypeError{location, TypeMismatch{superTy, subTy, "different number of generic type pack parameters"}});
     }
 
     for (size_t i = 0; i < numGenerics; i++)
@@ -1567,48 +1549,35 @@ void Unifier::tryUnifyFunctions(TypeId subTy, TypeId superTy, bool isFunctionCal
     {
         Unifier innerState = makeChildUnifier();
 
-        if (FFlag::LuauExtendedFunctionMismatchError)
+        innerState.ctx = CountMismatch::Arg;
+        innerState.tryUnify_(superFunction->argTypes, subFunction->argTypes, isFunctionCall);
+
+        bool reported = !innerState.errors.empty();
+
+        if (auto e = hasUnificationTooComplex(innerState.errors))
+            reportError(*e);
+        else if (!innerState.errors.empty() && innerState.firstPackErrorPos)
+            reportError(
+                TypeError{location, TypeMismatch{superTy, subTy, format("Argument #%d type is not compatible.", *innerState.firstPackErrorPos),
+                                        innerState.errors.front()}});
+        else if (!innerState.errors.empty())
+            reportError(TypeError{location, TypeMismatch{superTy, subTy, "", innerState.errors.front()}});
+
+        innerState.ctx = CountMismatch::Result;
+        innerState.tryUnify_(subFunction->retType, superFunction->retType);
+
+        if (!reported)
         {
-            innerState.ctx = CountMismatch::Arg;
-            innerState.tryUnify_(superFunction->argTypes, subFunction->argTypes, isFunctionCall);
-
-            bool reported = !innerState.errors.empty();
-
             if (auto e = hasUnificationTooComplex(innerState.errors))
-                errors.push_back(*e);
+                reportError(*e);
+            else if (!innerState.errors.empty() && size(superFunction->retType) == 1 && finite(superFunction->retType))
+                reportError(TypeError{location, TypeMismatch{superTy, subTy, "Return type is not compatible.", innerState.errors.front()}});
             else if (!innerState.errors.empty() && innerState.firstPackErrorPos)
-                errors.push_back(
-                    TypeError{location, TypeMismatch{superTy, subTy, format("Argument #%d type is not compatible.", *innerState.firstPackErrorPos),
+                reportError(
+                    TypeError{location, TypeMismatch{superTy, subTy, format("Return #%d type is not compatible.", *innerState.firstPackErrorPos),
                                             innerState.errors.front()}});
             else if (!innerState.errors.empty())
-                errors.push_back(TypeError{location, TypeMismatch{superTy, subTy, "", innerState.errors.front()}});
-
-            innerState.ctx = CountMismatch::Result;
-            innerState.tryUnify_(subFunction->retType, superFunction->retType);
-
-            if (!reported)
-            {
-                if (auto e = hasUnificationTooComplex(innerState.errors))
-                    errors.push_back(*e);
-                else if (!innerState.errors.empty() && size(superFunction->retType) == 1 && finite(superFunction->retType))
-                    errors.push_back(TypeError{location, TypeMismatch{superTy, subTy, "Return type is not compatible.", innerState.errors.front()}});
-                else if (!innerState.errors.empty() && innerState.firstPackErrorPos)
-                    errors.push_back(
-                        TypeError{location, TypeMismatch{superTy, subTy, format("Return #%d type is not compatible.", *innerState.firstPackErrorPos),
-                                                innerState.errors.front()}});
-                else if (!innerState.errors.empty())
-                    errors.push_back(TypeError{location, TypeMismatch{superTy, subTy, "", innerState.errors.front()}});
-            }
-        }
-        else
-        {
-            ctx = CountMismatch::Arg;
-            innerState.tryUnify_(superFunction->argTypes, subFunction->argTypes, isFunctionCall);
-
-            ctx = CountMismatch::Result;
-            innerState.tryUnify_(subFunction->retType, superFunction->retType);
-
-            checkChildUnifierTypeMismatch(innerState.errors, superTy, subTy);
+                reportError(TypeError{location, TypeMismatch{superTy, subTy, "", innerState.errors.front()}});
         }
 
         if (FFlag::LuauUseCommittingTxnLog)
@@ -1716,7 +1685,7 @@ void Unifier::tryUnifyTables(TypeId subTy, TypeId superTy, bool isIntersection)
 
         if (!missingProperties.empty())
         {
-            errors.push_back(TypeError{location, MissingProperties{superTy, subTy, std::move(missingProperties)}});
+            reportError(TypeError{location, MissingProperties{superTy, subTy, std::move(missingProperties)}});
             return;
         }
     }
@@ -1734,7 +1703,7 @@ void Unifier::tryUnifyTables(TypeId subTy, TypeId superTy, bool isIntersection)
 
         if (!extraProperties.empty())
         {
-            errors.push_back(TypeError{location, MissingProperties{superTy, subTy, std::move(extraProperties), MissingProperties::Extra}});
+            reportError(TypeError{location, MissingProperties{superTy, subTy, std::move(extraProperties), MissingProperties::Extra}});
             return;
         }
     }
@@ -1957,13 +1926,13 @@ void Unifier::tryUnifyTables(TypeId subTy, TypeId superTy, bool isIntersection)
 
     if (!missingProperties.empty())
     {
-        errors.push_back(TypeError{location, MissingProperties{superTy, subTy, std::move(missingProperties)}});
+        reportError(TypeError{location, MissingProperties{superTy, subTy, std::move(missingProperties)}});
         return;
     }
 
     if (!extraProperties.empty())
     {
-        errors.push_back(TypeError{location, MissingProperties{superTy, subTy, std::move(extraProperties), MissingProperties::Extra}});
+        reportError(TypeError{location, MissingProperties{superTy, subTy, std::move(extraProperties), MissingProperties::Extra}});
         return;
     }
 
@@ -2051,7 +2020,7 @@ void Unifier::DEPRECATED_tryUnifyTables(TypeId subTy, TypeId superTy, bool isInt
         return tryUnifySealedTables(subTy, superTy, isIntersection);
     else if ((superTable->state == TableState::Sealed && subTable->state == TableState::Generic) ||
              (superTable->state == TableState::Generic && subTable->state == TableState::Sealed))
-        errors.push_back(TypeError{location, TypeMismatch{superTy, subTy}});
+        reportError(TypeError{location, TypeMismatch{superTy, subTy}});
     else if ((superTable->state == TableState::Free) != (subTable->state == TableState::Free)) // one table is free and the other is not
     {
         TypeId freeTypeId = subTable->state == TableState::Free ? subTy : superTy;
@@ -2090,7 +2059,7 @@ void Unifier::DEPRECATED_tryUnifyTables(TypeId subTy, TypeId superTy, bool isInt
         {
             const auto& r = subTable->props.find(name);
             if (r == subTable->props.end())
-                errors.push_back(TypeError{location, UnknownProperty{subTy, name}});
+                reportError(TypeError{location, UnknownProperty{subTy, name}});
             else
                 tryUnify_(r->second.type, prop.type);
         }
@@ -2113,7 +2082,7 @@ void Unifier::DEPRECATED_tryUnifyTables(TypeId subTy, TypeId superTy, bool isInt
                 }
             }
             else
-                errors.push_back(TypeError{location, CannotExtendTable{subTy, CannotExtendTable::Indexer}});
+                reportError(TypeError{location, CannotExtendTable{subTy, CannotExtendTable::Indexer}});
         }
     }
     else if (superTable->state == TableState::Sealed)
@@ -2194,7 +2163,7 @@ void Unifier::tryUnifyFreeTable(TypeId subTy, TypeId superTy)
                 }
             }
             else
-                errors.push_back(TypeError{location, UnknownProperty{subTy, freeName}});
+                reportError(TypeError{location, UnknownProperty{subTy, freeName}});
         }
     }
 
@@ -2268,7 +2237,7 @@ void Unifier::tryUnifySealedTables(TypeId subTy, TypeId superTy, bool isIntersec
 
         if (!missingPropertiesInSuper.empty())
         {
-            errors.push_back(TypeError{location, MissingProperties{superTy, subTy, std::move(missingPropertiesInSuper)}});
+            reportError(TypeError{location, MissingProperties{superTy, subTy, std::move(missingPropertiesInSuper)}});
             return;
         }
     }
@@ -2284,7 +2253,7 @@ void Unifier::tryUnifySealedTables(TypeId subTy, TypeId superTy, bool isIntersec
 
             missingPropertiesInSuper.push_back(it.first);
 
-            innerState.errors.push_back(TypeError{location, TypeMismatch{superTy, subTy}});
+            innerState.reportError(TypeError{location, TypeMismatch{superTy, subTy}});
         }
         else
         {
@@ -2299,7 +2268,7 @@ void Unifier::tryUnifySealedTables(TypeId subTy, TypeId superTy, bool isIntersec
                 if (oldErrorSize != innerState.errors.size() && !errorReported)
                 {
                     errorReported = true;
-                    errors.push_back(innerState.errors.back());
+                    reportError(innerState.errors.back());
                 }
             }
             else
@@ -2340,7 +2309,7 @@ void Unifier::tryUnifySealedTables(TypeId subTy, TypeId superTy, bool isIntersec
                 }
             }
             else
-                innerState.errors.push_back(TypeError{location, TypeMismatch{superTy, subTy}});
+                innerState.reportError(TypeError{location, TypeMismatch{superTy, subTy}});
         }
         else
         {
@@ -2369,7 +2338,7 @@ void Unifier::tryUnifySealedTables(TypeId subTy, TypeId superTy, bool isIntersec
                 }
             }
             else
-                innerState.errors.push_back(TypeError{location, TypeMismatch{superTy, subTy}});
+                innerState.reportError(TypeError{location, TypeMismatch{superTy, subTy}});
         }
     }
 
@@ -2386,7 +2355,7 @@ void Unifier::tryUnifySealedTables(TypeId subTy, TypeId superTy, bool isIntersec
 
     if (!missingPropertiesInSuper.empty())
     {
-        errors.push_back(TypeError{location, MissingProperties{superTy, subTy, std::move(missingPropertiesInSuper)}});
+        reportError(TypeError{location, MissingProperties{superTy, subTy, std::move(missingPropertiesInSuper)}});
         return;
     }
 
@@ -2413,7 +2382,7 @@ void Unifier::tryUnifySealedTables(TypeId subTy, TypeId superTy, bool isIntersec
 
         if (!extraPropertiesInSub.empty())
         {
-            errors.push_back(TypeError{location, MissingProperties{superTy, subTy, std::move(extraPropertiesInSub), MissingProperties::Extra}});
+            reportError(TypeError{location, MissingProperties{superTy, subTy, std::move(extraPropertiesInSub), MissingProperties::Extra}});
             return;
         }
     }
@@ -2437,9 +2406,9 @@ void Unifier::tryUnifyWithMetatable(TypeId subTy, TypeId superTy, bool reversed)
         innerState.tryUnify_(subMetatable->metatable, superMetatable->metatable);
 
         if (auto e = hasUnificationTooComplex(innerState.errors))
-            errors.push_back(*e);
+            reportError(*e);
         else if (!innerState.errors.empty())
-            errors.push_back(
+            reportError(
                 TypeError{location, TypeMismatch{reversed ? subTy : superTy, reversed ? superTy : subTy, "", innerState.errors.front()}});
 
         if (FFlag::LuauUseCommittingTxnLog)
@@ -2470,7 +2439,7 @@ void Unifier::tryUnifyWithMetatable(TypeId subTy, TypeId superTy, bool reversed)
         case TableState::Sealed:
         case TableState::Unsealed:
         case TableState::Generic:
-            errors.push_back(mismatchError);
+            reportError(mismatchError);
         }
     }
     else if (FFlag::LuauUseCommittingTxnLog ? (log.getMutable<AnyTypeVar>(subTy) || log.getMutable<ErrorTypeVar>(subTy))
@@ -2479,7 +2448,7 @@ void Unifier::tryUnifyWithMetatable(TypeId subTy, TypeId superTy, bool reversed)
     }
     else
     {
-        errors.push_back(mismatchError);
+        reportError(mismatchError);
     }
 }
 
@@ -2491,9 +2460,9 @@ void Unifier::tryUnifyWithClass(TypeId subTy, TypeId superTy, bool reversed)
 
     auto fail = [&]() {
         if (!reversed)
-            errors.push_back(TypeError{location, TypeMismatch{superTy, subTy}});
+            reportError(TypeError{location, TypeMismatch{superTy, subTy}});
         else
-            errors.push_back(TypeError{location, TypeMismatch{subTy, superTy}});
+            reportError(TypeError{location, TypeMismatch{subTy, superTy}});
     };
 
     const ClassTypeVar* superClass = get<ClassTypeVar>(superTy);
@@ -2538,7 +2507,7 @@ void Unifier::tryUnifyWithClass(TypeId subTy, TypeId superTy, bool reversed)
             if (!classProp)
             {
                 ok = false;
-                errors.push_back(TypeError{location, UnknownProperty{superTy, propName}});
+                reportError(TypeError{location, UnknownProperty{superTy, propName}});
             }
             else
             {
@@ -2577,7 +2546,7 @@ void Unifier::tryUnifyWithClass(TypeId subTy, TypeId superTy, bool reversed)
         {
             ok = false;
             std::string msg = "Class " + superClass->name + " does not have an indexer";
-            errors.push_back(TypeError{location, GenericError{msg}});
+            reportError(TypeError{location, GenericError{msg}});
         }
 
         if (!ok)
@@ -2695,7 +2664,7 @@ void Unifier::tryUnifyVariadics(TypePackId subTp, TypePackId superTp, bool rever
             }
             else if (get<Unifiable::Generic>(tail))
             {
-                errors.push_back(TypeError{location, GenericError{"Cannot unify variadic and generic packs"}});
+                reportError(TypeError{location, GenericError{"Cannot unify variadic and generic packs"}});
             }
             else if (get<Unifiable::Error>(tail))
             {
@@ -2709,7 +2678,7 @@ void Unifier::tryUnifyVariadics(TypePackId subTp, TypePackId superTp, bool rever
     }
     else
     {
-        errors.push_back(TypeError{location, GenericError{"Failed to unify variadic packs"}});
+        reportError(TypeError{location, GenericError{"Failed to unify variadic packs"}});
     }
 }
 
@@ -2886,7 +2855,7 @@ void Unifier::occursCheck(DenseHashSet<TypeId>& seen, TypeId needle, TypeId hays
 
         if (needle == haystack)
         {
-            errors.push_back(TypeError{location, OccursCheckFailed{}});
+            reportError(TypeError{location, OccursCheckFailed{}});
             log.replace(needle, *getSingletonTypes().errorRecoveryType());
 
             return;
@@ -2894,17 +2863,6 @@ void Unifier::occursCheck(DenseHashSet<TypeId>& seen, TypeId needle, TypeId hays
 
         if (log.getMutable<FreeTypeVar>(haystack))
             return;
-        else if (auto a = log.getMutable<FunctionTypeVar>(haystack))
-        {
-            if (!FFlag::LuauOccursCheckOkWithRecursiveFunctions)
-            {
-                for (TypePackIterator it(a->argTypes, &log); it != end(a->argTypes); ++it)
-                    check(*it);
-
-                for (TypePackIterator it(a->retType, &log); it != end(a->retType); ++it)
-                    check(*it);
-            }
-        }
         else if (auto a = log.getMutable<UnionTypeVar>(haystack))
         {
             for (TypeId ty : a->options)
@@ -2934,7 +2892,7 @@ void Unifier::occursCheck(DenseHashSet<TypeId>& seen, TypeId needle, TypeId hays
 
         if (needle == haystack)
         {
-            errors.push_back(TypeError{location, OccursCheckFailed{}});
+            reportError(TypeError{location, OccursCheckFailed{}});
             DEPRECATED_log(needle);
             *asMutable(needle) = *getSingletonTypes().errorRecoveryType();
             return;
@@ -2942,17 +2900,6 @@ void Unifier::occursCheck(DenseHashSet<TypeId>& seen, TypeId needle, TypeId hays
 
         if (get<FreeTypeVar>(haystack))
             return;
-        else if (auto a = get<FunctionTypeVar>(haystack))
-        {
-            if (!FFlag::LuauOccursCheckOkWithRecursiveFunctions)
-            {
-                for (TypeId ty : a->argTypes)
-                    check(ty);
-
-                for (TypeId ty : a->retType)
-                    check(ty);
-            }
-        }
         else if (auto a = get<UnionTypeVar>(haystack))
         {
             for (TypeId ty : a->options)
@@ -2988,7 +2935,7 @@ void Unifier::occursCheck(DenseHashSet<TypePackId>& seen, TypePackId needle, Typ
         if (log.getMutable<Unifiable::Error>(needle))
             return;
 
