Sync to upstream/release/595

Analysis/include/Luau/ConstraintGraphBuilder.h

@@ -101,9 +101,10 @@ struct ConstraintGraphBuilder
 
     DcrLogger* logger;
 
-    ConstraintGraphBuilder(ModulePtr module, NotNull<Normalizer> normalizer, NotNull<ModuleResolver> moduleResolver, NotNull<BuiltinTypes> builtinTypes,
+    ConstraintGraphBuilder(ModulePtr module, NotNull<Normalizer> normalizer, NotNull<ModuleResolver> moduleResolver,
-        NotNull<InternalErrorReporter> ice, const ScopePtr& globalScope, std::function<void(const ModuleName&, const ScopePtr&)> prepareModuleScope,
+        NotNull<BuiltinTypes> builtinTypes, NotNull<InternalErrorReporter> ice, const ScopePtr& globalScope,
-        DcrLogger* logger, NotNull<DataFlowGraph> dfg, std::vector<RequireCycle> requireCycles);
+        std::function<void(const ModuleName&, const ScopePtr&)> prepareModuleScope, DcrLogger* logger, NotNull<DataFlowGraph> dfg,
+        std::vector<RequireCycle> requireCycles);
 
     /**
      * Fabricates a new free type belonging to a given scope.

Analysis/include/Luau/GlobalTypes.h

@@ -23,4 +23,4 @@ struct GlobalTypes
     ScopePtr globalScope;     // shared by all modules
 };
 
-}
+} // namespace Luau

Analysis/include/Luau/Instantiation.h

@@ -17,8 +17,8 @@ struct TypeCheckLimits;
 // A substitution which replaces generic types in a given set by free types.
 struct ReplaceGenerics : Substitution
 {
-    ReplaceGenerics(const TxnLog* log, TypeArena* arena, NotNull<BuiltinTypes> builtinTypes, TypeLevel level, Scope* scope, const std::vector<TypeId>& generics,
+    ReplaceGenerics(const TxnLog* log, TypeArena* arena, NotNull<BuiltinTypes> builtinTypes, TypeLevel level, Scope* scope,
-        const std::vector<TypePackId>& genericPacks)
+        const std::vector<TypeId>& generics, const std::vector<TypePackId>& genericPacks)
         : Substitution(log, arena)
         , builtinTypes(builtinTypes)
         , level(level)
@@ -77,6 +77,7 @@ struct Instantiation : Substitution
  * Instantiation fails only when processing the type causes internal recursion
  * limits to be exceeded.
  */
-std::optional<TypeId> instantiate(NotNull<BuiltinTypes> builtinTypes, NotNull<TypeArena> arena, NotNull<TypeCheckLimits> limits, NotNull<Scope> scope, TypeId ty);
+std::optional<TypeId> instantiate(
+    NotNull<BuiltinTypes> builtinTypes, NotNull<TypeArena> arena, NotNull<TypeCheckLimits> limits, NotNull<Scope> scope, TypeId ty);
 
 } // namespace Luau

Analysis/include/Luau/Subtyping.h

@@ -19,20 +19,18 @@ class TypeIds;
 class Normalizer;
 struct NormalizedType;
 struct NormalizedClassType;
+struct NormalizedStringType;
 struct NormalizedFunctionType;
 
+
 struct SubtypingResult
 {
-    // Did the test succeed?
     bool isSubtype = false;
     bool isErrorSuppressing = false;
     bool normalizationTooComplex = false;
 
-    // If so, what constraints are implied by this relation?
+    SubtypingResult& andAlso(const SubtypingResult& other);
-    // If not, what happened?
+    SubtypingResult& orElse(const SubtypingResult& other);
-
-    void andAlso(const SubtypingResult& other);
-    void orElse(const SubtypingResult& other);
 
     // Only negates the `isSubtype`.
     static SubtypingResult negate(const SubtypingResult& result);
@@ -47,6 +45,8 @@ struct Subtyping
     NotNull<Normalizer> normalizer;
     NotNull<InternalErrorReporter> iceReporter;
 
+    NotNull<Scope> scope;
+
     enum class Variance
     {
         Covariant,
@@ -72,6 +72,12 @@ struct Subtyping
 
     SeenSet seenTypes;
 
+    Subtyping(const Subtyping&) = delete;
+    Subtyping& operator=(const Subtyping&) = delete;
+
+    Subtyping(Subtyping&&) = default;
+    Subtyping& operator=(Subtyping&&) = default;
+
     // TODO cache
     // TODO cyclic types
     // TODO recursion limits
@@ -80,43 +86,61 @@ struct Subtyping
     SubtypingResult isSubtype(TypePackId subTy, TypePackId superTy);
 
 private:
-    SubtypingResult isSubtype_(TypeId subTy, TypeId superTy);
+    SubtypingResult isCovariantWith(TypeId subTy, TypeId superTy);
-    SubtypingResult isSubtype_(TypePackId subTy, TypePackId superTy);
+    SubtypingResult isCovariantWith(TypePackId subTy, TypePackId superTy);
 
     template<typename SubTy, typename SuperTy>
-    SubtypingResult isSubtype_(const TryPair<const SubTy*, const SuperTy*>& pair);
+    SubtypingResult isContravariantWith(SubTy&& subTy, SuperTy&& superTy);
 
-    SubtypingResult isSubtype_(TypeId subTy, const UnionType* superUnion);
+    template<typename SubTy, typename SuperTy>
-    SubtypingResult isSubtype_(const UnionType* subUnion, TypeId superTy);
+    SubtypingResult isInvariantWith(SubTy&& subTy, SuperTy&& superTy);
-    SubtypingResult isSubtype_(TypeId subTy, const IntersectionType* superIntersection);
-    SubtypingResult isSubtype_(const IntersectionType* subIntersection, TypeId superTy);
-    SubtypingResult isSubtype_(const PrimitiveType* subPrim, const PrimitiveType* superPrim);
-    SubtypingResult isSubtype_(const SingletonType* subSingleton, const PrimitiveType* superPrim);
-    SubtypingResult isSubtype_(const SingletonType* subSingleton, const SingletonType* superSingleton);
-    SubtypingResult isSubtype_(const TableType* subTable, const TableType* superTable);
-    SubtypingResult isSubtype_(const MetatableType* subMt, const MetatableType* superMt);
-    SubtypingResult isSubtype_(const MetatableType* subMt, const TableType* superTable);
-    SubtypingResult isSubtype_(const ClassType* subClass, const ClassType* superClass);
-    SubtypingResult isSubtype_(const ClassType* subClass, const TableType* superTable); // Actually a class <: shape.
-    SubtypingResult isSubtype_(const FunctionType* subFunction, const FunctionType* superFunction);
-    SubtypingResult isSubtype_(const PrimitiveType* subPrim, const TableType* superTable);
-    SubtypingResult isSubtype_(const SingletonType* subSingleton, const TableType* superTable);
 
-    SubtypingResult isSubtype_(const NormalizedType* subNorm, const NormalizedType* superNorm);
+    template<typename SubTy, typename SuperTy>
-    SubtypingResult isSubtype_(const NormalizedClassType& subClass, const NormalizedClassType& superClass, const TypeIds& superTables);
+    SubtypingResult isCovariantWith(const TryPair<const SubTy*, const SuperTy*>& pair);
-    SubtypingResult isSubtype_(const NormalizedFunctionType& subFunction, const NormalizedFunctionType& superFunction);
-    SubtypingResult isSubtype_(const TypeIds& subTypes, const TypeIds& superTypes);
 
-    SubtypingResult isSubtype_(const VariadicTypePack* subVariadic, const VariadicTypePack* superVariadic);
+    template<typename SubTy, typename SuperTy>
+    SubtypingResult isContravariantWith(const TryPair<const SubTy*, const SuperTy*>& pair);
+
+    template<typename SubTy, typename SuperTy>
+    SubtypingResult isInvariantWith(const TryPair<const SubTy*, const SuperTy*>& pair);
+
+    SubtypingResult isCovariantWith(TypeId subTy, const UnionType* superUnion);
+    SubtypingResult isCovariantWith(const UnionType* subUnion, TypeId superTy);
+    SubtypingResult isCovariantWith(TypeId subTy, const IntersectionType* superIntersection);
+    SubtypingResult isCovariantWith(const IntersectionType* subIntersection, TypeId superTy);
+
+    SubtypingResult isCovariantWith(const NegationType* subNegation, TypeId superTy);
+    SubtypingResult isCovariantWith(const TypeId subTy, const NegationType* superNegation);
+
+    SubtypingResult isCovariantWith(const PrimitiveType* subPrim, const PrimitiveType* superPrim);
+    SubtypingResult isCovariantWith(const SingletonType* subSingleton, const PrimitiveType* superPrim);
+    SubtypingResult isCovariantWith(const SingletonType* subSingleton, const SingletonType* superSingleton);
+    SubtypingResult isCovariantWith(const TableType* subTable, const TableType* superTable);
+    SubtypingResult isCovariantWith(const MetatableType* subMt, const MetatableType* superMt);
+    SubtypingResult isCovariantWith(const MetatableType* subMt, const TableType* superTable);
+    SubtypingResult isCovariantWith(const ClassType* subClass, const ClassType* superClass);
+    SubtypingResult isCovariantWith(const ClassType* subClass, const TableType* superTable);
+    SubtypingResult isCovariantWith(const FunctionType* subFunction, const FunctionType* superFunction);
+    SubtypingResult isCovariantWith(const PrimitiveType* subPrim, const TableType* superTable);
+    SubtypingResult isCovariantWith(const SingletonType* subSingleton, const TableType* superTable);
+
+    SubtypingResult isCovariantWith(const NormalizedType* subNorm, const NormalizedType* superNorm);
+    SubtypingResult isCovariantWith(const NormalizedClassType& subClass, const NormalizedClassType& superClass);
+    SubtypingResult isCovariantWith(const NormalizedClassType& subClass, const TypeIds& superTables);
+    SubtypingResult isCovariantWith(const NormalizedStringType& subString, const NormalizedStringType& superString);
+    SubtypingResult isCovariantWith(const NormalizedStringType& subString, const TypeIds& superTables);
+    SubtypingResult isCovariantWith(const NormalizedFunctionType& subFunction, const NormalizedFunctionType& superFunction);
+    SubtypingResult isCovariantWith(const TypeIds& subTypes, const TypeIds& superTypes);
+
+    SubtypingResult isCovariantWith(const VariadicTypePack* subVariadic, const VariadicTypePack* superVariadic);
 
     bool bindGeneric(TypeId subTp, TypeId superTp);
     bool bindGeneric(TypePackId subTp, TypePackId superTp);
 
-    template <typename T, typename Container>
+    template<typename T, typename Container>
     TypeId makeAggregateType(const Container& container, TypeId orElse);
 
-    [[noreturn]]
+    [[noreturn]] void unexpected(TypePackId tp);
-    void unexpected(TypePackId tp);
 };
 
 } // namespace Luau

Analysis/include/Luau/Type.h

@@ -849,6 +849,18 @@ bool isSubclass(const ClassType* cls, const ClassType* parent);
 
 Type* asMutable(TypeId ty);
 
+template<typename... Ts, typename T>
+bool is(T&& tv)
+{
+    if (!tv)
+        return false;
+
+    if constexpr (std::is_same_v<TypeId, T> && !(std::is_same_v<BoundType, Ts> || ...))
+        LUAU_ASSERT(get_if<BoundType>(&tv->ty) == nullptr);
+
+    return (get<Ts>(tv) || ...);
+}
+
 template<typename T>
 const T* get(TypeId tv)
 {

Analysis/include/Luau/TypeChecker2.h

@@ -14,7 +14,7 @@ struct DcrLogger;
 struct TypeCheckLimits;
 struct UnifierSharedState;
 
-void check(NotNull<BuiltinTypes> builtinTypes, NotNull<UnifierSharedState> sharedState, NotNull<TypeCheckLimits> limits, DcrLogger* logger, const SourceModule& sourceModule,
+void check(NotNull<BuiltinTypes> builtinTypes, NotNull<UnifierSharedState> sharedState, NotNull<TypeCheckLimits> limits, DcrLogger* logger,
-    Module* module);
+    const SourceModule& sourceModule, Module* module);
 
 } // namespace Luau

Analysis/include/Luau/TypeUtils.h

@@ -104,7 +104,8 @@ ErrorSuppression shouldSuppressErrors(NotNull<Normalizer> normalizer, TypePackId
 // Similar to `std::optional<std::pair<A, B>>`, but whose `sizeof()` is the same as `std::pair<A, B>`
 // and cooperates with C++'s `if (auto p = ...)` syntax without the extra fatness of `std::optional`.
 template<typename A, typename B>
-struct TryPair {
+struct TryPair
+{
     A first;
     B second;
 

Analysis/include/Luau/Unifier.h

@@ -105,10 +105,12 @@ struct Unifier
      * Populate the vector errors with any type errors that may arise.
      * Populate the transaction log with the set of TypeIds that need to be reset to undo the unification attempt.
      */
-    void tryUnify(TypeId subTy, TypeId superTy, bool isFunctionCall = false, bool isIntersection = false, const LiteralProperties* aliasableMap = nullptr);
+    void tryUnify(
+        TypeId subTy, TypeId superTy, bool isFunctionCall = false, bool isIntersection = false, const LiteralProperties* aliasableMap = nullptr);
 
 private:
-    void tryUnify_(TypeId subTy, TypeId superTy, bool isFunctionCall = false, bool isIntersection = false, const LiteralProperties* aliasableMap = nullptr);
+    void tryUnify_(
+        TypeId subTy, TypeId superTy, bool isFunctionCall = false, bool isIntersection = false, const LiteralProperties* aliasableMap = nullptr);
     void tryUnifyUnionWithType(TypeId subTy, const UnionType* uv, TypeId superTy);
 
     // Traverse the two types provided and block on any BlockedTypes we find.

Analysis/include/Luau/Unifier2.h

@@ -55,8 +55,8 @@ struct Unifier2
     bool unify(TypePackId subTp, TypePackId superTp);
 
     std::optional<TypeId> generalize(NotNull<Scope> scope, TypeId ty);
-private:
 
+private:
     /**
      * @returns simplify(left | right)
      */
@@ -72,4 +72,4 @@ private:
     OccursCheckResult occursCheck(DenseHashSet<TypePackId>& seen, TypePackId needle, TypePackId haystack);
 };
 
-}
+} // namespace Luau

Analysis/include/Luau/VisitType.h

@@ -10,6 +10,7 @@
 
 LUAU_FASTINT(LuauVisitRecursionLimit)
 LUAU_FASTFLAG(LuauBoundLazyTypes2)
+LUAU_FASTFLAG(DebugLuauDeferredConstraintResolution)
 LUAU_FASTFLAG(DebugLuauReadWriteProperties)
 
 namespace Luau
@@ -220,7 +221,21 @@ struct GenericTypeVisitor
                 traverse(btv->boundTo);
         }
         else if (auto ftv = get<FreeType>(ty))
-            visit(ty, *ftv);
+        {
+            if (FFlag::DebugLuauDeferredConstraintResolution)
+            {
+                if (visit(ty, *ftv))
+                {
+                    LUAU_ASSERT(ftv->lowerBound);
+                    traverse(ftv->lowerBound);
+
+                    LUAU_ASSERT(ftv->upperBound);
+                    traverse(ftv->upperBound);
+                }
+            }
+            else
+                visit(ty, *ftv);
+        }
         else if (auto gtv = get<GenericType>(ty))
             visit(ty, *gtv);
         else if (auto etv = get<ErrorType>(ty))

Analysis/src/Autocomplete.cpp

@@ -282,20 +282,8 @@ static void autocompleteProps(const Module& module, TypeArena* typeArena, NotNul
                 ParenthesesRecommendation parens =
                     indexType == PropIndexType::Key ? ParenthesesRecommendation::None : getParenRecommendation(type, nodes, typeCorrect);
 
-                result[name] = AutocompleteEntry{
+                result[name] = AutocompleteEntry{AutocompleteEntryKind::Property, type, prop.deprecated, isWrongIndexer(type), typeCorrect,
-                    AutocompleteEntryKind::Property,
+                    containingClass, &prop, prop.documentationSymbol, {}, parens, {}, indexType == PropIndexType::Colon};
-                    type,
-                    prop.deprecated,
-                    isWrongIndexer(type),
-                    typeCorrect,
-                    containingClass,
-                    &prop,
-                    prop.documentationSymbol,
-                    {},
-                    parens,
-                    {},
-                    indexType == PropIndexType::Colon
-                };
             }
         }
     };
@@ -606,7 +594,7 @@ std::optional<TypeId> getLocalTypeInScopeAt(const Module& module, Position posit
     return {};
 }
 
-template <typename T>
+template<typename T>
 static std::optional<std::string> tryToStringDetailed(const ScopePtr& scope, T ty, bool functionTypeArguments)
 {
     ToStringOptions opts;
@@ -1461,7 +1449,8 @@ static std::string makeAnonymous(const ScopePtr& scope, const FunctionType& func
     return result;
 }
 
-static std::optional<AutocompleteEntry> makeAnonymousAutofilled(const ModulePtr& module, Position position, const AstNode* node, const std::vector<AstNode*>& ancestry)
+static std::optional<AutocompleteEntry> makeAnonymousAutofilled(
+    const ModulePtr& module, Position position, const AstNode* node, const std::vector<AstNode*>& ancestry)
 {
     const AstExprCall* call = node->as<AstExprCall>();
     if (!call && ancestry.size() > 1)

Analysis/src/Clone.cpp

@@ -780,7 +780,7 @@ void TypeCloner::operator()(const UnionType& t)
         // We're just using this FreeType as a placeholder until we've finished
         // cloning the parts of this union so it is okay that its bounds are
         // nullptr.  We'll never indirect them.
-        TypeId result = dest.addType(FreeType{nullptr, /*lowerBound*/nullptr, /*upperBound*/nullptr});
+        TypeId result = dest.addType(FreeType{nullptr, /*lowerBound*/ nullptr, /*upperBound*/ nullptr});
         seenTypes[typeId] = result;
 
         std::vector<TypeId> options;

Analysis/src/ConstraintGraphBuilder.cpp

@@ -420,17 +420,17 @@ void ConstraintGraphBuilder::applyRefinements(const ScopePtr& scope, Location lo
                 {
                     switch (shouldSuppressErrors(normalizer, ty))
                     {
-                        case ErrorSuppression::DoNotSuppress:
+                    case ErrorSuppression::DoNotSuppress:
-                            ty = simplifyIntersection(builtinTypes, arena, ty, dt).result;
+                        ty = simplifyIntersection(builtinTypes, arena, ty, dt).result;
-                            break;
+                        break;
-                        case ErrorSuppression::Suppress:
+                    case ErrorSuppression::Suppress:
-                            ty = simplifyIntersection(builtinTypes, arena, ty, dt).result;
+                        ty = simplifyIntersection(builtinTypes, arena, ty, dt).result;
-                            ty = simplifyUnion(builtinTypes, arena, ty, builtinTypes->errorType).result;
+                        ty = simplifyUnion(builtinTypes, arena, ty, builtinTypes->errorType).result;
-                            break;
+                        break;
-                        case ErrorSuppression::NormalizationFailed:
+                    case ErrorSuppression::NormalizationFailed:
-                            reportError(location, NormalizationTooComplex{});
+                        reportError(location, NormalizationTooComplex{});
-                            ty = simplifyIntersection(builtinTypes, arena, ty, dt).result;
+                        ty = simplifyIntersection(builtinTypes, arena, ty, dt).result;
-                            break;
+                        break;
                     }
                 }
             }

Analysis/src/ConstraintSolver.cpp

@@ -430,6 +430,35 @@ bool ConstraintSolver::isDone()
     return unsolvedConstraints.empty();
 }
 
+namespace
+{
+
+struct TypeAndLocation
+{
+    TypeId typeId;
+    Location location;
+};
+
+struct FreeTypeSearcher : TypeOnceVisitor
+{
+    std::deque<TypeAndLocation>* result;
+    Location location;
+
+    FreeTypeSearcher(std::deque<TypeAndLocation>* result, Location location)
+        : result(result)
+        , location(location)
+    {
+    }
+
+    bool visit(TypeId ty, const FreeType&) override
+    {
+        result->push_back({ty, location});
+        return false;
+    }
+};
+
+} // namespace
+
 void ConstraintSolver::finalizeModule()
 {
     Anyification a{arena, rootScope, builtinTypes, &iceReporter, builtinTypes->anyType, builtinTypes->anyTypePack};
@@ -446,12 +475,28 @@ void ConstraintSolver::finalizeModule()
 
     Unifier2 u2{NotNull{arena}, builtinTypes, NotNull{&iceReporter}};
 
+    std::deque<TypeAndLocation> queue;
     for (auto& [name, binding] : rootScope->bindings)
+        queue.push_back({binding.typeId, binding.location});
+
+    DenseHashSet<TypeId> seen{nullptr};
+
+    while (!queue.empty())
     {
-        auto generalizedTy = u2.generalize(rootScope, binding.typeId);
+        TypeAndLocation binding = queue.front();
-        if (generalizedTy)
+        queue.pop_front();
-            binding.typeId = *generalizedTy;
+
-        else
+        TypeId ty = follow(binding.typeId);
+
+        if (seen.find(ty))
+            continue;
+        seen.insert(ty);
+
+        FreeTypeSearcher fts{&queue, binding.location};
+        fts.traverse(ty);
+
+        auto result = u2.generalize(rootScope, ty);
+        if (!result)
             reportError(CodeTooComplex{}, binding.location);
     }
 }
@@ -2642,20 +2687,14 @@ ErrorVec ConstraintSolver::unify(NotNull<Scope> scope, Location location, TypeId
 