-        if (!get<Unifiable::Free>(needle))
+        if (!log.getMutable<Unifiable::Free>(needle))
             ice("Expected needle pack to be free");
 
         RecursionLimiter _ra(&sharedState.counters.recursionCount, FInt::LuauTypeInferRecursionLimit);
@@ -2997,32 +2944,18 @@ void Unifier::occursCheck(DenseHashSet<TypePackId>& seen, TypePackId needle, Typ
         {
             if (needle == haystack)
             {
-                errors.push_back(TypeError{location, OccursCheckFailed{}});
+                reportError(TypeError{location, OccursCheckFailed{}});
                 log.replace(needle, *getSingletonTypes().errorRecoveryTypePack());
 
                 return;
             }
 
-            if (auto a = get<TypePack>(haystack))
+            if (auto a = get<TypePack>(haystack); a && a->tail)
             {
-                for (const auto& ty : a->head)
-                {
-                    if (!FFlag::LuauOccursCheckOkWithRecursiveFunctions)
-                    {
-                        if (auto f = log.getMutable<FunctionTypeVar>(log.follow(ty)))
-                        {
-                            occursCheck(seen, needle, f->argTypes);
-                            occursCheck(seen, needle, f->retType);
-                        }
-                    }
-                }
-
-                if (a->tail)
-                {
-                    haystack = follow(*a->tail);
-                    continue;
-                }
+                haystack = log.follow(*a->tail);
+                continue;
             }
+
             break;
         }
     }
@@ -3048,31 +2981,17 @@ void Unifier::occursCheck(DenseHashSet<TypePackId>& seen, TypePackId needle, Typ
         {
             if (needle == haystack)
             {
-                errors.push_back(TypeError{location, OccursCheckFailed{}});
+                reportError(TypeError{location, OccursCheckFailed{}});
                 DEPRECATED_log(needle);
                 *asMutable(needle) = *getSingletonTypes().errorRecoveryTypePack();
             }
 
-            if (auto a = get<TypePack>(haystack))
+            if (auto a = get<TypePack>(haystack); a && a->tail)
             {
-                if (!FFlag::LuauOccursCheckOkWithRecursiveFunctions)
-                {
-                    for (const auto& ty : a->head)
-                    {
-                        if (auto f = get<FunctionTypeVar>(follow(ty)))
-                        {
-                            occursCheck(seen, needle, f->argTypes);
-                            occursCheck(seen, needle, f->retType);
-                        }
-                    }
-                }
-
-                if (a->tail)
-                {
-                    haystack = follow(*a->tail);
-                    continue;
-                }
+                haystack = follow(*a->tail);
+                continue;
             }
+
             break;
         }
     }
@@ -3094,17 +3013,17 @@ bool Unifier::isNonstrictMode() const
 void Unifier::checkChildUnifierTypeMismatch(const ErrorVec& innerErrors, TypeId wantedType, TypeId givenType)
 {
     if (auto e = hasUnificationTooComplex(innerErrors))
-        errors.push_back(*e);
+        reportError(*e);
     else if (!innerErrors.empty())
-        errors.push_back(TypeError{location, TypeMismatch{wantedType, givenType}});
+        reportError(TypeError{location, TypeMismatch{wantedType, givenType}});
 }
 
 void Unifier::checkChildUnifierTypeMismatch(const ErrorVec& innerErrors, const std::string& prop, TypeId wantedType, TypeId givenType)
 {
     if (auto e = hasUnificationTooComplex(innerErrors))
-        errors.push_back(*e);
+        reportError(*e);
     else if (!innerErrors.empty())
-        errors.push_back(
+        reportError(
             TypeError{location, TypeMismatch{wantedType, givenType, format("Property '%s' is not compatible.", prop.c_str()), innerErrors.front()}});
 }
 

CLI/Analyze.cpp

@@ -43,7 +43,7 @@ static void report(ReportFormat format, const char* name, const Luau::Location&
     }
 }
 
-static void reportError(Luau::Frontend& frontend, ReportFormat format, const Luau::TypeError& error)
+static void reportError(const Luau::Frontend& frontend, ReportFormat format, const Luau::TypeError& error)
 {
     std::string humanReadableName = frontend.fileResolver->getHumanReadableModuleName(error.moduleName);
 

CLI/FileUtils.cpp

@@ -15,8 +15,6 @@
 
 #include <string.h>
 
-#define READ_BUFFER_SIZE 4096
-
 #ifdef _WIN32
 static std::wstring fromUtf8(const std::string& path)
 {
@@ -79,9 +77,9 @@ std::optional<std::string> readFile(const std::string& name)
 std::optional<std::string> readStdin()
 {
     std::string result;
-    char buffer[READ_BUFFER_SIZE] = { };
+    char buffer[4096] = { };
 
-    while (fgets(buffer, READ_BUFFER_SIZE, stdin) != nullptr)
+    while (fgets(buffer, sizeof(buffer), stdin) != nullptr)
         result.append(buffer);
 
     // If eof was not reached for stdin, then a read error occurred

CLI/Repl.cpp

@@ -158,7 +158,7 @@ static int lua_collectgarbage(lua_State* L)
     luaL_error(L, "collectgarbage must be called with 'count' or 'collect'");
 }
 
-static void setupState(lua_State* L)
+void setupState(lua_State* L)
 {
     luaL_openlibs(L);
 
@@ -176,7 +176,7 @@ static void setupState(lua_State* L)
     luaL_sandbox(L);
 }
 
-static std::string runCode(lua_State* L, const std::string& source)
+std::string runCode(lua_State* L, const std::string& source)
 {
     std::string bytecode = Luau::compile(source, copts());
 
@@ -206,7 +206,13 @@ static std::string runCode(lua_State* L, const std::string& source)
         if (n)
         {
             luaL_checkstack(T, LUA_MINSTACK, "too many results to print");
-            lua_getglobal(T, "print");
+            lua_getglobal(T, "_PRETTYPRINT");
+            // If _PRETTYPRINT is nil, then use the standard print function instead
+            if (lua_isnil(T, -1))
+            {
+                lua_pop(T, 1);
+                lua_getglobal(T, "print");
+            }
             lua_insert(T, 1);
             lua_pcall(T, n, 0, 0);
         }
@@ -545,7 +551,7 @@ static int assertionHandler(const char* expr, const char* file, int line, const
     return 1;
 }
 
-int main(int argc, char** argv)
+int replMain(int argc, char** argv)
 {
     Luau::assertHandler() = assertionHandler;
 
@@ -696,5 +702,6 @@ int main(int argc, char** argv)
     case CliMode::Unknown:
     default:
         LUAU_ASSERT(!"Unhandled cli mode.");
+        return 1;
     }
 }

CLI/Repl.h

@@ -0,0 +1,12 @@
+// This file is part of the Luau programming language and is licensed under MIT License; see LICENSE.txt for details
+#pragma once
+
+#include "lua.h"
+
+#include <string>
+
+// Note: These are internal functions which are being exposed in a header
+// so they can be included by unit tests.
+int replMain(int argc, char** argv);
+void setupState(lua_State* L);
+std::string runCode(lua_State* L, const std::string& source);

CLI/ReplEntry.cpp

@@ -0,0 +1,10 @@
+// This file is part of the Luau programming language and is licensed under MIT License; see LICENSE.txt for details
+
+#include "Repl.h"
+
+
+
+int main(int argc, char** argv)
+{
+    return replMain(argc, argv);
+}

CMakeLists.txt

@@ -29,6 +29,7 @@ endif()
 if(LUAU_BUILD_TESTS)
     add_executable(Luau.UnitTest)
     add_executable(Luau.Conformance)
+    add_executable(Luau.CLI.Test)
 endif()
 
 if(LUAU_BUILD_WEB)
@@ -109,6 +110,17 @@ if(LUAU_BUILD_TESTS)
     target_compile_options(Luau.Conformance PRIVATE ${LUAU_OPTIONS})
     target_include_directories(Luau.Conformance PRIVATE extern)
     target_link_libraries(Luau.Conformance PRIVATE Luau.Analysis Luau.Compiler Luau.VM)
+
+    target_compile_options(Luau.CLI.Test PRIVATE ${LUAU_OPTIONS})
+    target_include_directories(Luau.CLI.Test PRIVATE extern CLI)
+    target_link_libraries(Luau.CLI.Test PRIVATE Luau.Compiler Luau.VM)
+    if(UNIX)
+        find_library(LIBPTHREAD pthread)
+        if (LIBPTHREAD)
+            target_link_libraries(Luau.CLI.Test PRIVATE pthread)
+        endif()
+    endif()
+
 endif()
 
 if(LUAU_BUILD_WEB)

Compiler/include/Luau/Bytecode.h

@@ -472,6 +472,9 @@ enum LuauBuiltinFunction
     // bit32.count
     LBF_BIT32_COUNTLZ,
     LBF_BIT32_COUNTRZ,
+
+    // select(_, ...)
+    LBF_SELECT_VARARG,
 };
 
 // Capture type, used in LOP_CAPTURE

Compiler/src/Builtins.cpp

@@ -4,6 +4,8 @@
 #include "Luau/Bytecode.h"
 #include "Luau/Compiler.h"
 
+LUAU_FASTFLAGVARIABLE(LuauCompileSelectBuiltin, false)
+
 namespace Luau
 {
 namespace Compile
@@ -62,6 +64,9 @@ int getBuiltinFunctionId(const Builtin& builtin, const CompileOptions& options)
     if (builtin.isGlobal("unpack"))
         return LBF_TABLE_UNPACK;
 
+    if (FFlag::LuauCompileSelectBuiltin && builtin.isGlobal("select"))
+        return LBF_SELECT_VARARG;
+
     if (builtin.object == "math")
     {
         if (builtin.method == "abs")

Compiler/src/Compiler.cpp

@@ -15,6 +15,9 @@
 #include <bitset>
 #include <math.h>
 
+LUAU_FASTFLAGVARIABLE(LuauCompileTableIndexOpt, false)
+LUAU_FASTFLAG(LuauCompileSelectBuiltin)
+
 namespace Luau
 {
 
@@ -261,6 +264,122 @@ struct Compiler
         bytecode.emitABC(LOP_GETVARARGS, target, multRet ? 0 : uint8_t(targetCount + 1), 0);
     }
 
+    void compileExprSelectVararg(AstExprCall* expr, uint8_t target, uint8_t targetCount, bool targetTop, bool multRet, uint8_t regs)
+    {
+        LUAU_ASSERT(FFlag::LuauCompileSelectBuiltin);
+        LUAU_ASSERT(targetCount == 1);
+        LUAU_ASSERT(!expr->self);
+        LUAU_ASSERT(expr->args.size == 2 && expr->args.data[1]->is<AstExprVarargs>());
+
+        AstExpr* arg = expr->args.data[0];
+
+        uint8_t argreg;
+
+        if (isExprLocalReg(arg))
+            argreg = getLocal(arg->as<AstExprLocal>()->local);
+        else
+        {
+            argreg = uint8_t(regs + 1);
+            compileExprTempTop(arg, argreg);
+        }
+
+        size_t fastcallLabel = bytecode.emitLabel();
+
+        bytecode.emitABC(LOP_FASTCALL1, LBF_SELECT_VARARG, argreg, 0);
+
+        // note, these instructions are normally not executed and are used as a fallback for FASTCALL
+        // we can't use TempTop variant here because we need to make sure the arguments we already computed aren't overwritten
+        compileExprTemp(expr->func, regs);
+
+        bytecode.emitABC(LOP_GETVARARGS, uint8_t(regs + 2), 0, 0);
+
+        size_t callLabel = bytecode.emitLabel();
+        if (!bytecode.patchSkipC(fastcallLabel, callLabel))
+            CompileError::raise(expr->func->location, "Exceeded jump distance limit; simplify the code to compile");
+
+        // note, this is always multCall (last argument is variadic)
+        bytecode.emitABC(LOP_CALL, regs, 0, multRet ? 0 : uint8_t(targetCount + 1));
+
+        // if we didn't output results directly to target, we need to move them
+        if (!targetTop)
+        {
+            for (size_t i = 0; i < targetCount; ++i)
+                bytecode.emitABC(LOP_MOVE, uint8_t(target + i), uint8_t(regs + i), 0);
+        }
+    }
+
+    void compileExprFastcallN(AstExprCall* expr, uint8_t target, uint8_t targetCount, bool targetTop, bool multRet, uint8_t regs, int bfid)
+    {
+        LUAU_ASSERT(!expr->self);
+        LUAU_ASSERT(expr->args.size <= 2);
+
+        LuauOpcode opc = expr->args.size == 1 ? LOP_FASTCALL1 : LOP_FASTCALL2;
+
+        uint32_t args[2] = {};
+
+        for (size_t i = 0; i < expr->args.size; ++i)
+        {
+            if (i > 0)
+            {
+                if (int32_t cid = getConstantIndex(expr->args.data[i]); cid >= 0)
+                {
+                    opc = LOP_FASTCALL2K;
+                    args[i] = cid;
+                    break;
+                }
+            }
+
+            if (isExprLocalReg(expr->args.data[i]))
+                args[i] = getLocal(expr->args.data[i]->as<AstExprLocal>()->local);
+            else
+            {
+                args[i] = uint8_t(regs + 1 + i);
+                compileExprTempTop(expr->args.data[i], uint8_t(args[i]));
+            }
+        }
+
+        size_t fastcallLabel = bytecode.emitLabel();
+
+        bytecode.emitABC(opc, uint8_t(bfid), uint8_t(args[0]), 0);
+        if (opc != LOP_FASTCALL1)
+            bytecode.emitAux(args[1]);
+
+        // Set up a traditional Lua stack for the subsequent LOP_CALL.
+        // Note, as with other instructions that immediately follow FASTCALL, these are normally not executed and are used as a fallback for
+        // these FASTCALL variants.
+        for (size_t i = 0; i < expr->args.size; ++i)
+        {
+            if (i > 0 && opc == LOP_FASTCALL2K)
+            {
+                emitLoadK(uint8_t(regs + 1 + i), args[i]);
+                break;
+            }
+
+            if (args[i] != regs + 1 + i)
+                bytecode.emitABC(LOP_MOVE, uint8_t(regs + 1 + i), uint8_t(args[i]), 0);
+        }
+
+        // note, these instructions are normally not executed and are used as a fallback for FASTCALL
+        // we can't use TempTop variant here because we need to make sure the arguments we already computed aren't overwritten
+        compileExprTemp(expr->func, regs);
+
+        size_t callLabel = bytecode.emitLabel();
+
+        // FASTCALL will skip over the instructions needed to compute function and jump over CALL which must immediately follow the instruction
+        // sequence after FASTCALL
+        if (!bytecode.patchSkipC(fastcallLabel, callLabel))
+            CompileError::raise(expr->func->location, "Exceeded jump distance limit; simplify the code to compile");
+
+        bytecode.emitABC(LOP_CALL, regs, uint8_t(expr->args.size + 1), multRet ? 0 : uint8_t(targetCount + 1));
+
+        // if we didn't output results directly to target, we need to move them
+        if (!targetTop)
+        {
+            for (size_t i = 0; i < targetCount; ++i)
+                bytecode.emitABC(LOP_MOVE, uint8_t(target + i), uint8_t(regs + i), 0);
+        }
+    }
+
     void compileExprCall(AstExprCall* expr, uint8_t target, uint8_t targetCount, bool targetTop = false, bool multRet = false)
     {
         LUAU_ASSERT(!targetTop || unsigned(target + targetCount) == regTop);
@@ -284,6 +403,25 @@ struct Compiler
             bfid = getBuiltinFunctionId(builtin, options);
         }
 