 ErrorVec ConstraintSolver::unify(NotNull<Scope> scope, Location location, TypePackId subPack, TypePackId superPack)
 {
-    UnifierSharedState sharedState{&iceReporter};
+    Unifier2 u{arena, builtinTypes, NotNull{&iceReporter}};
-    Unifier u{normalizer, scope, Location{}, Covariant};
-    u.enableNewSolver();
 
-    u.tryUnify(subPack, superPack);
+    u.unify(subPack, superPack);
 
-    const auto [changedTypes, changedPacks] = u.log.getChanges();
+    unblock(subPack, Location{});
+    unblock(superPack, Location{});
 
-    u.log.commit();
+    return {};
-
-    unblock(changedTypes, Location{});
-    unblock(changedPacks, Location{});
-
-    return std::move(u.errors);
 }
 
 NotNull<Constraint> ConstraintSolver::pushConstraint(NotNull<Scope> scope, const Location& location, ConstraintV cv)

Analysis/src/Differ.cpp

@@ -117,9 +117,7 @@ std::string DiffError::toStringALeaf(std::string rootName, const DiffPathNodeLea
     case DiffError::Kind::Normal:
     {
         checkNonMissingPropertyLeavesHaveNulloptTableProperty();
-        return pathStr + conditionalNewline
+        return pathStr + conditionalNewline + "has type" + conditionalNewline + conditionalIndent + Luau::toString(*leaf.ty);
-            + "has type" + conditionalNewline
-            + conditionalIndent + Luau::toString(*leaf.ty);
     }
     case DiffError::Kind::MissingTableProperty:
     {
@@ -127,17 +125,14 @@ std::string DiffError::toStringALeaf(std::string rootName, const DiffPathNodeLea
         {
             if (!leaf.tableProperty.has_value())
                 throw InternalCompilerError{"leaf.tableProperty is nullopt"};
-            return pathStr + "." + *leaf.tableProperty + conditionalNewline
+            return pathStr + "." + *leaf.tableProperty + conditionalNewline + "has type" + conditionalNewline + conditionalIndent +
-                + "has type" + conditionalNewline
+                   Luau::toString(*leaf.ty);
-                + conditionalIndent + Luau::toString(*leaf.ty);
         }
         else if (otherLeaf.ty.has_value())
         {
             if (!otherLeaf.tableProperty.has_value())
                 throw InternalCompilerError{"otherLeaf.tableProperty is nullopt"};
-            return pathStr + conditionalNewline
+            return pathStr + conditionalNewline + "is missing the property" + conditionalNewline + conditionalIndent + *otherLeaf.tableProperty;
-                + "is missing the property" + conditionalNewline
-                + conditionalIndent + *otherLeaf.tableProperty;
         }
         throw InternalCompilerError{"Both leaf.ty and otherLeaf.ty is nullopt"};
     }
@@ -148,15 +143,11 @@ std::string DiffError::toStringALeaf(std::string rootName, const DiffPathNodeLea
         {
             if (!leaf.unionIndex.has_value())
                 throw InternalCompilerError{"leaf.unionIndex is nullopt"};
-            return pathStr + conditionalNewline
+            return pathStr + conditionalNewline + "is a union containing type" + conditionalNewline + conditionalIndent + Luau::toString(*leaf.ty);
-                + "is a union containing type" + conditionalNewline
-                + conditionalIndent + Luau::toString(*leaf.ty);
         }
         else if (otherLeaf.ty.has_value())
         {
-            return pathStr + conditionalNewline
+            return pathStr + conditionalNewline + "is a union missing type" + conditionalNewline + conditionalIndent + Luau::toString(*otherLeaf.ty);
-                + "is a union missing type" + conditionalNewline
-                + conditionalIndent + Luau::toString(*otherLeaf.ty);
         }
         throw InternalCompilerError{"Both leaf.ty and otherLeaf.ty is nullopt"};
     }
@@ -169,15 +160,13 @@ std::string DiffError::toStringALeaf(std::string rootName, const DiffPathNodeLea
         {
             if (!leaf.unionIndex.has_value())
                 throw InternalCompilerError{"leaf.unionIndex is nullopt"};
-            return pathStr + conditionalNewline
+            return pathStr + conditionalNewline + "is an intersection containing type" + conditionalNewline + conditionalIndent +
-                + "is an intersection containing type" + conditionalNewline
+                   Luau::toString(*leaf.ty);
-                + conditionalIndent + Luau::toString(*leaf.ty);
         }
         else if (otherLeaf.ty.has_value())
         {
-            return pathStr + conditionalNewline
+            return pathStr + conditionalNewline + "is an intersection missing type" + conditionalNewline + conditionalIndent +
-                + "is an intersection missing type" + conditionalNewline
+                   Luau::toString(*otherLeaf.ty);
-                + conditionalIndent + Luau::toString(*otherLeaf.ty);
         }
         throw InternalCompilerError{"Both leaf.ty and otherLeaf.ty is nullopt"};
     }
@@ -185,15 +174,13 @@ std::string DiffError::toStringALeaf(std::string rootName, const DiffPathNodeLea
     {
         if (!leaf.minLength.has_value())
             throw InternalCompilerError{"leaf.minLength is nullopt"};
-        return pathStr + conditionalNewline
+        return pathStr + conditionalNewline + "takes " + std::to_string(*leaf.minLength) + (leaf.isVariadic ? " or more" : "") + " arguments";
-            + "takes " + std::to_string(*leaf.minLength) + (leaf.isVariadic ? " or more" : "") + " arguments";
     }
     case DiffError::Kind::LengthMismatchInFnRets:
     {
         if (!leaf.minLength.has_value())
             throw InternalCompilerError{"leaf.minLength is nullopt"};
-        return pathStr + conditionalNewline
+        return pathStr + conditionalNewline + "returns " + std::to_string(*leaf.minLength) + (leaf.isVariadic ? " or more" : "") + " values";
-            + "returns " + std::to_string(*leaf.minLength) + (leaf.isVariadic ? " or more" : "") + " values";
     }
     default:
     {
@@ -249,17 +236,15 @@ std::string DiffError::toString(bool multiLine) const
     case DiffError::Kind::IncompatibleGeneric:
     {
         std::string diffPathStr{diffPath.toString(true)};
-        return "DiffError: these two types are not equal because the left generic at" + conditionalNewline
+        return "DiffError: these two types are not equal because the left generic at" + conditionalNewline + conditionalIndent + leftRootName +
-                + conditionalIndent + leftRootName + diffPathStr + conditionalNewline
+               diffPathStr + conditionalNewline + "cannot be the same type parameter as the right generic at" + conditionalNewline +
-                + "cannot be the same type parameter as the right generic at" + conditionalNewline
+               conditionalIndent + rightRootName + diffPathStr;
-                + conditionalIndent + rightRootName + diffPathStr;
     }
     default:
     {
-        return "DiffError: these two types are not equal because the left type at" + conditionalNewline
+        return "DiffError: these two types are not equal because the left type at" + conditionalNewline + conditionalIndent +
-                + conditionalIndent + toStringALeaf(leftRootName, left, right, multiLine) + "," + conditionalNewline +
+               toStringALeaf(leftRootName, left, right, multiLine) + "," + conditionalNewline + "while the right type at" + conditionalNewline +
-                "while the right type at" + conditionalNewline
+               conditionalIndent + toStringALeaf(rightRootName, right, left, multiLine);
-                + conditionalIndent + toStringALeaf(rightRootName, right, left, multiLine);
     }
     }
 }

Analysis/src/Frontend.cpp

@@ -1289,7 +1289,8 @@ ModulePtr check(const SourceModule& sourceModule, const std::vector<RequireCycle
 
     if (result->timeout || result->cancelled)
     {
-        // If solver was interrupted, skip typechecking and replace all module results with error-supressing types to avoid leaking blocked/pending types
+        // If solver was interrupted, skip typechecking and replace all module results with error-supressing types to avoid leaking blocked/pending
+        // types
         ScopePtr moduleScope = result->getModuleScope();
         moduleScope->returnType = builtinTypes->errorRecoveryTypePack();
 

Analysis/src/GlobalTypes.cpp

@@ -31,4 +31,4 @@ GlobalTypes::GlobalTypes(NotNull<BuiltinTypes> builtinTypes)
     }
 }
 
-}
+} // namespace Luau

Analysis/src/Instantiation.cpp

@@ -174,7 +174,8 @@ struct Replacer : Substitution
     }
 };
 
-std::optional<TypeId> instantiate(NotNull<BuiltinTypes> builtinTypes, NotNull<TypeArena> arena, NotNull<TypeCheckLimits> limits, NotNull<Scope> scope, TypeId ty)
+std::optional<TypeId> instantiate(
+    NotNull<BuiltinTypes> builtinTypes, NotNull<TypeArena> arena, NotNull<TypeCheckLimits> limits, NotNull<Scope> scope, TypeId ty)
 {
     ty = follow(ty);
 

Analysis/src/Linter.cpp

@@ -2791,8 +2791,8 @@ static void lintComments(LintContext& context, const std::vector<HotComment>& ho
             else if (first == "native")
             {
                 if (space != std::string::npos)
-                    emitWarning(context, LintWarning::Code_CommentDirective, hc.location,
+                    emitWarning(
-                        "native directive has extra symbols at the end of the line");
+                        context, LintWarning::Code_CommentDirective, hc.location, "native directive has extra symbols at the end of the line");
             }
             else
             {