+        if (bfid == LBF_SELECT_VARARG)
+        {
+            LUAU_ASSERT(FFlag::LuauCompileSelectBuiltin);
+            // Optimization: compile select(_, ...) as FASTCALL1; the builtin will read variadic arguments directly
+            // note: for now we restrict this to single-return expressions since our runtime code doesn't deal with general cases
+            if (multRet == false && targetCount == 1 && expr->args.size == 2 && expr->args.data[1]->is<AstExprVarargs>())
+                return compileExprSelectVararg(expr, target, targetCount, targetTop, multRet, regs);
+            else
+                bfid = -1;
+        }
+
+        // Optimization: for 1/2 argument fast calls use specialized opcodes
+        if (!expr->self && bfid >= 0 && expr->args.size >= 1 && expr->args.size <= 2)
+        {
+            AstExpr* last = expr->args.data[expr->args.size - 1];
+            if (!last->is<AstExprCall>() && !last->is<AstExprVarargs>())
+                return compileExprFastcallN(expr, target, targetCount, targetTop, multRet, regs, bfid);
+        }
+
         if (expr->self)
         {
             AstExprIndexName* fi = expr->func->as<AstExprIndexName>();
@@ -309,24 +447,13 @@ struct Compiler
             compileExprTempTop(expr->func, regs);
         }
 
-        // Note: if the last argument is ExprVararg or ExprCall, we need to route that directly to the called function preserving the # of args
         bool multCall = false;
-        bool skipArgs = false;
 
-        if (!expr->self && bfid >= 0 && expr->args.size >= 1 && expr->args.size <= 2)
-        {
-            AstExpr* last = expr->args.data[expr->args.size - 1];
-            skipArgs = !(last->is<AstExprCall>() || last->is<AstExprVarargs>());
-        }
-
-        if (!skipArgs)
-        {
-            for (size_t i = 0; i < expr->args.size; ++i)
-                if (i + 1 == expr->args.size)
-                    multCall = compileExprTempMultRet(expr->args.data[i], uint8_t(regs + 1 + expr->self + i));
-                else
-                    compileExprTempTop(expr->args.data[i], uint8_t(regs + 1 + expr->self + i));
-        }
+        for (size_t i = 0; i < expr->args.size; ++i)
+            if (i + 1 == expr->args.size)
+                multCall = compileExprTempMultRet(expr->args.data[i], uint8_t(regs + 1 + expr->self + i));
+            else
+                compileExprTempTop(expr->args.data[i], uint8_t(regs + 1 + expr->self + i));
 
         setDebugLineEnd(expr->func);
 
@@ -347,59 +474,8 @@ struct Compiler
         }
         else if (bfid >= 0)
         {
-            size_t fastcallLabel;
-
-            if (skipArgs)
-            {
-                LuauOpcode opc = expr->args.size == 1 ? LOP_FASTCALL1 : LOP_FASTCALL2;
-
-                uint32_t args[2] = {};
-                for (size_t i = 0; i < expr->args.size; ++i)
-                {
-                    if (i > 0)
-                    {
-                        if (int32_t cid = getConstantIndex(expr->args.data[i]); cid >= 0)
-                        {
-                            opc = LOP_FASTCALL2K;
-                            args[i] = cid;
-                            break;
-                        }
-                    }
-
-                    if (isExprLocalReg(expr->args.data[i]))
-                        args[i] = getLocal(expr->args.data[i]->as<AstExprLocal>()->local);
-                    else
-                    {
-                        args[i] = uint8_t(regs + 1 + i);
-                        compileExprTempTop(expr->args.data[i], uint8_t(args[i]));
-                    }
-                }
-
-                fastcallLabel = bytecode.emitLabel();
-                bytecode.emitABC(opc, uint8_t(bfid), uint8_t(args[0]), 0);
-                if (opc != LOP_FASTCALL1)
-                    bytecode.emitAux(args[1]);
-
-                // Set up a traditional Lua stack for the subsequent LOP_CALL.
-                // Note, as with other instructions that immediately follow FASTCALL, these are normally not executed and are used as a fallback for
-                // these FASTCALL variants.
-                for (size_t i = 0; i < expr->args.size; ++i)
-                {
-                    if (i > 0 && opc == LOP_FASTCALL2K)
-                    {
-                        emitLoadK(uint8_t(regs + 1 + i), args[i]);
-                        break;
-                    }
-
-                    if (args[i] != regs + 1 + i)
-                        bytecode.emitABC(LOP_MOVE, uint8_t(regs + 1 + i), uint8_t(args[i]), 0);
-                }
-            }
-            else
-            {
-                fastcallLabel = bytecode.emitLabel();
-                bytecode.emitABC(LOP_FASTCALL, uint8_t(bfid), 0, 0);
-            }
+            size_t fastcallLabel = bytecode.emitLabel();
+            bytecode.emitABC(LOP_FASTCALL, uint8_t(bfid), 0, 0);
 
             // note, these instructions are normally not executed and are used as a fallback for FASTCALL
             // we can't use TempTop variant here because we need to make sure the arguments we already computed aren't overwritten
@@ -1101,9 +1177,20 @@ struct Compiler
             for (size_t i = 0; i < expr->items.size; ++i)
             {
                 const AstExprTable::Item& item = expr->items.data[i];
-                AstExprConstantNumber* ckey = item.key->as<AstExprConstantNumber>();
+                LUAU_ASSERT(item.key); // no list portion => all items have keys
 
-                indexSize += (ckey && ckey->value == double(indexSize + 1));
+                if (FFlag::LuauCompileTableIndexOpt)
+                {
+                    const Constant* ckey = constants.find(item.key);
+
+                    indexSize += (ckey && ckey->type == Constant::Type_Number && ckey->valueNumber == double(indexSize + 1));
+                }
+                else
+                {
+                    AstExprConstantNumber* ckey = item.key->as<AstExprConstantNumber>();
+
+                    indexSize += (ckey && ckey->value == double(indexSize + 1));
+                }
             }
 
             // we only perform the optimization if we don't have any other []-keys
@@ -1200,37 +1287,47 @@ struct Compiler
                 arrayChunkCurrent = 0;
             }
 
-            // items with a key are set one by one via SETTABLE/SETTABLEKS
+            // items with a key are set one by one via SETTABLE/SETTABLEKS/SETTABLEN
             if (key)
             {
                 RegScope rsi(this);
 
-                // Optimization: use SETTABLEKS/SETTABLEN for literal keys, this happens often as part of usual table construction syntax
-                if (AstExprConstantString* ckey = key->as<AstExprConstantString>())
+                if (FFlag::LuauCompileTableIndexOpt)
                 {
-                    BytecodeBuilder::StringRef cname = sref(ckey->value);
-                    int32_t cid = bytecode.addConstantString(cname);
-                    if (cid < 0)
-                        CompileError::raise(expr->location, "Exceeded constant limit; simplify the code to compile");
-
+                    LValue lv = compileLValueIndex(reg, key, rsi);
                     uint8_t rv = compileExprAuto(value, rsi);
 
-                    bytecode.emitABC(LOP_SETTABLEKS, rv, reg, uint8_t(BytecodeBuilder::getStringHash(cname)));
-                    bytecode.emitAux(cid);
-                }
-                else if (AstExprConstantNumber* ckey = key->as<AstExprConstantNumber>();
-                         ckey && ckey->value >= 1 && ckey->value <= 256 && double(int(ckey->value)) == ckey->value)
-                {
-                    uint8_t rv = compileExprAuto(value, rsi);
-
-                    bytecode.emitABC(LOP_SETTABLEN, rv, reg, uint8_t(int(ckey->value) - 1));
+                    compileAssign(lv, rv);
                 }
                 else
                 {
-                    uint8_t rk = compileExprAuto(key, rsi);
-                    uint8_t rv = compileExprAuto(value, rsi);
+                    // Optimization: use SETTABLEKS/SETTABLEN for literal keys, this happens often as part of usual table construction syntax
+                    if (AstExprConstantString* ckey = key->as<AstExprConstantString>())
+                    {
+                        BytecodeBuilder::StringRef cname = sref(ckey->value);
+                        int32_t cid = bytecode.addConstantString(cname);
+                        if (cid < 0)
+                            CompileError::raise(expr->location, "Exceeded constant limit; simplify the code to compile");
 
-                    bytecode.emitABC(LOP_SETTABLE, rv, reg, rk);
+                        uint8_t rv = compileExprAuto(value, rsi);
+
+                        bytecode.emitABC(LOP_SETTABLEKS, rv, reg, uint8_t(BytecodeBuilder::getStringHash(cname)));
+                        bytecode.emitAux(cid);
+                    }
+                    else if (AstExprConstantNumber* ckey = key->as<AstExprConstantNumber>();
+                             ckey && ckey->value >= 1 && ckey->value <= 256 && double(int(ckey->value)) == ckey->value)
+                    {
+                        uint8_t rv = compileExprAuto(value, rsi);
+
+                        bytecode.emitABC(LOP_SETTABLEN, rv, reg, uint8_t(int(ckey->value) - 1));
+                    }
+                    else
+                    {
+                        uint8_t rk = compileExprAuto(key, rsi);
+                        uint8_t rv = compileExprAuto(value, rsi);
+
+                        bytecode.emitABC(LOP_SETTABLE, rv, reg, rk);
+                    }
                 }
             }
             // items without a key are set using SETLIST so that we can initialize large arrays quickly
@@ -1339,6 +1436,9 @@ struct Compiler
             uint8_t rt = compileExprAuto(expr->expr, rs);
             uint8_t i = uint8_t(int(cv->valueNumber) - 1);
 
+            if (FFlag::LuauCompileTableIndexOpt)
+                setDebugLine(expr->index);
+
             bytecode.emitABC(LOP_GETTABLEN, target, rt, i);
         }
         else if (cv && cv->type == Constant::Type_String)
@@ -1350,6 +1450,9 @@ struct Compiler
             if (cid < 0)
                 CompileError::raise(expr->location, "Exceeded constant limit; simplify the code to compile");
 
+            if (FFlag::LuauCompileTableIndexOpt)
+                setDebugLine(expr->index);
+
             bytecode.emitABC(LOP_GETTABLEKS, target, rt, uint8_t(BytecodeBuilder::getStringHash(iname)));
             bytecode.emitAux(cid);
         }
@@ -1657,6 +1760,40 @@ struct Compiler
         Location location;
     };
 
+    LValue compileLValueIndex(uint8_t reg, AstExpr* index, RegScope& rs)
+    {
+        const Constant* cv = constants.find(index);
+
+        if (cv && cv->type == Constant::Type_Number && cv->valueNumber >= 1 && cv->valueNumber <= 256 &&
+            double(int(cv->valueNumber)) == cv->valueNumber)
+        {
+            LValue result = {LValue::Kind_IndexNumber};
+            result.reg = reg;
+            result.number = uint8_t(int(cv->valueNumber) - 1);
+            result.location = index->location;
+
+            return result;
+        }
+        else if (cv && cv->type == Constant::Type_String)
+        {
+            LValue result = {LValue::Kind_IndexName};
+            result.reg = reg;
+            result.name = sref(cv->getString());
+            result.location = index->location;
+
+            return result;
+        }
+        else
+        {
+            LValue result = {LValue::Kind_IndexExpr};
+            result.reg = reg;
+            result.index = compileExprAuto(index, rs);
+            result.location = index->location;
+
+            return result;
+        }
+    }
+
     LValue compileLValue(AstExpr* node, RegScope& rs)
     {
         setDebugLine(node);
@@ -1699,36 +1836,9 @@ struct Compiler
         }
         else if (AstExprIndexExpr* expr = node->as<AstExprIndexExpr>())
         {
-            const Constant* cv = constants.find(expr->index);
+            uint8_t reg = compileExprAuto(expr->expr, rs);
 
-            if (cv && cv->type == Constant::Type_Number && cv->valueNumber >= 1 && cv->valueNumber <= 256 &&
-                double(int(cv->valueNumber)) == cv->valueNumber)
-            {
-                LValue result = {LValue::Kind_IndexNumber};
-                result.reg = compileExprAuto(expr->expr, rs);
-                result.number = uint8_t(int(cv->valueNumber) - 1);
-                result.location = node->location;
-
-                return result;
-            }
-            else if (cv && cv->type == Constant::Type_String)
-            {
-                LValue result = {LValue::Kind_IndexName};
-                result.reg = compileExprAuto(expr->expr, rs);
-                result.name = sref(cv->getString());
-                result.location = node->location;
-
-                return result;
-            }
-            else
-            {
-                LValue result = {LValue::Kind_IndexExpr};
-                result.reg = compileExprAuto(expr->expr, rs);
-                result.index = compileExprAuto(expr->index, rs);
-                result.location = node->location;
-
-                return result;
-            }
+            return compileLValueIndex(reg, expr->index, rs);
         }
         else
         {
@@ -1740,6 +1850,9 @@ struct Compiler
 
     void compileLValueUse(const LValue& lv, uint8_t reg, bool set)
     {
+        if (FFlag::LuauCompileTableIndexOpt)
+            setDebugLine(lv.location);
+
         switch (lv.kind)
         {
         case LValue::Kind_Local:

Makefile

@@ -23,11 +23,11 @@ VM_SOURCES=$(wildcard VM/src/*.cpp)
 VM_OBJECTS=$(VM_SOURCES:%=$(BUILD)/%.o)
 VM_TARGET=$(BUILD)/libluauvm.a
 
-TESTS_SOURCES=$(wildcard tests/*.cpp)
+TESTS_SOURCES=$(wildcard tests/*.cpp) CLI/FileUtils.cpp CLI/Profiler.cpp CLI/Coverage.cpp CLI/Repl.cpp
 TESTS_OBJECTS=$(TESTS_SOURCES:%=$(BUILD)/%.o)
 TESTS_TARGET=$(BUILD)/luau-tests
 
-REPL_CLI_SOURCES=CLI/FileUtils.cpp CLI/Profiler.cpp CLI/Coverage.cpp CLI/Repl.cpp
+REPL_CLI_SOURCES=CLI/FileUtils.cpp CLI/Profiler.cpp CLI/Coverage.cpp CLI/Repl.cpp CLI/ReplEntry.cpp
 REPL_CLI_OBJECTS=$(REPL_CLI_SOURCES:%=$(BUILD)/%.o)
 REPL_CLI_TARGET=$(BUILD)/luau
 
@@ -90,11 +90,12 @@ $(AST_OBJECTS): CXXFLAGS+=-std=c++17 -IAst/include
 $(COMPILER_OBJECTS): CXXFLAGS+=-std=c++17 -ICompiler/include -IAst/include
 $(ANALYSIS_OBJECTS): CXXFLAGS+=-std=c++17 -IAst/include -IAnalysis/include
 $(VM_OBJECTS): CXXFLAGS+=-std=c++11 -IVM/include
-$(TESTS_OBJECTS): CXXFLAGS+=-std=c++17 -IAst/include -ICompiler/include -IAnalysis/include -IVM/include -Iextern
+$(TESTS_OBJECTS): CXXFLAGS+=-std=c++17 -IAst/include -ICompiler/include -IAnalysis/include -IVM/include -ICLI -Iextern
 $(REPL_CLI_OBJECTS): CXXFLAGS+=-std=c++17 -IAst/include -ICompiler/include -IVM/include -Iextern
 $(ANALYZE_CLI_OBJECTS): CXXFLAGS+=-std=c++17 -IAst/include -IAnalysis/include -Iextern
 $(FUZZ_OBJECTS): CXXFLAGS+=-std=c++17 -IAst/include -ICompiler/include -IAnalysis/include -IVM/include
 
+$(TESTS_TARGET): LDFLAGS+=-lpthread
 $(REPL_CLI_TARGET): LDFLAGS+=-lpthread
 fuzz-proto fuzz-prototest: LDFLAGS+=build/libprotobuf-mutator/src/libfuzzer/libprotobuf-mutator-libfuzzer.a build/libprotobuf-mutator/src/libprotobuf-mutator.a build/libprotobuf-mutator/external.protobuf/lib/libprotobuf.a
 