Analysis/src/Normalize.cpp

@@ -176,7 +176,7 @@ const NormalizedStringType NormalizedStringType::never;
 
 bool isSubtype(const NormalizedStringType& subStr, const NormalizedStringType& superStr)
 {
-    if (subStr.isUnion() && superStr.isUnion())
+    if (subStr.isUnion() && (superStr.isUnion() && !superStr.isNever()))
     {
         for (auto [name, ty] : subStr.singletons)
         {
@@ -1983,18 +1983,68 @@ void Normalizer::intersectClassesWithClass(NormalizedClassType& heres, TypeId th
 
 void Normalizer::intersectStrings(NormalizedStringType& here, const NormalizedStringType& there)
 {
+    /* There are 9 cases to worry about here
+         Normalized Left    | Normalized Right
+       C1 string            | string              ===> trivial
+       C2 string - {u_1,..} | string              ===> trivial
+       C3 {u_1, ..}         | string              ===> trivial
+       C4 string            | string - {v_1, ..}  ===> string - {v_1, ..}
+       C5 string - {u_1,..} | string - {v_1, ..}  ===> string - ({u_s} U {v_s})
+       C6 {u_1, ..}         | string - {v_1, ..}  ===> {u_s} - {v_s}
+       C7 string            | {v_1, ..}           ===> {v_s}
+       C8 string - {u_1,..} | {v_1, ..}           ===> {v_s} - {u_s}
+       C9 {u_1, ..}         | {v_1, ..}           ===> {u_s} ∩ {v_s}
+    */
+    // Case 1,2,3
     if (there.isString())
         return;
-    if (here.isString())
+    // Case 4, Case 7
-        here.resetToNever();
+    else if (here.isString())
-
-    for (auto it = here.singletons.begin(); it != here.singletons.end();)
     {
-        if (there.singletons.count(it->first))
+        here.singletons.clear();
-            it++;
+        for (const auto& [key, type] : there.singletons)
-        else
+            here.singletons[key] = type;
-            it = here.singletons.erase(it);
+        here.isCofinite = here.isCofinite && there.isCofinite;
     }
+    // Case 5
+    else if (here.isIntersection() && there.isIntersection())
+    {
+        here.isCofinite = true;
+        for (const auto& [key, type] : there.singletons)
+            here.singletons[key] = type;
+    }
+    // Case 6
+    else if (here.isUnion() && there.isIntersection())
+    {
+        here.isCofinite = false;
+        for (const auto& [key, _] : there.singletons)
+            here.singletons.erase(key);
+    }
+    // Case 8
+    else if (here.isIntersection() && there.isUnion())
+    {
+        here.isCofinite = false;
+        std::map<std::string, TypeId> result(there.singletons);
+        for (const auto& [key, _] : here.singletons)
+            result.erase(key);
+        here.singletons = result;
+    }
+    // Case 9
+    else if (here.isUnion() && there.isUnion())
+    {
+        here.isCofinite = false;
+        std::map<std::string, TypeId> result;
+        result.insert(here.singletons.begin(), here.singletons.end());
+        result.insert(there.singletons.begin(), there.singletons.end());
+        for (auto it = result.begin(); it != result.end();)
+            if (!here.singletons.count(it->first) || !there.singletons.count(it->first))
+                it = result.erase(it);
+            else
+                ++it;
+        here.singletons = result;
+    }
+    else
+        LUAU_ASSERT(0 && "Internal Error - unrecognized case");
 }
 
 std::optional<TypePackId> Normalizer::intersectionOfTypePacks(TypePackId here, TypePackId there)

Analysis/src/Subtyping.cpp

@@ -5,6 +5,7 @@
 #include "Luau/Common.h"
 #include "Luau/Error.h"
 #include "Luau/Normalize.h"
+#include "Luau/Scope.h"
 #include "Luau/StringUtils.h"
 #include "Luau/ToString.h"
 #include "Luau/Type.h"
@@ -28,12 +29,12 @@ struct VarianceFlipper
     {
         switch (oldValue)
         {
-            case Subtyping::Variance::Covariant:
+        case Subtyping::Variance::Covariant:
-                *variance = Subtyping::Variance::Contravariant;
+            *variance = Subtyping::Variance::Contravariant;
-                break;
+            break;
-            case Subtyping::Variance::Contravariant:
+        case Subtyping::Variance::Contravariant:
-                *variance = Subtyping::Variance::Covariant;
+            *variance = Subtyping::Variance::Covariant;
-                break;
+            break;
         }
     }
 
@@ -43,19 +44,21 @@ struct VarianceFlipper
     }
 };
 
-void SubtypingResult::andAlso(const SubtypingResult& other)
+SubtypingResult& SubtypingResult::andAlso(const SubtypingResult& other)
 {
     isSubtype &= other.isSubtype;
     // `|=` is intentional here, we want to preserve error related flags.
     isErrorSuppressing |= other.isErrorSuppressing;
     normalizationTooComplex |= other.normalizationTooComplex;
+    return *this;
 }
 
-void SubtypingResult::orElse(const SubtypingResult& other)
+SubtypingResult& SubtypingResult::orElse(const SubtypingResult& other)
 {
     isSubtype |= other.isSubtype;
     isErrorSuppressing |= other.isErrorSuppressing;
     normalizationTooComplex |= other.normalizationTooComplex;
+    return *this;
 }
 
 SubtypingResult SubtypingResult::negate(const SubtypingResult& result)
@@ -88,9 +91,9 @@ SubtypingResult Subtyping::isSubtype(TypeId subTy, TypeId superTy)
     mappedGenerics.clear();
     mappedGenericPacks.clear();
 
-    SubtypingResult result = isSubtype_(subTy, superTy);
+    SubtypingResult result = isCovariantWith(subTy, superTy);
 
-    for (const auto& [subTy, bounds]: mappedGenerics)
+    for (const auto& [subTy, bounds] : mappedGenerics)
     {
         const auto& lb = bounds.lowerBound;
         const auto& ub = bounds.upperBound;
@@ -98,7 +101,7 @@ SubtypingResult Subtyping::isSubtype(TypeId subTy, TypeId superTy)
         TypeId lowerBound = makeAggregateType<UnionType>(lb, builtinTypes->neverType);
         TypeId upperBound = makeAggregateType<IntersectionType>(ub, builtinTypes->unknownType);
 
-        result.andAlso(isSubtype_(lowerBound, upperBound));
+        result.andAlso(isCovariantWith(lowerBound, upperBound));
     }
 
     return result;
@@ -106,7 +109,7 @@ SubtypingResult Subtyping::isSubtype(TypeId subTy, TypeId superTy)
 
 SubtypingResult Subtyping::isSubtype(TypePackId subTp, TypePackId superTp)
 {
-    return isSubtype_(subTp, superTp);
+    return isCovariantWith(subTp, superTp);
 }
 
 namespace
@@ -119,16 +122,17 @@ struct SeenSetPopper
     SeenSetPopper(Subtyping::SeenSet* seenTypes, std::pair<TypeId, TypeId> pair)
         : seenTypes(seenTypes)
         , pair(pair)
-    {}
+    {
+    }
 
     ~SeenSetPopper()
     {
         seenTypes->erase(pair);
     }
 };
-}
+} // namespace
 
-SubtypingResult Subtyping::isSubtype_(TypeId subTy, TypeId superTy)
+SubtypingResult Subtyping::isCovariantWith(TypeId subTy, TypeId superTy)
 {
     subTy = follow(subTy);
     superTy = follow(superTy);
@@ -146,19 +150,27 @@ SubtypingResult Subtyping::isSubtype_(TypeId subTy, TypeId superTy)
 
     SeenSetPopper ssp{&seenTypes, typePair};
 
+    // Within the scope to which a generic belongs, that generic should be
+    // tested as though it were its upper bounds.  We do not yet support bounded
+    // generics, so the upper bound is always unknown.
+    if (auto subGeneric = get<GenericType>(subTy); subGeneric && subsumes(subGeneric->scope, scope))
+        return isCovariantWith(builtinTypes->unknownType, superTy);
+    if (auto superGeneric = get<GenericType>(superTy); superGeneric && subsumes(superGeneric->scope, scope))
+        return isCovariantWith(subTy, builtinTypes->unknownType);
+
     if (auto subUnion = get<UnionType>(subTy))
-        return isSubtype_(subUnion, superTy);
+        return isCovariantWith(subUnion, superTy);
     else if (auto superUnion = get<UnionType>(superTy))
-        return isSubtype_(subTy, superUnion);
+        return isCovariantWith(subTy, superUnion);
     else if (auto superIntersection = get<IntersectionType>(superTy))
-        return isSubtype_(subTy, superIntersection);
+        return isCovariantWith(subTy, superIntersection);
     else if (auto subIntersection = get<IntersectionType>(subTy))
     {
-        SubtypingResult result = isSubtype_(subIntersection, superTy);
+        SubtypingResult result = isCovariantWith(subIntersection, superTy);
         if (result.isSubtype || result.isErrorSuppressing || result.normalizationTooComplex)
             return result;
         else
-            return isSubtype_(normalizer->normalize(subTy), normalizer->normalize(superTy));
+            return isCovariantWith(normalizer->normalize(subTy), normalizer->normalize(superTy));
     }
     else if (get<AnyType>(superTy))
         return {true}; // This is always true.
@@ -166,14 +178,12 @@ SubtypingResult Subtyping::isSubtype_(TypeId subTy, TypeId superTy)
     {
         // any = unknown | error, so we rewrite this to match.
         // As per TAPL: A | B <: T iff A <: T && B <: T
-        SubtypingResult result = isSubtype_(builtinTypes->unknownType, superTy);
+        return isCovariantWith(builtinTypes->unknownType, superTy).andAlso(isCovariantWith(builtinTypes->errorType, superTy));
-        result.andAlso(isSubtype_(builtinTypes->errorType, superTy));
-        return result;
     }
     else if (get<UnknownType>(superTy))
     {
-        LUAU_ASSERT(!get<AnyType>(subTy)); // TODO: replace with ice.
+        LUAU_ASSERT(!get<AnyType>(subTy));          // TODO: replace with ice.
-        LUAU_ASSERT(!get<UnionType>(subTy)); // TODO: replace with ice.
+        LUAU_ASSERT(!get<UnionType>(subTy));        // TODO: replace with ice.
         LUAU_ASSERT(!get<IntersectionType>(subTy)); // TODO: replace with ice.
 
         bool errorSuppressing = get<ErrorType>(subTy);
@@ -185,6 +195,12 @@ SubtypingResult Subtyping::isSubtype_(TypeId subTy, TypeId superTy)
         return {false, true};
     else if (get<ErrorType>(subTy))
         return {false, true};
+    else if (auto p = get2<NegationType, NegationType>(subTy, superTy))
+        return isCovariantWith(p.first->ty, p.second->ty);
+    else if (auto subNegation = get<NegationType>(subTy))
+        return isCovariantWith(subNegation, superTy);
+    else if (auto superNegation = get<NegationType>(superTy))
+        return isCovariantWith(subTy, superNegation);
     else if (auto subGeneric = get<GenericType>(subTy); subGeneric && variance == Variance::Covariant)
     {
         bool ok = bindGeneric(subTy, superTy);
@@ -196,32 +212,32 @@ SubtypingResult Subtyping::isSubtype_(TypeId subTy, TypeId superTy)
         return {ok};
     }
     else if (auto p = get2<PrimitiveType, PrimitiveType>(subTy, superTy))
-        return isSubtype_(p);
+        return isCovariantWith(p);
     else if (auto p = get2<SingletonType, PrimitiveType>(subTy, superTy))
-        return isSubtype_(p);
+        return isCovariantWith(p);
     else if (auto p = get2<SingletonType, SingletonType>(subTy, superTy))
-        return isSubtype_(p);
+        return isCovariantWith(p);
     else if (auto p = get2<FunctionType, FunctionType>(subTy, superTy))
-        return isSubtype_(p);
+        return isCovariantWith(p);
     else if (auto p = get2<TableType, TableType>(subTy, superTy))
-        return isSubtype_(p);
+        return isCovariantWith(p);
     else if (auto p = get2<MetatableType, MetatableType>(subTy, superTy))
-        return isSubtype_(p);
+        return isCovariantWith(p);
     else if (auto p = get2<MetatableType, TableType>(subTy, superTy))
-        return isSubtype_(p);
+        return isCovariantWith(p);
     else if (auto p = get2<ClassType, ClassType>(subTy, superTy))
-        return isSubtype_(p);
+        return isCovariantWith(p);
     else if (auto p = get2<ClassType, TableType>(subTy, superTy))
-        return isSubtype_(p);
+        return isCovariantWith(p);
     else if (auto p = get2<PrimitiveType, TableType>(subTy, superTy))
-        return isSubtype_(p);
+        return isCovariantWith(p);
     else if (auto p = get2<SingletonType, TableType>(subTy, superTy))
-        return isSubtype_(p);
+        return isCovariantWith(p);
 
     return {false};
 }
 
-SubtypingResult Subtyping::isSubtype_(TypePackId subTp, TypePackId superTp)
+SubtypingResult Subtyping::isCovariantWith(TypePackId subTp, TypePackId superTp)
 {
     subTp = follow(subTp);
     superTp = follow(superTp);
@@ -241,7 +257,7 @@ SubtypingResult Subtyping::isSubtype_(TypePackId subTp, TypePackId superTp)
 
     for (size_t i = 0; i < headSize; ++i)
     {
-        results.push_back(isSubtype_(subHead[i], superHead[i]));
+        results.push_back(isCovariantWith(subHead[i], superHead[i]));
         if (!results.back().isSubtype)
             return {false};
     }
@@ -255,7 +271,7 @@ SubtypingResult Subtyping::isSubtype_(TypePackId subTp, TypePackId superTp)
             if (auto vt = get<VariadicTypePack>(*subTail))
             {
                 for (size_t i = headSize; i < superHead.size(); ++i)
-                    results.push_back(isSubtype_(vt->ty, superHead[i]));
+                    results.push_back(isCovariantWith(vt->ty, superHead[i]));
             }
             else if (auto gt = get<GenericTypePack>(*subTail))
             {
@@ -266,11 +282,11 @@ SubtypingResult Subtyping::isSubtype_(TypePackId subTp, TypePackId superTp)
                     // <X>(X) -> () <: (T) -> ()
 
                     // Possible optimization: If headSize == 0 then we can just use subTp as-is.
-                    std::vector<TypeId> headSlice(begin(superHead), end(superHead) + headSize);
+                    std::vector<TypeId> headSlice(begin(superHead), begin(superHead) + headSize);
                     TypePackId superTailPack = arena->addTypePack(std::move(headSlice), superTail);
 
                     if (TypePackId* other = mappedGenericPacks.find(*subTail))
-                        results.push_back(isSubtype_(*other, superTailPack));
+                        results.push_back(isCovariantWith(*other, superTailPack));
                     else
                         mappedGenericPacks.try_insert(*subTail, superTailPack);
 
@@ -300,7 +316,7 @@ SubtypingResult Subtyping::isSubtype_(TypePackId subTp, TypePackId superTp)
             if (auto vt = get<VariadicTypePack>(*superTail))
             {
                 for (size_t i = headSize; i < subHead.size(); ++i)
-                    results.push_back(isSubtype_(subHead[i], vt->ty));
+                    results.push_back(isCovariantWith(subHead[i], vt->ty));
             }
             else if (auto gt = get<GenericTypePack>(*superTail))
             {
@@ -311,11 +327,11 @@ SubtypingResult Subtyping::isSubtype_(TypePackId subTp, TypePackId superTp)
                     // <X...>(X...) -> () <: (T) -> ()
 
                     // Possible optimization: If headSize == 0 then we can just use subTp as-is.
-                    std::vector<TypeId> headSlice(begin(subHead), end(subHead) + headSize);
+                    std::vector<TypeId> headSlice(begin(subHead), begin(subHead) + headSize);
                     TypePackId subTailPack = arena->addTypePack(std::move(headSlice), subTail);
 
                     if (TypePackId* other = mappedGenericPacks.find(*superTail))
-                        results.push_back(isSubtype_(*other, subTailPack));
+                        results.push_back(isCovariantWith(*other, subTailPack));
                     else
                         mappedGenericPacks.try_insert(*superTail, subTailPack);
 
@@ -344,7 +360,7 @@ SubtypingResult Subtyping::isSubtype_(TypePackId subTp, TypePackId superTp)
     {
         if (auto p = get2<VariadicTypePack, VariadicTypePack>(*subTail, *superTail))
         {
-            results.push_back(isSubtype_(p));
+            results.push_back(isCovariantWith(p));
         }
         else if (auto p = get2<GenericTypePack, GenericTypePack>(*subTail, *superTail))
         {
@@ -380,7 +396,8 @@ SubtypingResult Subtyping::isSubtype_(TypePackId subTp, TypePackId superTp)
             }
         }
         else
-            iceReporter->ice(format("Subtyping::isSubtype got unexpected type packs %s and %s", toString(*subTail).c_str(), toString(*superTail).c_str()));
+            iceReporter->ice(
+                format("Subtyping::isSubtype got unexpected type packs %s and %s", toString(*subTail).c_str(), toString(*superTail).c_str()));
     }
     else if (subTail)
     {
@@ -428,9 +445,33 @@ SubtypingResult Subtyping::isSubtype_(TypePackId subTp, TypePackId superTp)
 }
 
 template<typename SubTy, typename SuperTy>
-SubtypingResult Subtyping::isSubtype_(const TryPair<const SubTy*, const SuperTy*>& pair)
+SubtypingResult Subtyping::isContravariantWith(SubTy&& subTy, SuperTy&& superTy)
 {
-    return isSubtype_(pair.first, pair.second);
+    return isCovariantWith(superTy, subTy);
+}
+
+template<typename SubTy, typename SuperTy>
+SubtypingResult Subtyping::isInvariantWith(SubTy&& subTy, SuperTy&& superTy)
+{
+    return isCovariantWith(subTy, superTy).andAlso(isContravariantWith(subTy, superTy));
+}
+
+template<typename SubTy, typename SuperTy>
+SubtypingResult Subtyping::isCovariantWith(const TryPair<const SubTy*, const SuperTy*>& pair)
+{
+    return isCovariantWith(pair.first, pair.second);
+}
+
+template<typename SubTy, typename SuperTy>
+SubtypingResult Subtyping::isContravariantWith(const TryPair<const SubTy*, const SuperTy*>& pair)
+{
+    return isCovariantWith(pair.second, pair.first);
+}
+
+template<typename SubTy, typename SuperTy>
+SubtypingResult Subtyping::isInvariantWith(const TryPair<const SubTy*, const SuperTy*>& pair)
+{
+    return isCovariantWith(pair).andAlso(isContravariantWith(pair));
 }
 
 /*
@@ -464,48 +505,219 @@ SubtypingResult Subtyping::isSubtype_(const TryPair<const SubTy*, const SuperTy*
  * other just asks for boolean ~ 'b. We can dispatch this and only commit
  * boolean ~ 'b.  This constraint does not teach us anything about 'a.
  */
-SubtypingResult Subtyping::isSubtype_(TypeId subTy, const UnionType* superUnion)
+SubtypingResult Subtyping::isCovariantWith(TypeId subTy, const UnionType* superUnion)
 {
     // As per TAPL: T <: A | B iff T <: A || T <: B
     std::vector<SubtypingResult> subtypings;
     for (TypeId ty : superUnion)
-        subtypings.push_back(isSubtype_(subTy, ty));
+        subtypings.push_back(isCovariantWith(subTy, ty));
     return SubtypingResult::any(subtypings);
 }
 
-SubtypingResult Subtyping::isSubtype_(const UnionType* subUnion, TypeId superTy)
+SubtypingResult Subtyping::isCovariantWith(const UnionType* subUnion, TypeId superTy)
 {
     // As per TAPL: A | B <: T iff A <: T && B <: T
     std::vector<SubtypingResult> subtypings;
     for (TypeId ty : subUnion)
-        subtypings.push_back(isSubtype_(ty, superTy));
+        subtypings.push_back(isCovariantWith(ty, superTy));
     return SubtypingResult::all(subtypings);
 }
 
-SubtypingResult Subtyping::isSubtype_(TypeId subTy, const IntersectionType* superIntersection)
+SubtypingResult Subtyping::isCovariantWith(TypeId subTy, const IntersectionType* superIntersection)
 {
     // As per TAPL: T <: A & B iff T <: A && T <: B
     std::vector<SubtypingResult> subtypings;
     for (TypeId ty : superIntersection)
-        subtypings.push_back(isSubtype_(subTy, ty));
+        subtypings.push_back(isCovariantWith(subTy, ty));
     return SubtypingResult::all(subtypings);
 }
 
-SubtypingResult Subtyping::isSubtype_(const IntersectionType* subIntersection, TypeId superTy)
+SubtypingResult Subtyping::isCovariantWith(const IntersectionType* subIntersection, TypeId superTy)
 {
     // As per TAPL: A & B <: T iff A <: T || B <: T
     std::vector<SubtypingResult> subtypings;
     for (TypeId ty : subIntersection)
-        subtypings.push_back(isSubtype_(ty, superTy));
+        subtypings.push_back(isCovariantWith(ty, superTy));
     return SubtypingResult::any(subtypings);
 }
 