Sources.cmake

@@ -176,7 +176,8 @@ if(TARGET Luau.Repl.CLI)
         CLI/FileUtils.cpp
         CLI/Profiler.h
         CLI/Profiler.cpp
-        CLI/Repl.cpp)
+        CLI/Repl.cpp
+        CLI/ReplEntry.cpp)
 endif()
 
 if(TARGET Luau.Analyze.CLI)
@@ -243,6 +244,21 @@ if(TARGET Luau.Conformance)
         tests/main.cpp)
 endif()
 
+if(TARGET Luau.CLI.Test)
+    # Luau.CLI.Test Sources
+    target_sources(Luau.CLI.Test PRIVATE
+        CLI/Coverage.h
+        CLI/Coverage.cpp
+        CLI/FileUtils.h
+        CLI/FileUtils.cpp
+        CLI/Profiler.h
+        CLI/Profiler.cpp
+        CLI/Repl.cpp
+    
+        tests/Repl.test.cpp
+        tests/main.cpp)
+endif()
+
 if(TARGET Luau.Web)
     # Luau.Web Sources
     target_sources(Luau.Web PRIVATE

VM/src/lapi.cpp

@@ -14,6 +14,8 @@
 
 #include <string.h>
 
+LUAU_FASTFLAGVARIABLE(LuauGcForwardMetatableBarrier, false)
+
 const char* lua_ident = "$Lua: Lua 5.1.4 Copyright (C) 1994-2008 Lua.org, PUC-Rio $\n"
                         "$Authors: R. Ierusalimschy, L. H. de Figueiredo & W. Celes $\n"
                         "$URL: www.lua.org $\n";
@@ -869,7 +871,16 @@ int lua_setmetatable(lua_State* L, int objindex)
             luaG_runerror(L, "Attempt to modify a readonly table");
         hvalue(obj)->metatable = mt;
         if (mt)
-            luaC_objbarriert(L, hvalue(obj), mt);
+        {
+            if (FFlag::LuauGcForwardMetatableBarrier)
+            {
+                luaC_objbarrier(L, hvalue(obj), mt);
+            }
+            else
+            {
+                luaC_objbarriert(L, hvalue(obj), mt);
+            }
+        }
         break;
     }
     case LUA_TUSERDATA:

VM/src/lbuiltins.cpp

@@ -1087,6 +1087,34 @@ static int luauF_countrz(lua_State* L, StkId res, TValue* arg0, int nresults, St
     return -1;
 }
 
+static int luauF_select(lua_State* L, StkId res, TValue* arg0, int nresults, StkId args, int nparams)
+{
+    if (nparams == 1 && nresults == 1)
+    {
+        int n = cast_int(L->base - L->ci->func) - clvalue(L->ci->func)->l.p->numparams - 1;
+
+        if (ttisnumber(arg0))
+        {
+            int i = int(nvalue(arg0));
+
+            // i >= 1 && i <= n
+            if (unsigned(i - 1) <= unsigned(n))
+            {
+                setobj2s(L, res, L->base - n + (i - 1));
+                return 1;
+            }
+            // note: for now we don't handle negative case (wrap around) and defer to fallback
+        }
+        else if (ttisstring(arg0) && *svalue(arg0) == '#')
+        {
+            setnvalue(res, double(n));
+            return 1;
+        }
+    }
+
+    return -1;
+}
+
 luau_FastFunction luauF_table[256] = {
     NULL,
     luauF_assert,
@@ -1156,4 +1184,6 @@ luau_FastFunction luauF_table[256] = {
 
     luauF_countlz,
     luauF_countrz,
+
+    luauF_select,
 };

VM/src/lcorolib.cpp

@@ -5,8 +5,6 @@
 #include "lstate.h"
 #include "lvm.h"
 
-LUAU_FASTFLAGVARIABLE(LuauCoroutineClose, false)
-
 #define CO_RUN 0 /* running */
 #define CO_SUS 1 /* suspended */
 #define CO_NOR 2 /* 'normal' (it resumed another coroutine) */
@@ -235,9 +233,6 @@ static int coyieldable(lua_State* L)
 
 static int coclose(lua_State* L)
 {
-    if (!FFlag::LuauCoroutineClose)
-        luaL_error(L, "coroutine.close is not enabled");
-
     lua_State* co = lua_tothread(L, 1);
     luaL_argexpected(L, co, 1, "thread");
 

VM/src/ldo.cpp

@@ -17,8 +17,6 @@
 
 #include <string.h>
 
-LUAU_FASTFLAG(LuauCoroutineClose)
-
 /*
 ** {======================================================
 ** Error-recovery functions
@@ -300,7 +298,7 @@ static void resume(lua_State* L, void* ud)
     {
         // start coroutine
         LUAU_ASSERT(L->ci == L->base_ci && firstArg >= L->base);
-        if (FFlag::LuauCoroutineClose && firstArg == L->base)
+        if (firstArg == L->base)
             luaG_runerror(L, "cannot resume dead coroutine");
 
         if (luau_precall(L, firstArg - 1, LUA_MULTRET) != PCRLUA)

VM/src/lgc.cpp

@@ -93,10 +93,8 @@ static void finishGcCycleStats(global_State* g)
     g->gcstats.lastcycle = g->gcstats.currcycle;
     g->gcstats.currcycle = GCCycleStats();
 
-    g->gcstats.cyclestatsacc.markitems += g->gcstats.lastcycle.markitems;
     g->gcstats.cyclestatsacc.marktime += g->gcstats.lastcycle.marktime;
     g->gcstats.cyclestatsacc.atomictime += g->gcstats.lastcycle.atomictime;
-    g->gcstats.cyclestatsacc.sweepitems += g->gcstats.lastcycle.sweepitems;
     g->gcstats.cyclestatsacc.sweeptime += g->gcstats.lastcycle.sweeptime;
 }
 
@@ -492,23 +490,22 @@ static void freeobj(lua_State* L, GCObject* o, lua_Page* page)
     }
 }
 
-#define sweepwholelist(L, p, tc) sweeplist(L, p, SIZE_MAX, tc)
+#define sweepwholelist(L, p) sweeplist(L, p, SIZE_MAX)
 
-static GCObject** sweeplist(lua_State* L, GCObject** p, size_t count, size_t* traversedcount)
+static GCObject** sweeplist(lua_State* L, GCObject** p, size_t count)
 {
     LUAU_ASSERT(!FFlag::LuauGcPagedSweep);
 
     GCObject* curr;
     global_State* g = L->global;
     int deadmask = otherwhite(g);
-    size_t startcount = count;
     LUAU_ASSERT(testbit(deadmask, FIXEDBIT)); /* make sure we never sweep fixed objects */
     while ((curr = *p) != NULL && count-- > 0)
     {
         int alive = (curr->gch.marked ^ WHITEBITS) & deadmask;
         if (curr->gch.tt == LUA_TTHREAD)
         {
-            sweepwholelist(L, (GCObject**)&gco2th(curr)->openupval, traversedcount); /* sweep open upvalues */
+            sweepwholelist(L, (GCObject**)&gco2th(curr)->openupval); /* sweep open upvalues */
 
             lua_State* th = gco2th(curr);
 
@@ -534,10 +531,6 @@ static GCObject** sweeplist(lua_State* L, GCObject** p, size_t count, size_t* tr
         }
     }
 
-    // if we didn't reach the end of the list it means that we've stopped because the count dropped below zero
-    if (traversedcount)
-        *traversedcount += startcount - (curr ? count + 1 : count);
-
     return p;
 }
 
@@ -721,8 +714,6 @@ static bool sweepgco(lua_State* L, lua_Page* page, GCObject* gco)
 
     int alive = (gco->gch.marked ^ WHITEBITS) & deadmask;
 
-    g->gcstats.currcycle.sweepitems++;
-
     if (gco->gch.tt == LUA_TTHREAD)
     {
         lua_State* th = gco2th(gco);
@@ -793,8 +784,6 @@ static size_t gcstep(lua_State* L, size_t limit)
     {
         while (g->gray && cost < limit)
         {
-            g->gcstats.currcycle.markitems++;
-
             cost += propagatemark(g);
         }
 
@@ -812,8 +801,6 @@ static size_t gcstep(lua_State* L, size_t limit)
     {
         while (g->gray && cost < limit)
         {
-            g->gcstats.currcycle.markitems++;
-
             cost += propagatemark(g);
         }
 
@@ -842,10 +829,8 @@ static size_t gcstep(lua_State* L, size_t limit)
 
         while (g->sweepstrgc < g->strt.size && cost < limit)
         {
-            size_t traversedcount = 0;
-            sweepwholelist(L, (GCObject**)&g->strt.hash[g->sweepstrgc++], &traversedcount);
+            sweepwholelist(L, (GCObject**)&g->strt.hash[g->sweepstrgc++]);
 
-            g->gcstats.currcycle.sweepitems += traversedcount;
             cost += GC_SWEEPCOST;
         }
 
@@ -855,12 +840,10 @@ static size_t gcstep(lua_State* L, size_t limit)
             // sweep string buffer list and preserve used string count
             uint32_t nuse = L->global->strt.nuse;
 
-            size_t traversedcount = 0;
-            sweepwholelist(L, (GCObject**)&g->strbufgc, &traversedcount);
+            sweepwholelist(L, (GCObject**)&g->strbufgc);
 
             L->global->strt.nuse = nuse;
 
-            g->gcstats.currcycle.sweepitems += traversedcount;
             g->gcstate = GCSsweep; // end sweep-string phase
         }
         break;
@@ -893,10 +876,8 @@ static size_t gcstep(lua_State* L, size_t limit)
         {
             while (*g->sweepgc && cost < limit)
             {
-                size_t traversedcount = 0;
-                g->sweepgc = sweeplist(L, g->sweepgc, GC_SWEEPMAX, &traversedcount);
+                g->sweepgc = sweeplist(L, g->sweepgc, GC_SWEEPMAX);
 
-                g->gcstats.currcycle.sweepitems += traversedcount;
                 cost += GC_SWEEPMAX * GC_SWEEPCOST;
             }
 

VM/src/lgc.h

@@ -113,6 +113,7 @@
             luaC_barrierf(L, obj2gco(p), obj2gco(o)); \
     }
 
+// TODO: remove with FFlagLuauGcForwardMetatableBarrier
 #define luaC_objbarriert(L, t, o) \
     { \
         if (isblack(obj2gco(t)) && iswhite(obj2gco(o))) \

VM/src/lstate.h

@@ -96,9 +96,6 @@ struct GCCycleStats
 
     double sweeptime = 0.0;
 
-    size_t markitems = 0;
-    size_t sweepitems = 0;
-
     size_t assistwork = 0;
     size_t explicitwork = 0;
 

tests/AstQuery.test.cpp

@@ -44,10 +44,6 @@ TEST_CASE_FIXTURE(DocumentationSymbolFixture, "prop")
 
 TEST_CASE_FIXTURE(DocumentationSymbolFixture, "event_callback_arg")
 {
-    ScopedFastFlag sffs[] = {
-        {"LuauPersistDefinitionFileTypes", true},
-    };
-
     loadDefinition(R"(
         declare function Connect(fn: (string) -> ())
     )");
@@ -63,8 +59,6 @@ TEST_CASE_FIXTURE(DocumentationSymbolFixture, "event_callback_arg")
 
 TEST_CASE_FIXTURE(DocumentationSymbolFixture, "overloaded_fn")
 {
-    ScopedFastFlag sffs{"LuauStoreMatchingOverloadFnType", true};
-
     loadDefinition(R"(
         declare foo: ((string) -> number) & ((number) -> string)
     )");

tests/Autocomplete.test.cpp

@@ -2626,7 +2626,6 @@ local a: A<(number, s@1>
 TEST_CASE_FIXTURE(ACFixture, "autocomplete_first_function_arg_expected_type")
 {
     ScopedFastFlag luauAutocompleteAvoidMutation("LuauAutocompleteAvoidMutation", true);
-    ScopedFastFlag luauAutocompleteFirstArg("LuauAutocompleteFirstArg", true);
 
     check(R"(
 local function foo1() return 1 end
@@ -2728,4 +2727,39 @@ end
     CHECK(ac.entryMap.count("getx"));
 }
 
+TEST_CASE_FIXTURE(ACFixture, "autocomplete_on_string_singletons")
+{
+    ScopedFastFlag sffs[] = {
+        {"LuauParseSingletonTypes", true},
+        {"LuauSingletonTypes", true},
+        {"LuauRefactorTypeVarQuestions", true},
+    };
+
+    check(R"(
+        --!strict
+        local foo: "hello" | "bye" = "hello"
+        foo:@1
+    )");
+
+    auto ac = autocomplete('1');
+
+    CHECK(ac.entryMap.count("format"));
+}
+
+TEST_CASE_FIXTURE(ACFixture, "function_in_assignment_has_parentheses_2")
+{
+    ScopedFastFlag luauAutocompleteAvoidMutation("LuauAutocompleteAvoidMutation", true);
+    ScopedFastFlag preferToCallFunctionsForIntersects("PreferToCallFunctionsForIntersects", true);
+
+    check(R"(
+local bar: ((number) -> number) & (number, number) -> number)
+local abc = b@1
+    )");
+
+    auto ac = autocomplete('1');
+
+    CHECK(ac.entryMap.count("bar"));
+    CHECK(ac.entryMap["bar"].parens == ParenthesesRecommendation::CursorInside);
+}
+
 TEST_SUITE_END();

tests/Compiler.test.cpp

@@ -603,6 +603,37 @@ RETURN R0 1
 )");
 }
 
+TEST_CASE("TableLiteralsIndexConstant")
+{
+    ScopedFastFlag sff("LuauCompileTableIndexOpt", true);
+
+    // validate that we use SETTTABLEKS for constant variable keys
+    CHECK_EQ("\n" + compileFunction0(R"(
+        local a, b = "key", "value"
+        return {[a] = 42, [b] = 0}
+)"), R"(
+NEWTABLE R0 2 0
+LOADN R1 42
+SETTABLEKS R1 R0 K0
+LOADN R1 0
+SETTABLEKS R1 R0 K1
+RETURN R0 1
+)");
+
+    // validate that we use SETTABLEN for constant variable keys *and* that we predict array size
+    CHECK_EQ("\n" + compileFunction0(R"(
+        local a, b = 1, 2
+        return {[a] = 42, [b] = 0}
+)"), R"(
+NEWTABLE R0 0 2
+LOADN R1 42
+SETTABLEN R1 R0 1
+LOADN R1 0
+SETTABLEN R1 R0 2
+RETURN R0 1
+)");
+}
+
 TEST_CASE("TableSizePredictionBasic")
 {
     CHECK_EQ("\n" + compileFunction0(R"(
@@ -2450,6 +2481,37 @@ return
 )");
 }
 
+TEST_CASE("DebugLineInfoAssignment")
+{
+    ScopedFastFlag sff("LuauCompileTableIndexOpt", true);
+
+    Luau::BytecodeBuilder bcb;
+    bcb.setDumpFlags(Luau::BytecodeBuilder::Dump_Code | Luau::BytecodeBuilder::Dump_Lines);
+    Luau::compileOrThrow(bcb, R"(
+   local a = { b = { c = { d = 3 } } }
+
+a
+["b"]
+["c"]
+["d"] = 4
+)");
+
+    CHECK_EQ("\n" + bcb.dumpFunction(0), R"(
+2: DUPTABLE R0 1
+2: DUPTABLE R1 3
+2: DUPTABLE R2 5
+2: LOADN R3 3
+2: SETTABLEKS R3 R2 K4
+2: SETTABLEKS R2 R1 K2
+2: SETTABLEKS R1 R0 K0
+5: GETTABLEKS R2 R0 K0
+6: GETTABLEKS R1 R2 K2
+7: LOADN R2 4
+7: SETTABLEKS R2 R1 K4
+8: RETURN R0 0
+)");
+}
+
 TEST_CASE("DebugSource")
 {
     const char* source = R"(
@@ -2763,6 +2825,75 @@ RETURN R1 -1
 )");
 }
 