-SubtypingResult Subtyping::isSubtype_(const PrimitiveType* subPrim, const PrimitiveType* superPrim)
+SubtypingResult Subtyping::isCovariantWith(const NegationType* subNegation, TypeId superTy)
+{
+    TypeId negatedTy = follow(subNegation->ty);
+
+    // In order to follow a consistent codepath, rather than folding the
+    // isCovariantWith test down to its conclusion here, we test the subtyping test
+    // of the result of negating the type for never, unknown, any, and error.
+    if (is<NeverType>(negatedTy))
+    {
+        // ¬never ~ unknown
+        return isCovariantWith(builtinTypes->unknownType, superTy);
+    }
+    else if (is<UnknownType>(negatedTy))
+    {
+        // ¬unknown ~ never
+        return isCovariantWith(builtinTypes->neverType, superTy);
+    }
+    else if (is<AnyType>(negatedTy))
+    {
+        // ¬any ~ any
+        return isCovariantWith(negatedTy, superTy);
+    }
+    else if (auto u = get<UnionType>(negatedTy))
+    {
+        // ¬(A ∪ B) ~ ¬A ∩ ¬B
+        // follow intersection rules: A & B <: T iff A <: T && B <: T
+        std::vector<SubtypingResult> subtypings;
+
+        for (TypeId ty : u)
+        {
+            NegationType negatedTmp{ty};
+            subtypings.push_back(isCovariantWith(&negatedTmp, superTy));
+        }
+
+        return SubtypingResult::all(subtypings);
+    }
+    else if (auto i = get<IntersectionType>(negatedTy))
+    {
+        // ¬(A ∩ B) ~ ¬A ∪ ¬B
+        // follow union rules: A | B <: T iff A <: T || B <: T
+        std::vector<SubtypingResult> subtypings;
+
+        for (TypeId ty : i)
+        {
+            if (auto negatedPart = get<NegationType>(follow(ty)))
+                subtypings.push_back(isCovariantWith(negatedPart->ty, superTy));
+            else
+            {
+                NegationType negatedTmp{ty};
+                subtypings.push_back(isCovariantWith(&negatedTmp, superTy));
+            }
+        }
+
+        return SubtypingResult::any(subtypings);
+    }
+    else if (is<ErrorType, FunctionType, TableType, MetatableType>(negatedTy))
+    {
+        iceReporter->ice("attempting to negate a non-testable type");
+    }
+    // negating a different subtype will get you a very wide type that's not a
+    // subtype of other stuff.
+    else
+    {
+        return {false};
+    }
+}
+
+SubtypingResult Subtyping::isCovariantWith(const TypeId subTy, const NegationType* superNegation)
+{
+    TypeId negatedTy = follow(superNegation->ty);
+
+    if (is<NeverType>(negatedTy))
+    {
+        // ¬never ~ unknown
+        return isCovariantWith(subTy, builtinTypes->unknownType);
+    }
+    else if (is<UnknownType>(negatedTy))
+    {
+        // ¬unknown ~ never
+        return isCovariantWith(subTy, builtinTypes->neverType);
+    }
+    else if (is<AnyType>(negatedTy))
+    {
+        // ¬any ~ any
+        return isSubtype(subTy, negatedTy);
+    }
+    else if (auto u = get<UnionType>(negatedTy))
+    {
+        // ¬(A ∪ B) ~ ¬A ∩ ¬B
+        // follow intersection rules: A & B <: T iff A <: T && B <: T
+        std::vector<SubtypingResult> subtypings;
+
+        for (TypeId ty : u)
+        {
+            if (auto negatedPart = get<NegationType>(follow(ty)))
+                subtypings.push_back(isCovariantWith(subTy, negatedPart->ty));
+            else
+            {
+                NegationType negatedTmp{ty};
+                subtypings.push_back(isCovariantWith(subTy, &negatedTmp));
+            }
+        }
+
+        return SubtypingResult::all(subtypings);
+    }
+    else if (auto i = get<IntersectionType>(negatedTy))
+    {
+        // ¬(A ∩ B) ~ ¬A ∪ ¬B
+        // follow union rules: A | B <: T iff A <: T || B <: T
+        std::vector<SubtypingResult> subtypings;
+
+        for (TypeId ty : i)
+        {
+            if (auto negatedPart = get<NegationType>(follow(ty)))
+                subtypings.push_back(isCovariantWith(subTy, negatedPart->ty));
+            else
+            {
+                NegationType negatedTmp{ty};
+                subtypings.push_back(isCovariantWith(subTy, &negatedTmp));
+            }
+        }
+
+        return SubtypingResult::any(subtypings);
+    }
+    else if (auto p = get2<PrimitiveType, PrimitiveType>(subTy, negatedTy))
+    {
+        // number <: ¬boolean
+        // number </: ¬number
+        return {p.first->type != p.second->type};
+    }
+    else if (auto p = get2<SingletonType, PrimitiveType>(subTy, negatedTy))
+    {
+        // "foo" </: ¬string
+        if (get<StringSingleton>(p.first) && p.second->type == PrimitiveType::String)
+            return {false};
+        // false </: ¬boolean
+        else if (get<BooleanSingleton>(p.first) && p.second->type == PrimitiveType::Boolean)
+            return {false};
+        // other cases are true
+        else
+            return {true};
+    }
+    else if (auto p = get2<PrimitiveType, SingletonType>(subTy, negatedTy))
+    {
+        if (p.first->type == PrimitiveType::String && get<StringSingleton>(p.second))
+            return {false};
+        else if (p.first->type == PrimitiveType::Boolean && get<BooleanSingleton>(p.second))
+            return {false};
+        else
+            return {true};
+    }
+    // the top class type is not actually a primitive type, so the negation of
+    // any one of them includes the top class type.
+    else if (auto p = get2<ClassType, PrimitiveType>(subTy, negatedTy))
+        return {true};
+    else if (auto p = get<PrimitiveType>(negatedTy); p && is<TableType, MetatableType>(subTy))
+        return {p->type != PrimitiveType::Table};
+    else if (auto p = get2<FunctionType, PrimitiveType>(subTy, negatedTy))
+        return {p.second->type != PrimitiveType::Function};
+    else if (auto p = get2<SingletonType, SingletonType>(subTy, negatedTy))
+        return {*p.first != *p.second};
+    else if (auto p = get2<ClassType, ClassType>(subTy, negatedTy))
+        return SubtypingResult::negate(isCovariantWith(p.first, p.second));
+    else if (get2<FunctionType, ClassType>(subTy, negatedTy))
+        return {true};
+    else if (is<ErrorType, FunctionType, TableType, MetatableType>(negatedTy))
+        iceReporter->ice("attempting to negate a non-testable type");
+
+    return {false};
+}
+
+SubtypingResult Subtyping::isCovariantWith(const PrimitiveType* subPrim, const PrimitiveType* superPrim)
 {
     return {subPrim->type == superPrim->type};
 }
 
-SubtypingResult Subtyping::isSubtype_(const SingletonType* subSingleton, const PrimitiveType* superPrim)
+SubtypingResult Subtyping::isCovariantWith(const SingletonType* subSingleton, const PrimitiveType* superPrim)
 {
     if (get<StringSingleton>(subSingleton) && superPrim->type == PrimitiveType::String)
         return {true};
@@ -515,24 +727,20 @@ SubtypingResult Subtyping::isSubtype_(const SingletonType* subSingleton, const P
         return {false};
 }
 
-SubtypingResult Subtyping::isSubtype_(const SingletonType* subSingleton, const SingletonType* superSingleton)
+SubtypingResult Subtyping::isCovariantWith(const SingletonType* subSingleton, const SingletonType* superSingleton)
 {
     return {*subSingleton == *superSingleton};
 }
 
-SubtypingResult Subtyping::isSubtype_(const TableType* subTable, const TableType* superTable)
+SubtypingResult Subtyping::isCovariantWith(const TableType* subTable, const TableType* superTable)
 {
     SubtypingResult result{true};
 
-    for (const auto& [name, prop]: superTable->props)
+    for (const auto& [name, prop] : superTable->props)
     {
         auto it = subTable->props.find(name);
         if (it != subTable->props.end())
-        {
+            result.andAlso(isInvariantWith(prop.type(), it->second.type()));
-            // Table properties are invariant
-            result.andAlso(isSubtype(it->second.type(), prop.type()));
-            result.andAlso(isSubtype(prop.type(), it->second.type()));
-        }
         else
             return SubtypingResult{false};
     }
@@ -540,17 +748,18 @@ SubtypingResult Subtyping::isSubtype_(const TableType* subTable, const TableType
     return result;
 }
 
-SubtypingResult Subtyping::isSubtype_(const MetatableType* subMt, const MetatableType* superMt)
+SubtypingResult Subtyping::isCovariantWith(const MetatableType* subMt, const MetatableType* superMt)
 {
     return SubtypingResult::all({
-        isSubtype_(subMt->table, superMt->table),
+        isCovariantWith(subMt->table, superMt->table),
-        isSubtype_(subMt->metatable, superMt->metatable),
+        isCovariantWith(subMt->metatable, superMt->metatable),
     });
 }
 
-SubtypingResult Subtyping::isSubtype_(const MetatableType* subMt, const TableType* superTable)
+SubtypingResult Subtyping::isCovariantWith(const MetatableType* subMt, const TableType* superTable)
 {
-    if (auto subTable = get<TableType>(subMt->table)) {
+    if (auto subTable = get<TableType>(subMt->table))
+    {
         // Metatables cannot erase properties from the table they're attached to, so
         // the subtyping rule for this is just if the table component is a subtype
         // of the supertype table.
@@ -560,7 +769,7 @@ SubtypingResult Subtyping::isSubtype_(const MetatableType* subMt, const TableTyp
         // that the metatable isn't a subtype of the table, even though they have
         // compatible properties/shapes. We'll revisit this later when we have a
         // better understanding of how important this is.
-        return isSubtype_(subTable, superTable);
+        return isCovariantWith(subTable, superTable);
     }
     else
     {
@@ -569,23 +778,19 @@ SubtypingResult Subtyping::isSubtype_(const MetatableType* subMt, const TableTyp
     }
 }
 
-SubtypingResult Subtyping::isSubtype_(const ClassType* subClass, const ClassType* superClass)
+SubtypingResult Subtyping::isCovariantWith(const ClassType* subClass, const ClassType* superClass)
 {
     return {isSubclass(subClass, superClass)};
 }
 
-SubtypingResult Subtyping::isSubtype_(const ClassType* subClass, const TableType* superTable)
+SubtypingResult Subtyping::isCovariantWith(const ClassType* subClass, const TableType* superTable)
 {
     SubtypingResult result{true};
 
-    for (const auto& [name, prop]: superTable->props)
+    for (const auto& [name, prop] : superTable->props)
     {
         if (auto classProp = lookupClassProp(subClass, name))
-        {
+            result.andAlso(isInvariantWith(prop.type(), classProp->type()));
-            // Table properties are invariant
-            result.andAlso(isSubtype_(classProp->type(), prop.type()));
-            result.andAlso(isSubtype_(prop.type(), classProp->type()));
-        }
         else
             return SubtypingResult{false};
     }
@@ -593,20 +798,20 @@ SubtypingResult Subtyping::isSubtype_(const ClassType* subClass, const TableType
     return result;
 }
 
-SubtypingResult Subtyping::isSubtype_(const FunctionType* subFunction, const FunctionType* superFunction)
+SubtypingResult Subtyping::isCovariantWith(const FunctionType* subFunction, const FunctionType* superFunction)
 {
     SubtypingResult result;
     {
         VarianceFlipper vf{&variance};
-        result.orElse(isSubtype_(superFunction->argTypes, subFunction->argTypes));
+        result.orElse(isContravariantWith(subFunction->argTypes, superFunction->argTypes));
     }
 
-    result.andAlso(isSubtype_(subFunction->retTypes, superFunction->retTypes));
+    result.andAlso(isCovariantWith(subFunction->retTypes, superFunction->retTypes));
 
     return result;
 }
 
-SubtypingResult Subtyping::isSubtype_(const PrimitiveType* subPrim, const TableType* superTable)
+SubtypingResult Subtyping::isCovariantWith(const PrimitiveType* subPrim, const TableType* superTable)
 {
     SubtypingResult result{false};
     if (subPrim->type == PrimitiveType::String)
@@ -618,7 +823,7 @@ SubtypingResult Subtyping::isSubtype_(const PrimitiveType* subPrim, const TableT
                 if (auto it = mttv->props.find("__index"); it != mttv->props.end())
                 {
                     if (auto stringTable = get<TableType>(it->second.type()))
-                        result.orElse(isSubtype_(stringTable, superTable));
+                        result.orElse(isCovariantWith(stringTable, superTable));
                 }
             }
         }
@@ -627,7 +832,7 @@ SubtypingResult Subtyping::isSubtype_(const PrimitiveType* subPrim, const TableT
     return result;
 }
 
-SubtypingResult Subtyping::isSubtype_(const SingletonType* subSingleton, const TableType* superTable)
+SubtypingResult Subtyping::isCovariantWith(const SingletonType* subSingleton, const TableType* superTable)
 {
     SubtypingResult result{false};
     if (auto stringleton = get<StringSingleton>(subSingleton))
@@ -639,7 +844,7 @@ SubtypingResult Subtyping::isSubtype_(const SingletonType* subSingleton, const T
                 if (auto it = mttv->props.find("__index"); it != mttv->props.end())
                 {
                     if (auto stringTable = get<TableType>(it->second.type()))
-                        result.orElse(isSubtype_(stringTable, superTable));
+                        result.orElse(isCovariantWith(stringTable, superTable));
                 }
             }
         }
@@ -647,29 +852,27 @@ SubtypingResult Subtyping::isSubtype_(const SingletonType* subSingleton, const T
     return result;
 }
 
-SubtypingResult Subtyping::isSubtype_(const NormalizedType* subNorm, const NormalizedType* superNorm)
+SubtypingResult Subtyping::isCovariantWith(const NormalizedType* subNorm, const NormalizedType* superNorm)
 {
     if (!subNorm || !superNorm)
         return {false, true, true};
 
-    SubtypingResult result = isSubtype_(subNorm->tops, superNorm->tops);
+    SubtypingResult result = isCovariantWith(subNorm->tops, superNorm->tops);
-    result.andAlso(isSubtype_(subNorm->booleans, superNorm->booleans));
+    result.andAlso(isCovariantWith(subNorm->booleans, superNorm->booleans));
-    result.andAlso(isSubtype_(subNorm->classes, superNorm->classes, superNorm->tables));
+    result.andAlso(isCovariantWith(subNorm->classes, superNorm->classes).orElse(isCovariantWith(subNorm->classes, superNorm->tables)));
-    result.andAlso(isSubtype_(subNorm->errors, superNorm->errors));
+    result.andAlso(isCovariantWith(subNorm->errors, superNorm->errors));
-    result.andAlso(isSubtype_(subNorm->nils, superNorm->nils));
+    result.andAlso(isCovariantWith(subNorm->nils, superNorm->nils));
-    result.andAlso(isSubtype_(subNorm->numbers, superNorm->numbers));
+    result.andAlso(isCovariantWith(subNorm->numbers, superNorm->numbers));
-    result.isSubtype &= Luau::isSubtype(subNorm->strings, superNorm->strings);
+    result.andAlso(isCovariantWith(subNorm->strings, superNorm->strings));
-    // isSubtype_(subNorm->strings, superNorm->tables);
+    result.andAlso(isCovariantWith(subNorm->strings, superNorm->tables));
-    result.andAlso(isSubtype_(subNorm->threads, superNorm->threads));
+    result.andAlso(isCovariantWith(subNorm->threads, superNorm->threads));
-    result.andAlso(isSubtype_(subNorm->tables, superNorm->tables));
+    result.andAlso(isCovariantWith(subNorm->tables, superNorm->tables));
-    // isSubtype_(subNorm->tables, superNorm->strings);
+    result.andAlso(isCovariantWith(subNorm->functions, superNorm->functions));
-    // isSubtype_(subNorm->tables, superNorm->classes);
+    // isCovariantWith(subNorm->tyvars, superNorm->tyvars);
-    result.andAlso(isSubtype_(subNorm->functions, superNorm->functions));
-    // isSubtype_(subNorm->tyvars, superNorm->tyvars);
     return result;
 }
 
-SubtypingResult Subtyping::isSubtype_(const NormalizedClassType& subClass, const NormalizedClassType& superClass, const TypeIds& superTables)
+SubtypingResult Subtyping::isCovariantWith(const NormalizedClassType& subClass, const NormalizedClassType& superClass)
 {
     for (const auto& [subClassTy, _] : subClass.classes)
     {
@@ -677,24 +880,18 @@ SubtypingResult Subtyping::isSubtype_(const NormalizedClassType& subClass, const
 
         for (const auto& [superClassTy, superNegations] : superClass.classes)
         {
-            result.orElse(isSubtype_(subClassTy, superClassTy));
+            result.orElse(isCovariantWith(subClassTy, superClassTy));
             if (!result.isSubtype)
                 continue;
 
             for (TypeId negation : superNegations)
             {
-                result.andAlso(SubtypingResult::negate(isSubtype_(subClassTy, negation)));
+                result.andAlso(SubtypingResult::negate(isCovariantWith(subClassTy, negation)));
                 if (result.isSubtype)
                     break;
             }
         }
 
-        if (result.isSubtype)
-            continue;
-
-        for (TypeId superTableTy : superTables)
-            result.orElse(isSubtype_(subClassTy, superTableTy));
-
         if (!result.isSubtype)
             return result;
     }
@@ -702,17 +899,79 @@ SubtypingResult Subtyping::isSubtype_(const NormalizedClassType& subClass, const
     return {true};
 }
 
-SubtypingResult Subtyping::isSubtype_(const NormalizedFunctionType& subFunction, const NormalizedFunctionType& superFunction)
+SubtypingResult Subtyping::isCovariantWith(const NormalizedClassType& subClass, const TypeIds& superTables)
+{
+    for (const auto& [subClassTy, _] : subClass.classes)
+    {
+        SubtypingResult result;
+
+        for (TypeId superTableTy : superTables)
+            result.orElse(isCovariantWith(subClassTy, superTableTy));
+
+        if (!result.isSubtype)
+            return result;
+    }
+
+    return {true};
+}
+
+SubtypingResult Subtyping::isCovariantWith(const NormalizedStringType& subString, const NormalizedStringType& superString)
+{
+    bool isSubtype = Luau::isSubtype(subString, superString);
+    return {isSubtype};
+}
+
+SubtypingResult Subtyping::isCovariantWith(const NormalizedStringType& subString, const TypeIds& superTables)
+{
+    if (subString.isNever())
+        return {true};
+
+    if (subString.isCofinite)
+    {
+        SubtypingResult result;
+        for (const auto& superTable : superTables)
+        {
+            result.orElse(isCovariantWith(builtinTypes->stringType, superTable));
+            if (result.isSubtype)
+                return result;
+        }
+        return result;
+    }
+
+    // Finite case
+    // S = s1 | s2 | s3 ... sn <: t1 | t2 | ... | tn
+    // iff for some ti, S <: ti
+    // iff for all sj, sj <: ti
+    for (const auto& superTable : superTables)
+    {
+        SubtypingResult result{true};
+        for (const auto& [_, subString] : subString.singletons)
+        {
+            result.andAlso(isCovariantWith(subString, superTable));
+            if (!result.isSubtype)
+                break;
+        }
+
+        if (!result.isSubtype)
+            continue;
+        else
+            return result;
+    }
+
+    return {false};
+}
+
+SubtypingResult Subtyping::isCovariantWith(const NormalizedFunctionType& subFunction, const NormalizedFunctionType& superFunction)
 {
     if (subFunction.isNever())
         return {true};
     else if (superFunction.isTop)
         return {true};
     else
-        return isSubtype_(subFunction.parts, superFunction.parts);
+        return isCovariantWith(subFunction.parts, superFunction.parts);
 }
 
-SubtypingResult Subtyping::isSubtype_(const TypeIds& subTypes, const TypeIds& superTypes)
+SubtypingResult Subtyping::isCovariantWith(const TypeIds& subTypes, const TypeIds& superTypes)
 {
     std::vector<SubtypingResult> results;
 
@@ -720,15 +979,15 @@ SubtypingResult Subtyping::isSubtype_(const TypeIds& subTypes, const TypeIds& su
     {
         results.emplace_back();
         for (TypeId superTy : superTypes)
-            results.back().orElse(isSubtype_(subTy, superTy));
+            results.back().orElse(isCovariantWith(subTy, superTy));
     }
 
     return SubtypingResult::all(results);
 }
 
-SubtypingResult Subtyping::isSubtype_(const VariadicTypePack* subVariadic, const VariadicTypePack* superVariadic)
+SubtypingResult Subtyping::isCovariantWith(const VariadicTypePack* subVariadic, const VariadicTypePack* superVariadic)
 {
-    return isSubtype_(subVariadic->ty, superVariadic->ty);
+    return isCovariantWith(subVariadic->ty, superVariadic->ty);
 }
 
 bool Subtyping::bindGeneric(TypeId subTy, TypeId superTy)
@@ -772,7 +1031,7 @@ bool Subtyping::bindGeneric(TypePackId subTp, TypePackId superTp)
     return true;
 }
 
-template <typename T, typename Container>
+template<typename T, typename Container>
 TypeId Subtyping::makeAggregateType(const Container& container, TypeId orElse)
 {
     if (container.empty())

Analysis/src/ToDot.cpp

@@ -145,8 +145,7 @@ void StateDot::visitChildren(TypeId ty, int index)
     startNode(index);
     startNodeLabel();
 
-    auto go = [&](auto&& t)
+    auto go = [&](auto&& t) {
-    {
         using T = std::decay_t<decltype(t)>;
 
         if constexpr (std::is_same_v<T, BoundType>)

Analysis/src/TypeChecker2.cpp

@@ -242,8 +242,8 @@ struct TypeChecker2
 
     Normalizer normalizer;
 
-    TypeChecker2(NotNull<BuiltinTypes> builtinTypes, NotNull<UnifierSharedState> unifierState, NotNull<TypeCheckLimits> limits, DcrLogger* logger, const SourceModule* sourceModule,
+    TypeChecker2(NotNull<BuiltinTypes> builtinTypes, NotNull<UnifierSharedState> unifierState, NotNull<TypeCheckLimits> limits, DcrLogger* logger,
-        Module* module)
+        const SourceModule* sourceModule, Module* module)
         : builtinTypes(builtinTypes)
         , logger(logger)
         , limits(limits)
@@ -1295,13 +1295,8 @@ struct TypeChecker2
             else if (auto assertion = expr->as<AstExprTypeAssertion>())
                 return isLiteral(assertion->expr);
 
-            return
+            return expr->is<AstExprConstantNil>() || expr->is<AstExprConstantBool>() || expr->is<AstExprConstantNumber>() ||
-                expr->is<AstExprConstantNil>() ||
+                   expr->is<AstExprConstantString>() || expr->is<AstExprFunction>() || expr->is<AstExprTable>();
-                expr->is<AstExprConstantBool>() ||
-                expr->is<AstExprConstantNumber>() ||
-                expr->is<AstExprConstantString>() ||
-                expr->is<AstExprFunction>() ||
-                expr->is<AstExprTable>();
         }
 
         static std::unique_ptr<LiteralProperties> buildLiteralPropertiesSet(AstExpr* expr)
@@ -1423,7 +1418,7 @@ struct TypeChecker2
             LUAU_ASSERT(argOffset == args->head.size());
 
             const Location argLoc = argExprs->empty() ? Location{} // TODO
-                : argExprs->at(argExprs->size() - 1)->location;
+                                                      : argExprs->at(argExprs->size() - 1)->location;
 
             if (paramIter.tail() && args->tail)
             {
@@ -2686,8 +2681,8 @@ struct TypeChecker2
     }
 };
 