+TEST_CASE("FastcallSelect")
+{
+    ScopedFastFlag sff("LuauCompileSelectBuiltin", true);
+
+    // select(_, ...) compiles to a builtin call
+    CHECK_EQ("\n" + compileFunction0("return (select('#', ...))"), R"(
+LOADK R1 K0
+FASTCALL1 57 R1 +3
+GETIMPORT R0 2
+GETVARARGS R2 -1
+CALL R0 -1 1
+RETURN R0 1
+)");
+
+    // more complex example: select inside a for loop bound + select from a iterator
+    CHECK_EQ("\n" + compileFunction0(R"(
+local sum = 0
+for i=1, select('#', ...) do
+    sum += select(i, ...)
+end
+return sum
+)"), R"(
+LOADN R0 0
+LOADN R3 1
+LOADK R5 K0
+FASTCALL1 57 R5 +3
+GETIMPORT R4 2
+GETVARARGS R6 -1
+CALL R4 -1 1
+MOVE R1 R4
+LOADN R2 1
+FORNPREP R1 +7
+FASTCALL1 57 R3 +3
+GETIMPORT R4 2
+GETVARARGS R6 -1
+CALL R4 -1 1
+ADD R0 R0 R4
+FORNLOOP R1 -7
+RETURN R0 1
+)");
+
+    // currently we assume a single value return to avoid dealing with stack resizing
+    CHECK_EQ("\n" + compileFunction0("return select('#', ...)"), R"(
+GETIMPORT R0 1
+LOADK R1 K2
+GETVARARGS R2 -1
+CALL R0 -1 -1
+RETURN R0 -1
+)");
+
+    // note that select with a non-variadic second argument doesn't get optimized
+    CHECK_EQ("\n" + compileFunction0("return select('#')"), R"(
+GETIMPORT R0 1
+LOADK R1 K2
+CALL R0 1 -1
+RETURN R0 -1
+)");
+
+    // note that select with a non-variadic second argument doesn't get optimized
+    CHECK_EQ("\n" + compileFunction0("return select('#', foo())"), R"(
+GETIMPORT R0 1
+LOADK R1 K2
+GETIMPORT R2 4
+CALL R2 0 -1
+CALL R0 -1 -1
+RETURN R0 -1
+)");
+}
+
 TEST_CASE("LotsOfParameters")
 {
     const char* source = R"(

tests/Conformance.test.cpp

@@ -331,8 +331,6 @@ TEST_CASE("UTF8")
 
 TEST_CASE("Coroutine")
 {
-    ScopedFastFlag sff("LuauCoroutineClose", true);
-
     runConformance("coroutine.lua");
 }
 

tests/Frontend.test.cpp

@@ -956,7 +956,6 @@ TEST_CASE("no_use_after_free_with_type_fun_instantiation")
 {
     // This flag forces this test to crash if there's a UAF in this code.
     ScopedFastFlag sff_DebugLuauFreezeArena("DebugLuauFreezeArena", true);
-    ScopedFastFlag sff_LuauCloneCorrectlyBeforeMutatingTableType("LuauCloneCorrectlyBeforeMutatingTableType", true);
 
     FrontendFixture fix;
 

tests/Parser.test.cpp

@@ -2000,6 +2000,73 @@ TEST_CASE_FIXTURE(Fixture, "parse_type_alias_default_type_errors")
     matchParseError("type Y<T... = (string) -> number> = {}", "Expected type pack after '=', got type", Location{{0, 14}, {0, 32}});
 }
 
+TEST_CASE_FIXTURE(Fixture, "parse_if_else_expression")
+{
+    {
+        AstStat* stat = parse("return if true then 1 else 2");
+
+        REQUIRE(stat != nullptr);
+        AstStatReturn* str = stat->as<AstStatBlock>()->body.data[0]->as<AstStatReturn>();
+        REQUIRE(str != nullptr);
+        CHECK(str->list.size == 1);
+        auto* ifElseExpr = str->list.data[0]->as<AstExprIfElse>();
+        REQUIRE(ifElseExpr != nullptr);
+    }
+
+    {
+        AstStat* stat = parse("return if true then 1 elseif true then 2 else 3");
+
+        REQUIRE(stat != nullptr);
+        AstStatReturn* str = stat->as<AstStatBlock>()->body.data[0]->as<AstStatReturn>();
+        REQUIRE(str != nullptr);
+        CHECK(str->list.size == 1);
+        auto* ifElseExpr1 = str->list.data[0]->as<AstExprIfElse>();
+        REQUIRE(ifElseExpr1 != nullptr);
+        auto* ifElseExpr2 = ifElseExpr1->falseExpr->as<AstExprIfElse>();
+        REQUIRE(ifElseExpr2 != nullptr);
+    }
+
+    // Use "else if" as opposed to elseif
+    {
+        AstStat* stat = parse("return if true then 1 else if true then 2 else 3");
+
+        REQUIRE(stat != nullptr);
+        AstStatReturn* str = stat->as<AstStatBlock>()->body.data[0]->as<AstStatReturn>();
+        REQUIRE(str != nullptr);
+        CHECK(str->list.size == 1);
+        auto* ifElseExpr1 = str->list.data[0]->as<AstExprIfElse>();
+        REQUIRE(ifElseExpr1 != nullptr);
+        auto* ifElseExpr2 = ifElseExpr1->falseExpr->as<AstExprIfElse>();
+        REQUIRE(ifElseExpr2 != nullptr);
+    }
+
+    // Use an if-else expression as the conditional expression of an if-else expression
+    {
+        AstStat* stat = parse("return if if true then false else true then 1 else 2");
+
+        REQUIRE(stat != nullptr);
+        AstStatReturn* str = stat->as<AstStatBlock>()->body.data[0]->as<AstStatReturn>();
+        REQUIRE(str != nullptr);
+        CHECK(str->list.size == 1);
+        auto* ifElseExpr = str->list.data[0]->as<AstExprIfElse>();
+        REQUIRE(ifElseExpr != nullptr);
+        auto* nestedIfElseExpr = ifElseExpr->condition->as<AstExprIfElse>();
+        REQUIRE(nestedIfElseExpr != nullptr);
+    }
+}
+
+TEST_CASE_FIXTURE(Fixture, "parse_type_pack_type_parameters")
+{
+    AstStat* stat = parse(R"(
+type Packed<T...> = () -> T...
+
+type A<X...> = Packed<X...>
+type B<X...> = Packed<...number>
+type C<X...> = Packed<(number, X...)>
+    )");
+    REQUIRE(stat != nullptr);
+}
+
 TEST_SUITE_END();
 
 TEST_SUITE_BEGIN("ParseErrorRecovery");
@@ -2504,71 +2571,4 @@ type Y<T..., U = T...> = (T...) -> U...
     CHECK_EQ(1, result.errors.size());
 }
 
-TEST_CASE_FIXTURE(Fixture, "parse_if_else_expression")
-{
-    {
-        AstStat* stat = parse("return if true then 1 else 2");
-
-        REQUIRE(stat != nullptr);
-        AstStatReturn* str = stat->as<AstStatBlock>()->body.data[0]->as<AstStatReturn>();
-        REQUIRE(str != nullptr);
-        CHECK(str->list.size == 1);
-        auto* ifElseExpr = str->list.data[0]->as<AstExprIfElse>();
-        REQUIRE(ifElseExpr != nullptr);
-    }
-
-    {
-        AstStat* stat = parse("return if true then 1 elseif true then 2 else 3");
-
-        REQUIRE(stat != nullptr);
-        AstStatReturn* str = stat->as<AstStatBlock>()->body.data[0]->as<AstStatReturn>();
-        REQUIRE(str != nullptr);
-        CHECK(str->list.size == 1);
-        auto* ifElseExpr1 = str->list.data[0]->as<AstExprIfElse>();
-        REQUIRE(ifElseExpr1 != nullptr);
-        auto* ifElseExpr2 = ifElseExpr1->falseExpr->as<AstExprIfElse>();
-        REQUIRE(ifElseExpr2 != nullptr);
-    }
-
-    // Use "else if" as opposed to elseif
-    {
-        AstStat* stat = parse("return if true then 1 else if true then 2 else 3");
-
-        REQUIRE(stat != nullptr);
-        AstStatReturn* str = stat->as<AstStatBlock>()->body.data[0]->as<AstStatReturn>();
-        REQUIRE(str != nullptr);
-        CHECK(str->list.size == 1);
-        auto* ifElseExpr1 = str->list.data[0]->as<AstExprIfElse>();
-        REQUIRE(ifElseExpr1 != nullptr);
-        auto* ifElseExpr2 = ifElseExpr1->falseExpr->as<AstExprIfElse>();
-        REQUIRE(ifElseExpr2 != nullptr);
-    }
-
-    // Use an if-else expression as the conditional expression of an if-else expression
-    {
-        AstStat* stat = parse("return if if true then false else true then 1 else 2");
-
-        REQUIRE(stat != nullptr);
-        AstStatReturn* str = stat->as<AstStatBlock>()->body.data[0]->as<AstStatReturn>();
-        REQUIRE(str != nullptr);
-        CHECK(str->list.size == 1);
-        auto* ifElseExpr = str->list.data[0]->as<AstExprIfElse>();
-        REQUIRE(ifElseExpr != nullptr);
-        auto* nestedIfElseExpr = ifElseExpr->condition->as<AstExprIfElse>();
-        REQUIRE(nestedIfElseExpr != nullptr);
-    }
-}
-
-TEST_CASE_FIXTURE(Fixture, "parse_type_pack_type_parameters")
-{
-    AstStat* stat = parse(R"(
-type Packed<T...> = () -> T...
-
-type A<X...> = Packed<X...>
-type B<X...> = Packed<...number>
-type C<X...> = Packed<(number, X...)>
-    )");
-    REQUIRE(stat != nullptr);
-}
-
 TEST_SUITE_END();

tests/Repl.test.cpp

@@ -0,0 +1,117 @@
+// This file is part of the Luau programming language and is licensed under MIT License; see LICENSE.txt for details
+#include "lua.h"
+#include "lualib.h"
+
+#include "Repl.h"
+
+#include "doctest.h"
+
+#include <iostream>
+#include <memory>
+#include <string>
+#include <vector>
+
+
+class ReplFixture
+{
+public:
+    ReplFixture()
+        : luaState(luaL_newstate(), lua_close)
+    {
+        L = luaState.get();
+        setupState(L);
+        luaL_sandboxthread(L);
+
+        std::string result = runCode(L, prettyPrintSource);
+    }
+
+    // Returns all of the output captured from the pretty printer
+    std::string getCapturedOutput()
+    {
+        lua_getglobal(L, "capturedoutput");
+        const char* str = lua_tolstring(L, -1, nullptr);
+        std::string result(str);
+        lua_pop(L, 1);
+        return result;
+    }
+    lua_State* L;
+
+private:
+    std::unique_ptr<lua_State, void (*)(lua_State*)> luaState;
+
+    // This is a simplicitic and incomplete pretty printer.
+    // It is included here to test that the pretty printer hook is being called.
+    // More elaborate tests to ensure correct output can be added if we introduce
+    // a more feature rich pretty printer.
+    std::string prettyPrintSource = R"(
+-- Accumulate pretty printer output in `capturedoutput`
+capturedoutput = ""
+
+function arraytostring(arr)
+    local strings = {} 
+    table.foreachi(arr, function(k,v) table.insert(strings, pptostring(v)) end )
+    return "{" .. table.concat(strings, ", ") .. "}"
+end
+
+function pptostring(x)
+    if type(x) == "table" then
+        -- Just assume array-like tables for now.
+        return arraytostring(x)
+    elseif type(x) == "string" then
+        return '"' .. x .. '"'
+    else
+        return tostring(x)
+    end
+end
+
+-- Note: Instead of calling print, the pretty printer just stores the output
+-- in `capturedoutput` so we can check for the correct results.
+function _PRETTYPRINT(...)
+    local args = table.pack(...)
+    local strings = {}
+    for i=1, args.n do
+        local item = args[i]
+        local str = pptostring(item, customoptions)
+        if i == 1 then
+            capturedoutput = capturedoutput .. str
+        else
+            capturedoutput = capturedoutput .. "\t" .. str
+        end
+    end
+end
+)";
+};
+
+TEST_SUITE_BEGIN("ReplPrettyPrint");
+
+TEST_CASE_FIXTURE(ReplFixture, "AdditionStatement")
+{
+    runCode(L, "return 30 + 12");
+    CHECK(getCapturedOutput() == "42");
+}
+
+TEST_CASE_FIXTURE(ReplFixture, "TableLiteral")
+{
+    runCode(L, "return {1, 2, 3, 4}");
+    CHECK(getCapturedOutput() == "{1, 2, 3, 4}");
+}
+
+TEST_CASE_FIXTURE(ReplFixture, "StringLiteral")
+{
+    runCode(L, "return 'str'");
+    CHECK(getCapturedOutput() == "\"str\"");
+}
+
+TEST_CASE_FIXTURE(ReplFixture, "TableWithStringLiterals")
+{
+    runCode(L, "return {1, 'two', 3, 'four'}");
+    CHECK(getCapturedOutput() == "{1, \"two\", 3, \"four\"}");
+}
+
+TEST_CASE_FIXTURE(ReplFixture, "MultipleArguments")
+{
+    runCode(L, "return 3, 'three'");
+    CHECK(getCapturedOutput() == "3\t\"three\"");
+}
+
+TEST_SUITE_END();

tests/ToString.test.cpp

@@ -435,8 +435,6 @@ TEST_CASE_FIXTURE(Fixture, "toString_the_boundTo_table_type_contained_within_a_T
 
 TEST_CASE_FIXTURE(Fixture, "no_parentheses_around_cyclic_function_type_in_union")
 {
-    ScopedFastFlag sff{"LuauOccursCheckOkWithRecursiveFunctions", true};
-
     CheckResult result = check(R"(
         type F = ((() -> number)?) -> F?
         local function f(p) return f end
@@ -450,8 +448,6 @@ TEST_CASE_FIXTURE(Fixture, "no_parentheses_around_cyclic_function_type_in_union"
 
 TEST_CASE_FIXTURE(Fixture, "no_parentheses_around_cyclic_function_type_in_intersection")
 {
-    ScopedFastFlag sff{"LuauOccursCheckOkWithRecursiveFunctions", true};
-
     CheckResult result = check(R"(
         function f() return f end
         local a: ((number) -> ()) & typeof(f)

tests/TypeInfer.aliases.test.cpp

@@ -11,8 +11,6 @@ TEST_SUITE_BEGIN("TypeAliases");
 
 TEST_CASE_FIXTURE(Fixture, "cyclic_function_type_in_type_alias")
 {
-    ScopedFastFlag sff{"LuauOccursCheckOkWithRecursiveFunctions", true};
-
     CheckResult result = check(R"(
         type F = () -> F?
         local function f()
@@ -194,8 +192,6 @@ TEST_CASE_FIXTURE(Fixture, "corecursive_types_generic")
 
 TEST_CASE_FIXTURE(Fixture, "corecursive_function_types")
 {
-    ScopedFastFlag sff{"LuauOccursCheckOkWithRecursiveFunctions", true};
-
     CheckResult result = check(R"(
         type A = () -> (number, B)
         type B = () -> (string, A)

tests/TypeInfer.generics.test.cpp

@@ -9,8 +9,6 @@
 
 using namespace Luau;
 
-LUAU_FASTFLAG(LuauExtendedFunctionMismatchError)
-
 TEST_SUITE_BEGIN("GenericsTests");
 
 TEST_CASE_FIXTURE(Fixture, "check_generic_function")
@@ -656,11 +654,7 @@ local d: D = c
 
     LUAU_REQUIRE_ERROR_COUNT(1, result);
 
-    if (FFlag::LuauExtendedFunctionMismatchError)
-        CHECK_EQ(
-            toString(result.errors[0]), R"(Type '() -> ()' could not be converted into '<T>() -> ()'; different number of generic type parameters)");
-    else
-        CHECK_EQ(toString(result.errors[0]), R"(Type '() -> ()' could not be converted into '<T>() -> ()')");
+    CHECK_EQ(toString(result.errors[0]), R"(Type '() -> ()' could not be converted into '<T>() -> ()'; different number of generic type parameters)");
 }
 
 TEST_CASE_FIXTURE(Fixture, "error_detailed_function_mismatch_generic_pack")
@@ -675,11 +669,8 @@ local d: D = c
 
     LUAU_REQUIRE_ERROR_COUNT(1, result);
 
-    if (FFlag::LuauExtendedFunctionMismatchError)
-        CHECK_EQ(toString(result.errors[0]),
-            R"(Type '() -> ()' could not be converted into '<T...>() -> ()'; different number of generic type pack parameters)");
-    else
-        CHECK_EQ(toString(result.errors[0]), R"(Type '() -> ()' could not be converted into '<T...>() -> ()')");
+    CHECK_EQ(toString(result.errors[0]),
+        R"(Type '() -> ()' could not be converted into '<T...>() -> ()'; different number of generic type pack parameters)");
 }
 
 TEST_SUITE_END();

tests/TypeInfer.provisional.test.cpp

@@ -271,6 +271,32 @@ TEST_CASE_FIXTURE(Fixture, "lvalue_equals_another_lvalue_with_no_overlap")
     CHECK_EQ(toString(requireTypeAtPosition({5, 36})), "boolean?"); // a ~= b
 }
 
+// Also belongs in TypeInfer.refinements.test.cpp.
+// Just needs to fully support equality refinement. Which is annoying without type states.
+TEST_CASE_FIXTURE(Fixture, "discriminate_from_x_not_equal_to_nil")
+{
+    ScopedFastFlag sff{"LuauDiscriminableUnions", true};
+
+    CheckResult result = check(R"(
+        type T = {x: string, y: number} | {x: nil, y: nil}
+
+        local function f(t: T)
+            if t.x ~= nil then
+                local foo = t
+            else
+                local bar = t
+            end
+        end
+    )");
+
+    LUAU_REQUIRE_NO_ERRORS(result);
+
+    CHECK_EQ("{| x: string, y: number |}", toString(requireTypeAtPosition({5, 28})));
+
+    // Should be {| x: nil, y: nil |}
+    CHECK_EQ("{| x: nil, y: nil |} | {| x: string, y: number |}", toString(requireTypeAtPosition({7, 28})));
+}
+
 TEST_CASE_FIXTURE(Fixture, "bail_early_if_unification_is_too_complicated" * doctest::timeout(0.5))
 {
     ScopedFastInt sffi{"LuauTarjanChildLimit", 1};
@@ -590,8 +616,6 @@ TEST_CASE_FIXTURE(Fixture, "invariant_table_properties_means_instantiating_table
 
 TEST_CASE_FIXTURE(Fixture, "self_recursive_instantiated_param")
 {
-    ScopedFastFlag luauCloneCorrectlyBeforeMutatingTableType{"LuauCloneCorrectlyBeforeMutatingTableType", true};
-
     // Mutability in type function application right now can create strange recursive types
     // TODO: instantiation right now is problematic, in this example should either leave the Table type alone
     // or it should rename the type to 'Self' so that the result will be 'Self<Table>'

tests/TypeInfer.refinements.test.cpp

@@ -6,11 +6,77 @@
 
 #include "doctest.h"
 
+LUAU_FASTFLAG(LuauDiscriminableUnions)
 LUAU_FASTFLAG(LuauWeakEqConstraint)
 LUAU_FASTFLAG(LuauQuantifyInPlace2)
 
 using namespace Luau;
 
+namespace
+{
+std::optional<ExprResult<TypePackId>> magicFunctionInstanceIsA(
+    TypeChecker& typeChecker, const ScopePtr& scope, const AstExprCall& expr, ExprResult<TypePackId> exprResult)
+{
+    if (expr.args.size != 1)
+        return std::nullopt;
+
+    auto index = expr.func->as<Luau::AstExprIndexName>();
+    auto str = expr.args.data[0]->as<Luau::AstExprConstantString>();
+    if (!index || !str)
+        return std::nullopt;
+
+    std::optional<LValue> lvalue = tryGetLValue(*index->expr);
+    std::optional<TypeFun> tfun = scope->lookupType(std::string(str->value.data, str->value.size));
+    if (!lvalue || !tfun)
+        return std::nullopt;
+
+    unfreeze(typeChecker.globalTypes);
+    TypePackId booleanPack = typeChecker.globalTypes.addTypePack({typeChecker.booleanType});
+    freeze(typeChecker.globalTypes);
+    return ExprResult<TypePackId>{booleanPack, {IsAPredicate{std::move(*lvalue), expr.location, tfun->type}}};
+}
+
+struct RefinementClassFixture : Fixture
+{
+    RefinementClassFixture()
+    {
+        TypeArena& arena = typeChecker.globalTypes;
+
+        unfreeze(arena);
+        TypeId vec3 = arena.addType(ClassTypeVar{"Vector3", {}, std::nullopt, std::nullopt, {}, nullptr});
+        getMutable<ClassTypeVar>(vec3)->props = {
+            {"X", Property{typeChecker.numberType}},
+            {"Y", Property{typeChecker.numberType}},
+            {"Z", Property{typeChecker.numberType}},
+        };
+
+        TypeId inst = arena.addType(ClassTypeVar{"Instance", {}, std::nullopt, std::nullopt, {}, nullptr});
+
+        TypePackId isAParams = arena.addTypePack({inst, typeChecker.stringType});
+        TypePackId isARets = arena.addTypePack({typeChecker.booleanType});
+        TypeId isA = arena.addType(FunctionTypeVar{isAParams, isARets});
+        getMutable<FunctionTypeVar>(isA)->magicFunction = magicFunctionInstanceIsA;
+
+        getMutable<ClassTypeVar>(inst)->props = {
+            {"Name", Property{typeChecker.stringType}},
+            {"IsA", Property{isA}},
+        };
+
+        TypeId folder = typeChecker.globalTypes.addType(ClassTypeVar{"Folder", {}, inst, std::nullopt, {}, nullptr});
+        TypeId part = typeChecker.globalTypes.addType(ClassTypeVar{"Part", {}, inst, std::nullopt, {}, nullptr});
+        getMutable<ClassTypeVar>(part)->props = {
+            {"Position", Property{vec3}},
+        };
+
+        typeChecker.globalScope->exportedTypeBindings["Vector3"] = TypeFun{{}, vec3};
+        typeChecker.globalScope->exportedTypeBindings["Instance"] = TypeFun{{}, inst};
+        typeChecker.globalScope->exportedTypeBindings["Folder"] = TypeFun{{}, folder};
+        typeChecker.globalScope->exportedTypeBindings["Part"] = TypeFun{{}, part};
+        freeze(typeChecker.globalTypes);
+    }
+};
+} // namespace
+
 TEST_SUITE_BEGIN("RefinementTest");
 
 TEST_CASE_FIXTURE(Fixture, "is_truthy_constraint")
@@ -196,8 +262,18 @@ TEST_CASE_FIXTURE(Fixture, "typeguard_only_look_up_types_from_global_scope")
         end
     )");
 
-    LUAU_REQUIRE_ERROR_COUNT(1, result);
-    CHECK_EQ("Type 'number' has no overlap with 'string'", toString(result.errors[0]));
+    if (FFlag::LuauDiscriminableUnions)
+    {
+        LUAU_REQUIRE_NO_ERRORS(result);
+
+        CHECK_EQ("*unknown*", toString(requireTypeAtPosition({8, 44})));
+        CHECK_EQ("*unknown*", toString(requireTypeAtPosition({9, 38})));
+    }
+    else
+    {
+        LUAU_REQUIRE_ERROR_COUNT(1, result);
+        CHECK_EQ("Type 'number' has no overlap with 'string'", toString(result.errors[0]));
+    }
 }
 
 TEST_CASE_FIXTURE(Fixture, "call_a_more_specific_function_using_typeguard")
@@ -237,7 +313,6 @@ TEST_CASE_FIXTURE(Fixture, "impossible_type_narrow_is_not_an_error")
 
 TEST_CASE_FIXTURE(Fixture, "truthy_constraint_on_properties")
 {
-
     CheckResult result = check(R"(
         local t: {x: number?} = {x = 1}
 
@@ -254,7 +329,6 @@ TEST_CASE_FIXTURE(Fixture, "truthy_constraint_on_properties")
 
 TEST_CASE_FIXTURE(Fixture, "index_on_a_refined_property")
 {
-
     CheckResult result = check(R"(
         local t: {x: {y: string}?} = {x = {y = "hello!"}}
 
@@ -360,7 +434,10 @@ TEST_CASE_FIXTURE(Fixture, "lvalue_is_equal_to_a_term")
 
 TEST_CASE_FIXTURE(Fixture, "term_is_equal_to_an_lvalue")
 {
-    ScopedFastFlag sff1{"LuauEqConstraint", true};
+    ScopedFastFlag sff[] = {
+        {"LuauDiscriminableUnions", true},
+        {"LuauSingletonTypes", true},
+    };
 
     CheckResult result = check(R"(
         local function f(a: (string | number)?)
@@ -374,16 +451,8 @@ TEST_CASE_FIXTURE(Fixture, "term_is_equal_to_an_lvalue")
 
     LUAU_REQUIRE_NO_ERRORS(result);
 
-    if (FFlag::LuauWeakEqConstraint)
-    {
-        CHECK_EQ(toString(requireTypeAtPosition({3, 28})), "(number | string)?"); // a == "hello"
-        CHECK_EQ(toString(requireTypeAtPosition({5, 28})), "(number | string)?"); // a ~= "hello"
-    }
-    else
-    {
-        CHECK_EQ(toString(requireTypeAtPosition({3, 28})), "string");             // a == "hello"
-        CHECK_EQ(toString(requireTypeAtPosition({5, 28})), "(number | string)?"); // a ~= "hello"
-    }
+    CHECK_EQ(toString(requireTypeAtPosition({3, 28})), R"("hello")");         // a == "hello"
+    CHECK_EQ(toString(requireTypeAtPosition({5, 28})), "(number | string)?"); // a ~= "hello"
 }
 
 TEST_CASE_FIXTURE(Fixture, "lvalue_is_not_nil")
@@ -416,7 +485,8 @@ TEST_CASE_FIXTURE(Fixture, "lvalue_is_not_nil")
 
 TEST_CASE_FIXTURE(Fixture, "free_type_is_equal_to_an_lvalue")
 {
-    ScopedFastFlag sff1{"LuauEqConstraint", true};
+    ScopedFastFlag sff{"LuauDiscriminableUnions", true};
+    ScopedFastFlag sff2{"LuauWeakEqConstraint", true};
 
     CheckResult result = check(R"(
         local function f(a, b: string?)
@@ -428,16 +498,8 @@ TEST_CASE_FIXTURE(Fixture, "free_type_is_equal_to_an_lvalue")
 
     LUAU_REQUIRE_NO_ERRORS(result);
 
-    if (FFlag::LuauWeakEqConstraint)
-    {
-        CHECK_EQ(toString(requireTypeAtPosition({3, 33})), "a");       // a == b
-        CHECK_EQ(toString(requireTypeAtPosition({3, 36})), "string?"); // a == b
-    }
-    else
-    {
-        CHECK_EQ(toString(requireTypeAtPosition({3, 33})), "string?"); // a == b
-        CHECK_EQ(toString(requireTypeAtPosition({3, 36})), "string?"); // a == b
-    }
+    CHECK_EQ(toString(requireTypeAtPosition({3, 33})), "a");       // a == b
+    CHECK_EQ(toString(requireTypeAtPosition({3, 36})), "string?"); // a == b
 }
 
 TEST_CASE_FIXTURE(Fixture, "unknown_lvalue_is_not_synonymous_with_other_on_not_equal")
@@ -527,9 +589,17 @@ TEST_CASE_FIXTURE(Fixture, "type_narrow_to_vector")
         end
     )");
 
-    // This is kinda weird to see, but this actually only happens in Luau without Roblox type bindings because we don't have a Vector3 type.
-    LUAU_REQUIRE_ERROR_COUNT(1, result);
-    CHECK_EQ("Unknown type 'Vector3'", toString(result.errors[0]));
+    if (FFlag::LuauDiscriminableUnions)
+    {
+        LUAU_REQUIRE_NO_ERRORS(result);
+    }
+    else
+    {
+        // This is kinda weird to see, but this actually only happens in Luau without Roblox type bindings because we don't have a Vector3 type.
+        LUAU_REQUIRE_ERROR_COUNT(1, result);
+        CHECK_EQ("Unknown type 'Vector3'", toString(result.errors[0]));
+    }
+
     CHECK_EQ("*unknown*", toString(requireTypeAtPosition({3, 28})));
 }
 
@@ -614,214 +684,6 @@ TEST_CASE_FIXTURE(Fixture, "typeguard_narrows_for_functions")
     CHECK_EQ("string", toString(requireTypeAtPosition({5, 28})));             // type(x) ~= "function"
 }
 
-namespace
-{
-std::optional<ExprResult<TypePackId>> magicFunctionInstanceIsA(
-    TypeChecker& typeChecker, const ScopePtr& scope, const AstExprCall& expr, ExprResult<TypePackId> exprResult)
-{
-    if (expr.args.size != 1)
-        return std::nullopt;
-
-    auto index = expr.func->as<Luau::AstExprIndexName>();
-    auto str = expr.args.data[0]->as<Luau::AstExprConstantString>();
-    if (!index || !str)
-        return std::nullopt;
-
-    std::optional<LValue> lvalue = tryGetLValue(*index->expr);
-    std::optional<TypeFun> tfun = scope->lookupType(std::string(str->value.data, str->value.size));
-    if (!lvalue || !tfun)
-        return std::nullopt;
-
-    unfreeze(typeChecker.globalTypes);
-    TypePackId booleanPack = typeChecker.globalTypes.addTypePack({typeChecker.booleanType});
-    freeze(typeChecker.globalTypes);
-    return ExprResult<TypePackId>{booleanPack, {IsAPredicate{std::move(*lvalue), expr.location, tfun->type}}};
-}
-
-struct RefinementClassFixture : Fixture
-{
-    RefinementClassFixture()
-    {
-        TypeArena& arena = typeChecker.globalTypes;
-
-        unfreeze(arena);
-        TypeId vec3 = arena.addType(ClassTypeVar{"Vector3", {}, std::nullopt, std::nullopt, {}, nullptr});
-        getMutable<ClassTypeVar>(vec3)->props = {
-            {"X", Property{typeChecker.numberType}},
-            {"Y", Property{typeChecker.numberType}},
-            {"Z", Property{typeChecker.numberType}},
-        };
-
-        TypeId inst = arena.addType(ClassTypeVar{"Instance", {}, std::nullopt, std::nullopt, {}, nullptr});
-
-        TypePackId isAParams = arena.addTypePack({inst, typeChecker.stringType});
-        TypePackId isARets = arena.addTypePack({typeChecker.booleanType});
-        TypeId isA = arena.addType(FunctionTypeVar{isAParams, isARets});
-        getMutable<FunctionTypeVar>(isA)->magicFunction = magicFunctionInstanceIsA;
-
-        getMutable<ClassTypeVar>(inst)->props = {
-            {"Name", Property{typeChecker.stringType}},
-            {"IsA", Property{isA}},
-        };
-
-        TypeId folder = typeChecker.globalTypes.addType(ClassTypeVar{"Folder", {}, inst, std::nullopt, {}, nullptr});
-        TypeId part = typeChecker.globalTypes.addType(ClassTypeVar{"Part", {}, inst, std::nullopt, {}, nullptr});
-        getMutable<ClassTypeVar>(part)->props = {
-            {"Position", Property{vec3}},
-        };
-
-        typeChecker.globalScope->exportedTypeBindings["Vector3"] = TypeFun{{}, vec3};
-        typeChecker.globalScope->exportedTypeBindings["Instance"] = TypeFun{{}, inst};
-        typeChecker.globalScope->exportedTypeBindings["Folder"] = TypeFun{{}, folder};
-        typeChecker.globalScope->exportedTypeBindings["Part"] = TypeFun{{}, part};
-        freeze(typeChecker.globalTypes);
-    }
-};
-} // namespace
-
-TEST_CASE_FIXTURE(RefinementClassFixture, "typeguard_cast_free_table_to_vector")
-{
-    CheckResult result = check(R"(
-        local function f(vec)
-            local X, Y, Z = vec.X, vec.Y, vec.Z
-
-            if type(vec) == "vector" then
-                local foo = vec
-            elseif typeof(vec) == "Instance" then
-                local foo = vec
-            else
-                local foo = vec
-            end
-        end
-    )");
-
-    LUAU_REQUIRE_ERROR_COUNT(1, result);
-
-    CHECK_EQ("Vector3", toString(requireTypeAtPosition({5, 28}))); // type(vec) == "vector"
-
-    if (FFlag::LuauQuantifyInPlace2)
-        CHECK_EQ("Type '{+ X: a, Y: b, Z: c +}' could not be converted into 'Instance'", toString(result.errors[0]));
-    else
-        CHECK_EQ("Type '{- X: a, Y: b, Z: c -}' could not be converted into 'Instance'", toString(result.errors[0]));
-
-    CHECK_EQ("*unknown*", toString(requireTypeAtPosition({7, 28}))); // typeof(vec) == "Instance"
-
-    if (FFlag::LuauQuantifyInPlace2)
-        CHECK_EQ("{+ X: a, Y: b, Z: c +}", toString(requireTypeAtPosition({9, 28}))); // type(vec) ~= "vector" and typeof(vec) ~= "Instance"
-    else
-        CHECK_EQ("{- X: a, Y: b, Z: c -}", toString(requireTypeAtPosition({9, 28}))); // type(vec) ~= "vector" and typeof(vec) ~= "Instance"
-}
-
-TEST_CASE_FIXTURE(RefinementClassFixture, "typeguard_cast_instance_or_vector3_to_vector")
-{
-    CheckResult result = check(R"(
-        local function f(x: Instance | Vector3)
-            if typeof(x) == "Vector3" then
-                local foo = x
-            else
-                local foo = x
-            end
-        end
-    )");
-
-    LUAU_REQUIRE_NO_ERRORS(result);
-
-    CHECK_EQ("Vector3", toString(requireTypeAtPosition({3, 28})));
-    CHECK_EQ("Instance", toString(requireTypeAtPosition({5, 28})));
-}
-
-TEST_CASE_FIXTURE(RefinementClassFixture, "type_narrow_for_all_the_userdata")
-{
-    CheckResult result = check(R"(
-        local function f(x: string | number | Instance | Vector3)
-            if type(x) == "userdata" then
-                local foo = x
-            else
-                local foo = x
-            end
-        end
-    )");
-
-    LUAU_REQUIRE_NO_ERRORS(result);
-
-    CHECK_EQ("Instance | Vector3", toString(requireTypeAtPosition({3, 28})));
-    CHECK_EQ("number | string", toString(requireTypeAtPosition({5, 28})));
-}
-
-TEST_CASE_FIXTURE(RefinementClassFixture, "eliminate_subclasses_of_instance")
-{
-    CheckResult result = check(R"(
-        local function f(x: Part | Folder | string)
-            if typeof(x) == "Instance" then
-                local foo = x
-            else
-                local foo = x
-            end
-        end
-    )");
-
-    LUAU_REQUIRE_NO_ERRORS(result);
-
-    CHECK_EQ("Folder | Part", toString(requireTypeAtPosition({3, 28})));
-    CHECK_EQ("string", toString(requireTypeAtPosition({5, 28})));
-}
-
-TEST_CASE_FIXTURE(RefinementClassFixture, "narrow_this_large_union")
-{
-    CheckResult result = check(R"(
-        local function f(x: Part | Folder | Instance | string | Vector3 | any)
-            if typeof(x) == "Instance" then
-                local foo = x
-            else
-                local foo = x
-            end
-        end
-    )");
-
-    LUAU_REQUIRE_NO_ERRORS(result);
-
-    CHECK_EQ("Folder | Instance | Part", toString(requireTypeAtPosition({3, 28})));
-    CHECK_EQ("Vector3 | any | string", toString(requireTypeAtPosition({5, 28})));
-}
-
-TEST_CASE_FIXTURE(RefinementClassFixture, "x_as_any_if_x_is_instance_elseif_x_is_table")
-{
-    CheckResult result = check(R"(
-        --!nonstrict
-
-        local function f(x)
-            if typeof(x) == "Instance" and x:IsA("Folder") then
-                local foo = x
-            elseif typeof(x) == "table" then
-                local foo = x
-            end
-        end
-    )");
-
-    LUAU_REQUIRE_NO_ERRORS(result);
-
-    CHECK_EQ("Folder", toString(requireTypeAtPosition({5, 28})));
-    CHECK_EQ("any", toString(requireTypeAtPosition({7, 28})));
-}
-
-TEST_CASE_FIXTURE(RefinementClassFixture, "x_is_not_instance_or_else_not_part")
-{
-    CheckResult result = check(R"(
-        local function f(x: Part | Folder | string)
-            if typeof(x) ~= "Instance" or not x:IsA("Part") then
-                local foo = x
-            else
-                local foo = x
-            end
-        end
-    )");
-
-    LUAU_REQUIRE_NO_ERRORS(result);
-
-    CHECK_EQ("Folder | string", toString(requireTypeAtPosition({3, 28})));
-    CHECK_EQ("Part", toString(requireTypeAtPosition({5, 28})));
-}
-
 TEST_CASE_FIXTURE(Fixture, "type_guard_can_filter_for_intersection_of_tables")
 {
     CheckResult result = check(R"(
@@ -1145,4 +1007,259 @@ TEST_CASE_FIXTURE(Fixture, "apply_refinements_on_astexprindexexpr_whose_subscrip
     LUAU_REQUIRE_NO_ERRORS(result);
 }
 
+TEST_CASE_FIXTURE(Fixture, "discriminate_from_truthiness_of_x")
+{
+    ScopedFastFlag sff[] = {
+        {"LuauDiscriminableUnions", true},
+        {"LuauParseSingletonTypes", true},
+        {"LuauSingletonTypes", true},
+    };
+
+    CheckResult result = check(R"(
+        type T = {tag: "missing", x: nil} | {tag: "exists", x: string}
+
+        local function f(t: T)
+            if t.x then
+                local foo = t
+            else
+                local bar = t
+            end
+        end
+    )");
+
+    LUAU_REQUIRE_NO_ERRORS(result);
+
+    CHECK_EQ(R"({| tag: "exists", x: string |})", toString(requireTypeAtPosition({5, 28})));
+    CHECK_EQ(R"({| tag: "missing", x: nil |})", toString(requireTypeAtPosition({7, 28})));
+}
+
+TEST_CASE_FIXTURE(Fixture, "discriminate_tag")
+{
+    ScopedFastFlag sff[] = {
+        {"LuauDiscriminableUnions", true},
+        {"LuauParseSingletonTypes", true},
+        {"LuauSingletonTypes", true},
+    };
+
+    CheckResult result = check(R"(
+        type Cat = {tag: "Cat", name: string, catfood: string}
+        type Dog = {tag: "Dog", name: string, dogfood: string}
+        type Animal = Cat | Dog
+
+        local function f(animal: Animal)
+            if animal.tag == "Cat" then
+                local cat: Cat = animal
+            elseif animal.tag == "Dog" then
+                local dog: Dog = animal
+            end
+        end
+    )");
+
+    LUAU_REQUIRE_NO_ERRORS(result);
+
+    CHECK_EQ("Cat", toString(requireTypeAtPosition({7, 33})));
+    CHECK_EQ("Dog", toString(requireTypeAtPosition({9, 33})));
+}
+
+TEST_CASE_FIXTURE(Fixture, "apply_refinements_on_astexprindexexpr_whose_subscript_expr_is_constant_string")
+{
+    ScopedFastFlag sff{"LuauRefiLookupFromIndexExpr", true};
+
+    CheckResult result = check(R"(
+        type T = { [string]: { prop: number }? }
+        local t: T = {}
+
+        if t["hello"] then
+            local foo = t["hello"].prop
+        end
+    )");
+
+    LUAU_REQUIRE_NO_ERRORS(result);
+}
+
+TEST_CASE_FIXTURE(Fixture, "and_or_peephole_refinement")
+{
+    CheckResult result = check(R"(
+        local function len(a: {any})
+            return a and #a or nil
+        end
+    )");
+
+    LUAU_REQUIRE_NO_ERRORS(result);
+}
+
+TEST_CASE_FIXTURE(RefinementClassFixture, "discriminate_from_isa_of_x")
+{
+    ScopedFastFlag sff[] = {
+        {"LuauDiscriminableUnions", true},
+        {"LuauParseSingletonTypes", true},
+        {"LuauSingletonTypes", true},
+    };
+
+    CheckResult result = check(R"(
+        type T = {tag: "Part", x: Part} | {tag: "Folder", x: Folder}
+
+        local function f(t: T)
+            if t.x:IsA("Part") then
+                local foo = t
+            else
+                local bar = t
+            end
+        end
+    )");
+
+    LUAU_REQUIRE_NO_ERRORS(result);
+
+    CHECK_EQ(R"({| tag: "Part", x: Part |})", toString(requireTypeAtPosition({5, 28})));
+    CHECK_EQ(R"({| tag: "Folder", x: Folder |})", toString(requireTypeAtPosition({7, 28})));
+}
+
+TEST_CASE_FIXTURE(RefinementClassFixture, "typeguard_cast_free_table_to_vector")
+{
+    CheckResult result = check(R"(
+        local function f(vec)
+            local X, Y, Z = vec.X, vec.Y, vec.Z
+
+            if type(vec) == "vector" then
+                local foo = vec
+            elseif typeof(vec) == "Instance" then
+                local foo = vec
+            else
+                local foo = vec
+            end
+        end
+    )");
+
+    if (FFlag::LuauDiscriminableUnions)
+        LUAU_REQUIRE_NO_ERRORS(result);
+    else
+    {
+        LUAU_REQUIRE_ERROR_COUNT(1, result);
+
+        if (FFlag::LuauQuantifyInPlace2)
+            CHECK_EQ("Type '{+ X: a, Y: b, Z: c +}' could not be converted into 'Instance'", toString(result.errors[0]));
+        else
+            CHECK_EQ("Type '{- X: a, Y: b, Z: c -}' could not be converted into 'Instance'", toString(result.errors[0]));
+    }
+
+    CHECK_EQ("Vector3", toString(requireTypeAtPosition({5, 28}))); // type(vec) == "vector"
+
+    CHECK_EQ("*unknown*", toString(requireTypeAtPosition({7, 28}))); // typeof(vec) == "Instance"
+
+    if (FFlag::LuauQuantifyInPlace2)
+        CHECK_EQ("{+ X: a, Y: b, Z: c +}", toString(requireTypeAtPosition({9, 28}))); // type(vec) ~= "vector" and typeof(vec) ~= "Instance"
+    else
+        CHECK_EQ("{- X: a, Y: b, Z: c -}", toString(requireTypeAtPosition({9, 28}))); // type(vec) ~= "vector" and typeof(vec) ~= "Instance"
+}
+
+TEST_CASE_FIXTURE(RefinementClassFixture, "typeguard_cast_instance_or_vector3_to_vector")
+{
+    CheckResult result = check(R"(
+        local function f(x: Instance | Vector3)
+            if typeof(x) == "Vector3" then
+                local foo = x
+            else
+                local foo = x
+            end
+        end
+    )");
+
+    LUAU_REQUIRE_NO_ERRORS(result);
+
+    CHECK_EQ("Vector3", toString(requireTypeAtPosition({3, 28})));
+    CHECK_EQ("Instance", toString(requireTypeAtPosition({5, 28})));
+}
+
+TEST_CASE_FIXTURE(RefinementClassFixture, "type_narrow_for_all_the_userdata")
+{
+    CheckResult result = check(R"(
+        local function f(x: string | number | Instance | Vector3)
+            if type(x) == "userdata" then
+                local foo = x
+            else
+                local foo = x
+            end
+        end
+    )");
+
+    LUAU_REQUIRE_NO_ERRORS(result);
+
+    CHECK_EQ("Instance | Vector3", toString(requireTypeAtPosition({3, 28})));
+    CHECK_EQ("number | string", toString(requireTypeAtPosition({5, 28})));
+}
+
+TEST_CASE_FIXTURE(RefinementClassFixture, "eliminate_subclasses_of_instance")
+{
+    CheckResult result = check(R"(
+        local function f(x: Part | Folder | string)
+            if typeof(x) == "Instance" then
+                local foo = x
+            else
+                local foo = x
+            end
+        end
+    )");
+
+    LUAU_REQUIRE_NO_ERRORS(result);
+
+    CHECK_EQ("Folder | Part", toString(requireTypeAtPosition({3, 28})));
+    CHECK_EQ("string", toString(requireTypeAtPosition({5, 28})));
+}
+
+TEST_CASE_FIXTURE(RefinementClassFixture, "narrow_this_large_union")
+{
+    CheckResult result = check(R"(
+        local function f(x: Part | Folder | Instance | string | Vector3 | any)
+            if typeof(x) == "Instance" then
+                local foo = x
+            else
+                local foo = x
+            end
+        end
+    )");
+
+    LUAU_REQUIRE_NO_ERRORS(result);
+
+    CHECK_EQ("Folder | Instance | Part", toString(requireTypeAtPosition({3, 28})));
+    CHECK_EQ("Vector3 | any | string", toString(requireTypeAtPosition({5, 28})));
+}
+
+TEST_CASE_FIXTURE(RefinementClassFixture, "x_as_any_if_x_is_instance_elseif_x_is_table")
+{
+    CheckResult result = check(R"(
+        --!nonstrict
+
+        local function f(x)
+            if typeof(x) == "Instance" and x:IsA("Folder") then
+                local foo = x
+            elseif typeof(x) == "table" then
+                local foo = x
+            end
+        end
+    )");
+
+    LUAU_REQUIRE_NO_ERRORS(result);
+
+    CHECK_EQ("Folder", toString(requireTypeAtPosition({5, 28})));
+    CHECK_EQ("any", toString(requireTypeAtPosition({7, 28})));
+}
+
+TEST_CASE_FIXTURE(RefinementClassFixture, "x_is_not_instance_or_else_not_part")
+{
+    CheckResult result = check(R"(
+        local function f(x: Part | Folder | string)
+            if typeof(x) ~= "Instance" or not x:IsA("Part") then
+                local foo = x
+            else
+                local foo = x
+            end
+        end
+    )");
+
+    LUAU_REQUIRE_NO_ERRORS(result);
+
+    CHECK_EQ("Folder | string", toString(requireTypeAtPosition({3, 28})));
+    CHECK_EQ("Part", toString(requireTypeAtPosition({5, 28})));
+}
+
 TEST_SUITE_END();

tests/TypeInfer.singletons.test.cpp

@@ -379,9 +379,7 @@ TEST_CASE_FIXTURE(Fixture, "error_detailed_tagged_union_mismatch_string")
     ScopedFastFlag sffs[] = {
         {"LuauSingletonTypes", true},
         {"LuauParseSingletonTypes", true},
-        {"LuauUnionHeuristic", true},
         {"LuauExpectedTypesOfProperties", true},
-        {"LuauExtendedUnionMismatchError", true},
     };
 
     CheckResult result = check(R"(
@@ -404,9 +402,7 @@ TEST_CASE_FIXTURE(Fixture, "error_detailed_tagged_union_mismatch_bool")
     ScopedFastFlag sffs[] = {
         {"LuauSingletonTypes", true},
         {"LuauParseSingletonTypes", true},
-        {"LuauUnionHeuristic", true},
         {"LuauExpectedTypesOfProperties", true},
-        {"LuauExtendedUnionMismatchError", true},
     };
 
     CheckResult result = check(R"(
@@ -429,9 +425,7 @@ TEST_CASE_FIXTURE(Fixture, "if_then_else_expression_singleton_options")
     ScopedFastFlag sffs[] = {
         {"LuauSingletonTypes", true},
         {"LuauParseSingletonTypes", true},
-        {"LuauUnionHeuristic", true},
         {"LuauExpectedTypesOfProperties", true},
-        {"LuauExtendedUnionMismatchError", true},
         {"LuauIfElseExpectedType2", true},
         {"LuauIfElseBranchTypeUnion", true},
     };

tests/TypeInfer.tables.test.cpp

@@ -12,8 +12,6 @@
 
 using namespace Luau;
 
-LUAU_FASTFLAG(LuauExtendedFunctionMismatchError)
-
 TEST_SUITE_BEGIN("TableTests");
 
 TEST_CASE_FIXTURE(Fixture, "basic")
@@ -2075,22 +2073,11 @@ caused by:
 caused by:
   Property 'y' is not compatible. Type 'string' could not be converted into 'number')");
 
-    if (FFlag::LuauExtendedFunctionMismatchError)
-    {
-        CHECK_EQ(toString(result.errors[1]), R"(Type 'b2' could not be converted into 'a2'
+    CHECK_EQ(toString(result.errors[1]), R"(Type 'b2' could not be converted into 'a2'
 caused by:
   Type '{ __call: (a, b) -> () }' could not be converted into '{ __call: <a>(a) -> () }'
 caused by:
   Property '__call' is not compatible. Type '(a, b) -> ()' could not be converted into '<a>(a) -> ()'; different number of generic type parameters)");
-    }
-    else
-    {
-        CHECK_EQ(toString(result.errors[1]), R"(Type 'b2' could not be converted into 'a2'
-caused by:
-  Type '{ __call: (a, b) -> () }' could not be converted into '{ __call: <a>(a) -> () }'
-caused by:
-  Property '__call' is not compatible. Type '(a, b) -> ()' could not be converted into '<a>(a) -> ()')");
-    }
 }
 
 TEST_CASE_FIXTURE(Fixture, "explicitly_typed_table")
@@ -2166,7 +2153,6 @@ a.p = { x = 9 }
 TEST_CASE_FIXTURE(Fixture, "recursive_metatable_type_call")
 {
     ScopedFastFlag sff[]{
-        {"LuauFixRecursiveMetatableCall", true},
         {"LuauUnsealedTableLiteral", true},
     };
 

tests/TypeInfer.test.cpp

@@ -16,7 +16,6 @@
 
 LUAU_FASTFLAG(LuauFixLocationSpanTableIndexExpr)
 LUAU_FASTFLAG(LuauEqConstraint)
-LUAU_FASTFLAG(LuauExtendedFunctionMismatchError)
 
 using namespace Luau;
 
@@ -959,8 +958,6 @@ TEST_CASE_FIXTURE(Fixture, "another_recursive_local_function")
 
 TEST_CASE_FIXTURE(Fixture, "cyclic_function_type_in_rets")
 {
-    ScopedFastFlag sff{"LuauOccursCheckOkWithRecursiveFunctions", true};
-
     CheckResult result = check(R"(
         function f()
             return f
@@ -973,8 +970,6 @@ TEST_CASE_FIXTURE(Fixture, "cyclic_function_type_in_rets")
 
 TEST_CASE_FIXTURE(Fixture, "cyclic_function_type_in_args")
 {
-    ScopedFastFlag sff{"LuauOccursCheckOkWithRecursiveFunctions", true};
-
     CheckResult result = check(R"(
         function f(g)
             return f(f)
@@ -1699,8 +1694,6 @@ TEST_CASE_FIXTURE(Fixture, "first_argument_can_be_optional")
 
 TEST_CASE_FIXTURE(Fixture, "dont_ice_when_failing_the_occurs_check")
 {
-    ScopedFastFlag sff{"LuauOccursCheckOkWithRecursiveFunctions", true};
-
     CheckResult result = check(R"(
         --!strict
         local s
@@ -1711,8 +1704,6 @@ TEST_CASE_FIXTURE(Fixture, "dont_ice_when_failing_the_occurs_check")
 
 TEST_CASE_FIXTURE(Fixture, "occurs_check_does_not_recurse_forever_if_asked_to_traverse_a_cyclic_type")
 {
-    ScopedFastFlag sff{"LuauOccursCheckOkWithRecursiveFunctions", true};
-
     CheckResult result = check(R"(
          --!strict
         function u(t, w)
@@ -3326,11 +3317,12 @@ TEST_CASE_FIXTURE(Fixture, "unknown_type_in_comparison")
     LUAU_REQUIRE_NO_ERRORS(result);
 }
 
-TEST_CASE_FIXTURE(Fixture, "relation_op_on_any_lhs_where_rhs_maybe_has_metatable")
+TEST_CASE_FIXTURE(Fixture, "concat_op_on_free_lhs_and_string_rhs")
 {
     CheckResult result = check(R"(
-        local x
-        print((x == true and (x .. "y")) .. 1)
+        local function f(x)
+            return x .. "y"
+        end
     )");
 
     LUAU_REQUIRE_ERROR_COUNT(1, result);
@@ -3340,13 +3332,14 @@ TEST_CASE_FIXTURE(Fixture, "relation_op_on_any_lhs_where_rhs_maybe_has_metatable
 TEST_CASE_FIXTURE(Fixture, "concat_op_on_string_lhs_and_free_rhs")
 {
     CheckResult result = check(R"(
-        local x
-        print("foo" .. x)
+        local function f(x)
+            return "foo" .. x
+        end
     )");
 
     LUAU_REQUIRE_NO_ERRORS(result);
 
-    CHECK_EQ("string", toString(requireType("x")));
+    CHECK_EQ("(string) -> string", toString(requireType("f")));
 }
 
 TEST_CASE_FIXTURE(Fixture, "strict_binary_op_where_lhs_unknown")
@@ -4374,8 +4367,6 @@ TEST_CASE_FIXTURE(Fixture, "recursive_types_restriction_not_ok")
 
 TEST_CASE_FIXTURE(Fixture, "record_matching_overload")
 {
-    ScopedFastFlag sffs("LuauStoreMatchingOverloadFnType", true);
-
     CheckResult result = check(R"(
         type Overload = ((string) -> string) & ((number) -> number)
         local abc: Overload
@@ -4475,17 +4466,10 @@ f(function(a, b, c, ...) return a + b end)
 
     LUAU_REQUIRE_ERRORS(result);
 
-    if (FFlag::LuauExtendedFunctionMismatchError)
-    {
-        CHECK_EQ(R"(Type '(number, number, a) -> number' could not be converted into '(number, number) -> number'
+    CHECK_EQ(R"(Type '(number, number, a) -> number' could not be converted into '(number, number) -> number'
 caused by:
   Argument count mismatch. Function expects 3 arguments, but only 2 are specified)",
-            toString(result.errors[0]));
-    }
-    else
-    {
-        CHECK_EQ(R"(Type '(number, number, a) -> number' could not be converted into '(number, number) -> number')", toString(result.errors[0]));
-    }
+        toString(result.errors[0]));
 
     // Infer from variadic packs into elements
     result = check(R"(
@@ -4618,17 +4602,9 @@ local c = sumrec(function(x, y, f) return f(x, y) end) -- type binders are not i
     )");
 
     LUAU_REQUIRE_ERRORS(result);
-    if (FFlag::LuauExtendedFunctionMismatchError)
-    {
-        CHECK_EQ(
-            "Type '(a, b, (a, b) -> (c...)) -> (c...)' could not be converted into '<a>(a, a, (a, a) -> a) -> a'; different number of generic type "
-            "parameters",
-            toString(result.errors[0]));
-    }
-    else
-    {
-        CHECK_EQ("Type '(a, b, (a, b) -> (c...)) -> (c...)' could not be converted into '<a>(a, a, (a, a) -> a) -> a'", toString(result.errors[0]));
-    }
+    CHECK_EQ("Type '(a, b, (a, b) -> (c...)) -> (c...)' could not be converted into '<a>(a, a, (a, a) -> a) -> a'; different number of generic type "
+             "parameters",
+        toString(result.errors[0]));
 }
 
 TEST_CASE_FIXTURE(Fixture, "infer_return_value_type")
@@ -4741,8 +4717,6 @@ TEST_CASE_FIXTURE(Fixture, "accidentally_checked_prop_in_opposite_branch")
 
 TEST_CASE_FIXTURE(Fixture, "substitution_with_bound_table")
 {
-    ScopedFastFlag luauCloneCorrectlyBeforeMutatingTableType{"LuauCloneCorrectlyBeforeMutatingTableType", true};
-
     CheckResult result = check(R"(
 type A = { x: number }
 local a: A = { x = 1 }
@@ -4965,8 +4939,6 @@ TEST_CASE_FIXTURE(Fixture, "inferred_methods_of_free_tables_have_the_same_level_
 
 TEST_CASE_FIXTURE(Fixture, "error_detailed_function_mismatch_arg_count")
 {
-    ScopedFastFlag luauExtendedFunctionMismatchError{"LuauExtendedFunctionMismatchError", true};
-
     CheckResult result = check(R"(
 type A = (number, number) -> string
 type B = (number) -> string
@@ -4983,8 +4955,6 @@ caused by:
 
 TEST_CASE_FIXTURE(Fixture, "error_detailed_function_mismatch_arg")
 {
-    ScopedFastFlag luauExtendedFunctionMismatchError{"LuauExtendedFunctionMismatchError", true};
-
     CheckResult result = check(R"(
 type A = (number, number) -> string
 type B = (number, string) -> string
@@ -5001,8 +4971,6 @@ caused by:
 
 TEST_CASE_FIXTURE(Fixture, "error_detailed_function_mismatch_ret_count")
 {
-    ScopedFastFlag luauExtendedFunctionMismatchError{"LuauExtendedFunctionMismatchError", true};
-
     CheckResult result = check(R"(
 type A = (number, number) -> (number)
 type B = (number, number) -> (number, boolean)
@@ -5019,8 +4987,6 @@ caused by:
 
 TEST_CASE_FIXTURE(Fixture, "error_detailed_function_mismatch_ret")
 {
-    ScopedFastFlag luauExtendedFunctionMismatchError{"LuauExtendedFunctionMismatchError", true};
-
     CheckResult result = check(R"(
 type A = (number, number) -> string
 type B = (number, number) -> number
@@ -5037,8 +5003,6 @@ caused by:
 
 TEST_CASE_FIXTURE(Fixture, "error_detailed_function_mismatch_ret_mult")
 {
-    ScopedFastFlag luauExtendedFunctionMismatchError{"LuauExtendedFunctionMismatchError", true};
-
     CheckResult result = check(R"(
 type A = (number, number) -> (number, string)
 type B = (number, number) -> (number, boolean)
@@ -5069,8 +5033,6 @@ TEST_CASE_FIXTURE(Fixture, "prop_access_on_any_with_other_options")
 
 TEST_CASE_FIXTURE(Fixture, "table_function_check_use_after_free")
 {
-    ScopedFastFlag luauUnifyFunctionCheckResult{"LuauUpdateFunctionNameBinding", true};
-
     CheckResult result = check(R"(
 local t = {}
 

tests/TypeInfer.typePacks.cpp

@@ -931,7 +931,6 @@ type R = { m: F<R> }
 TEST_CASE_FIXTURE(Fixture, "pack_tail_unification_check")
 {
     ScopedFastFlag luauUnifyPackTails{"LuauUnifyPackTails", true};
-    ScopedFastFlag luauExtendedFunctionMismatchError{"LuauExtendedFunctionMismatchError", true};
 
     CheckResult result = check(R"(
 local a: () -> (number, ...string)

tests/TypeInfer.unionTypes.test.cpp

@@ -464,8 +464,6 @@ local a: XYZ = { w = 4 }
 
 TEST_CASE_FIXTURE(Fixture, "error_detailed_optional")
 {
-    ScopedFastFlag luauExtendedUnionMismatchError{"LuauExtendedUnionMismatchError", true};
-
     CheckResult result = check(R"(
 type X = { x: number }
 

tests/TypeVar.test.cpp

@@ -268,8 +268,6 @@ TEST_CASE_FIXTURE(Fixture, "substitution_skip_failure")
 
 TEST_CASE("tagging_tables")
 {
-    ScopedFastFlag sff{"LuauRefactorTagging", true};
-
     TypeVar ttv{TableTypeVar{}};
     CHECK(!Luau::hasTag(&ttv, "foo"));
     Luau::attachTag(&ttv, "foo");
@@ -278,8 +276,6 @@ TEST_CASE("tagging_tables")
 
 TEST_CASE("tagging_classes")
 {
-    ScopedFastFlag sff{"LuauRefactorTagging", true};
-
     TypeVar base{ClassTypeVar{"Base", {}, std::nullopt, std::nullopt, {}, nullptr}};
     CHECK(!Luau::hasTag(&base, "foo"));
     Luau::attachTag(&base, "foo");
@@ -288,8 +284,6 @@ TEST_CASE("tagging_classes")
 
 TEST_CASE("tagging_subclasses")
 {
-    ScopedFastFlag sff{"LuauRefactorTagging", true};
-
     TypeVar base{ClassTypeVar{"Base", {}, std::nullopt, std::nullopt, {}, nullptr}};
     TypeVar derived{ClassTypeVar{"Derived", {}, &base, std::nullopt, {}, nullptr}};
 
@@ -307,8 +301,6 @@ TEST_CASE("tagging_subclasses")
 
 TEST_CASE("tagging_functions")
 {
-    ScopedFastFlag sff{"LuauRefactorTagging", true};
-
     TypePackVar empty{TypePack{}};
     TypeVar ftv{FunctionTypeVar{&empty, &empty}};
     CHECK(!Luau::hasTag(&ftv, "foo"));
@@ -318,8 +310,6 @@ TEST_CASE("tagging_functions")
 
 TEST_CASE("tagging_props")
 {
-    ScopedFastFlag sff{"LuauRefactorTagging", true};
-
     Property prop{};
     CHECK(!Luau::hasTag(prop, "foo"));
     Luau::attachTag(prop, "foo");
@@ -370,4 +360,66 @@ local b: (T, T, T) -> T
         CHECK_EQ(count, 1);
 }
 
+TEST_CASE("isString_on_string_singletons")
+{
+    ScopedFastFlag sff{"LuauRefactorTypeVarQuestions", true};
+
+    TypeVar helloString{SingletonTypeVar{StringSingleton{"hello"}}};
+    CHECK(isString(&helloString));
+}
+
+TEST_CASE("isString_on_unions_of_various_string_singletons")
+{
+    ScopedFastFlag sff{"LuauRefactorTypeVarQuestions", true};
+
+    TypeVar helloString{SingletonTypeVar{StringSingleton{"hello"}}};
+    TypeVar byeString{SingletonTypeVar{StringSingleton{"bye"}}};
+    TypeVar union_{UnionTypeVar{{&helloString, &byeString}}};
+
+    CHECK(isString(&union_));
+}
+
+TEST_CASE("proof_that_isString_uses_all_of")
+{
+    ScopedFastFlag sff{"LuauRefactorTypeVarQuestions", true};
+
+    TypeVar helloString{SingletonTypeVar{StringSingleton{"hello"}}};
+    TypeVar byeString{SingletonTypeVar{StringSingleton{"bye"}}};
+    TypeVar booleanType{PrimitiveTypeVar{PrimitiveTypeVar::Boolean}};
+    TypeVar union_{UnionTypeVar{{&helloString, &byeString, &booleanType}}};
+
+    CHECK(!isString(&union_));
+}
+
+TEST_CASE("isBoolean_on_boolean_singletons")
+{
+    ScopedFastFlag sff{"LuauRefactorTypeVarQuestions", true};
+
+    TypeVar trueBool{SingletonTypeVar{BooleanSingleton{true}}};
+    CHECK(isBoolean(&trueBool));
+}
+
+TEST_CASE("isBoolean_on_unions_of_true_or_false_singletons")
+{
+    ScopedFastFlag sff{"LuauRefactorTypeVarQuestions", true};
+
+    TypeVar trueBool{SingletonTypeVar{BooleanSingleton{true}}};
+    TypeVar falseBool{SingletonTypeVar{BooleanSingleton{false}}};
+    TypeVar union_{UnionTypeVar{{&trueBool, &falseBool}}};
+
+    CHECK(isBoolean(&union_));
+}
+
+TEST_CASE("proof_that_isBoolean_uses_all_of")
+{
+    ScopedFastFlag sff{"LuauRefactorTypeVarQuestions", true};
+
+    TypeVar trueBool{SingletonTypeVar{BooleanSingleton{true}}};
+    TypeVar falseBool{SingletonTypeVar{BooleanSingleton{false}}};
+    TypeVar stringType{PrimitiveTypeVar{PrimitiveTypeVar::String}};
+    TypeVar union_{UnionTypeVar{{&trueBool, &falseBool, &stringType}}};
+
+    CHECK(!isBoolean(&union_));
+}
+
 TEST_SUITE_END();