-void check(
+void check(NotNull<BuiltinTypes> builtinTypes, NotNull<UnifierSharedState> unifierState, NotNull<TypeCheckLimits> limits, DcrLogger* logger,
-    NotNull<BuiltinTypes> builtinTypes, NotNull<UnifierSharedState> unifierState, NotNull<TypeCheckLimits> limits, DcrLogger* logger, const SourceModule& sourceModule, Module* module)
+    const SourceModule& sourceModule, Module* module)
 {
     TypeChecker2 typeChecker{builtinTypes, unifierState, limits, logger, &sourceModule, module};
 

Analysis/src/Unifier.cpp

@@ -25,6 +25,7 @@ LUAU_FASTFLAGVARIABLE(LuauOccursIsntAlwaysFailure, false)
 LUAU_FASTFLAG(LuauNormalizeBlockedTypes)
 LUAU_FASTFLAG(LuauAlwaysCommitInferencesOfFunctionCalls)
 LUAU_FASTFLAG(DebugLuauDeferredConstraintResolution)
+LUAU_FASTFLAGVARIABLE(LuauFixIndexerSubtypingOrdering, false)
 
 namespace Luau
 {
@@ -2285,7 +2286,7 @@ void Unifier::tryUnifyTables(TypeId subTy, TypeId superTy, bool isIntersection,
                 variance = Invariant;
 
             Unifier innerState = makeChildUnifier();
-            if (useNewSolver)
+            if (useNewSolver || FFlag::LuauFixIndexerSubtypingOrdering)
                 innerState.tryUnify_(prop.type(), superTable->indexer->indexResultType);
             else
             {

Analysis/src/Unifier2.cpp

@@ -26,7 +26,6 @@ Unifier2::Unifier2(NotNull<TypeArena> arena, NotNull<BuiltinTypes> builtinTypes,
     , ice(ice)
     , recursionLimit(FInt::LuauTypeInferRecursionLimit)
 {
-
 }
 
 bool Unifier2::unify(TypeId subTy, TypeId superTy)
@@ -99,10 +98,7 @@ bool Unifier2::unify(TypePackId subTp, TypePackId superTp)
         return true;
     }
 
-    size_t maxLength = std::max(
+    size_t maxLength = std::max(flatten(subTp).first.size(), flatten(superTp).first.size());
-        flatten(subTp).first.size(),
-        flatten(superTp).first.size()
-    );
 
     auto [subTypes, subTail] = extendTypePack(*arena, builtinTypes, subTp, maxLength);
     auto [superTypes, superTail] = extendTypePack(*arena, builtinTypes, superTp, maxLength);
@@ -123,16 +119,25 @@ struct FreeTypeSearcher : TypeVisitor
     explicit FreeTypeSearcher(NotNull<Scope> scope)
         : TypeVisitor(/*skipBoundTypes*/ true)
         , scope(scope)
-    {}
+    {
+    }
 
-    enum { Positive, Negative } polarity = Positive;
+    enum
+    {
+        Positive,
+        Negative
+    } polarity = Positive;
 
     void flip()
     {
         switch (polarity)
         {
-            case Positive: polarity = Negative; break;
+        case Positive:
-            case Negative: polarity = Positive; break;
+            polarity = Negative;
+            break;
+        case Negative:
+            polarity = Positive;
+            break;
         }
     }
 
@@ -152,8 +157,12 @@ struct FreeTypeSearcher : TypeVisitor
 
         switch (polarity)
         {
-            case Positive: positiveTypes.insert(ty); break;
+        case Positive:
-            case Negative: negativeTypes.insert(ty); break;
+            positiveTypes.insert(ty);
+            break;
+        case Negative:
+            negativeTypes.insert(ty);
+            break;
         }
 
         return true;
@@ -180,13 +189,17 @@ struct MutatingGeneralizer : TypeOnceVisitor
     std::unordered_set<TypeId> negativeTypes;
     std::vector<TypeId> generics;
 
-    MutatingGeneralizer(NotNull<BuiltinTypes> builtinTypes, NotNull<Scope> scope, std::unordered_set<TypeId> positiveTypes, std::unordered_set<TypeId> negativeTypes)
+    bool isWithinFunction = false;
+
+    MutatingGeneralizer(
+        NotNull<BuiltinTypes> builtinTypes, NotNull<Scope> scope, std::unordered_set<TypeId> positiveTypes, std::unordered_set<TypeId> negativeTypes)
         : TypeOnceVisitor(/* skipBoundTypes */ true)
         , builtinTypes(builtinTypes)
         , scope(scope)
         , positiveTypes(std::move(positiveTypes))
         , negativeTypes(std::move(negativeTypes))
-    {}
+    {
+    }
 
     static void replace(DenseHashSet<TypeId>& seen, TypeId haystack, TypeId needle, TypeId replacement)
     {
@@ -211,10 +224,7 @@ struct MutatingGeneralizer : TypeOnceVisitor
             // FIXME: I bet this function has reentrancy problems
             option = follow(option);
             if (option == needle)
-            {
-                LUAU_ASSERT(!seen.find(option));
                 option = replacement;
-            }
 
             // TODO seen set
             else if (get<UnionType>(option))
@@ -224,7 +234,21 @@ struct MutatingGeneralizer : TypeOnceVisitor
         }
     }
 
-    bool visit (TypeId ty, const FreeType&) override
+    bool visit(TypeId ty, const FunctionType& ft) override
+    {
+        const bool oldValue = isWithinFunction;
+
+        isWithinFunction = true;
+
+        traverse(ft.argTypes);
+        traverse(ft.retTypes);
+
+        isWithinFunction = oldValue;
+
+        return false;
+    }
+
+    bool visit(TypeId ty, const FreeType&) override
     {
         const FreeType* ft = get<FreeType>(ty);
         LUAU_ASSERT(ft);
@@ -232,7 +256,8 @@ struct MutatingGeneralizer : TypeOnceVisitor
         traverse(ft->lowerBound);
         traverse(ft->upperBound);
 
-        // ft is potentially invalid now.
+        // It is possible for the above traverse() calls to cause ty to be
+        // transmuted.  We must reaquire ft if this happens.
         ty = follow(ty);
         ft = get<FreeType>(ty);
         if (!ft)
@@ -252,8 +277,13 @@ struct MutatingGeneralizer : TypeOnceVisitor
 
         if (!hasLowerBound && !hasUpperBound)
         {
-            emplaceType<GenericType>(asMutable(ty), scope);
+            if (isWithinFunction)
-            generics.push_back(ty);
+            {
+                emplaceType<GenericType>(asMutable(ty), scope);
+                generics.push_back(ty);
+            }
+            else
+                emplaceType<BoundType>(asMutable(ty), builtinTypes->unknownType);
         }
 
         // It is possible that this free type has other free types in its upper
@@ -264,19 +294,27 @@ struct MutatingGeneralizer : TypeOnceVisitor
         // If we do not do this, we get tautological bounds like a <: a <: unknown.
         else if (isPositive && !hasUpperBound)
         {
-            if (FreeType* lowerFree = getMutable<FreeType>(ft->lowerBound); lowerFree && lowerFree->upperBound == ty)
+            TypeId lb = follow(ft->lowerBound);
+            if (FreeType* lowerFree = getMutable<FreeType>(lb); lowerFree && lowerFree->upperBound == ty)
                 lowerFree->upperBound = builtinTypes->unknownType;
             else
-                replace(seen, ft->lowerBound, ty, builtinTypes->unknownType);
+            {
-            emplaceType<BoundType>(asMutable(ty), ft->lowerBound);
+                DenseHashSet<TypeId> replaceSeen{nullptr};
+                replace(replaceSeen, lb, ty, builtinTypes->unknownType);
+            }
+            emplaceType<BoundType>(asMutable(ty), lb);
         }
         else
         {
-            if (FreeType* upperFree = getMutable<FreeType>(ft->upperBound); upperFree && upperFree->lowerBound == ty)
+            TypeId ub = follow(ft->upperBound);
+            if (FreeType* upperFree = getMutable<FreeType>(ub); upperFree && upperFree->lowerBound == ty)
                 upperFree->lowerBound = builtinTypes->neverType;
             else
-                replace(seen, ft->upperBound, ty, builtinTypes->neverType);
+            {
-            emplaceType<BoundType>(asMutable(ty), ft->upperBound);
+                DenseHashSet<TypeId> replaceSeen{nullptr};
+                replace(replaceSeen, ub, ty, builtinTypes->neverType);
+            }
+            emplaceType<BoundType>(asMutable(ty), ub);
         }
 
         return false;
@@ -363,4 +401,4 @@ OccursCheckResult Unifier2::occursCheck(DenseHashSet<TypePackId>& seen, TypePack
     return OccursCheckResult::Pass;
 }
 
-}
+} // namespace Luau

Ast/include/Luau/Ast.h

@@ -534,7 +534,7 @@ class AstStatBlock : public AstStat
 public:
     LUAU_RTTI(AstStatBlock)
 
-    AstStatBlock(const Location& location, const AstArray<AstStat*>& body, bool hasEnd=true);
+    AstStatBlock(const Location& location, const AstArray<AstStat*>& body, bool hasEnd = true);
 
     void visit(AstVisitor* visitor) override;
 

CLI/Compile.cpp

@@ -89,13 +89,14 @@ static void reportError(const char* name, const Luau::CompileError& error)
     report(name, error.getLocation(), "CompileError", error.what());
 }
 
-static std::string getCodegenAssembly(const char* name, const std::string& bytecode, Luau::CodeGen::AssemblyOptions options)
+static std::string getCodegenAssembly(
+    const char* name, const std::string& bytecode, Luau::CodeGen::AssemblyOptions options, Luau::CodeGen::LoweringStats* stats)
 {
     std::unique_ptr<lua_State, void (*)(lua_State*)> globalState(luaL_newstate(), lua_close);
     lua_State* L = globalState.get();
 
     if (luau_load(L, name, bytecode.data(), bytecode.size(), 0) == 0)
-        return Luau::CodeGen::getAssembly(L, -1, options);
+        return Luau::CodeGen::getAssembly(L, -1, options, stats);
 
     fprintf(stderr, "Error loading bytecode %s\n", name);
     return "";
@@ -119,6 +120,8 @@ struct CompileStats
     double parseTime;
     double compileTime;
     double codegenTime;
+
+    Luau::CodeGen::LoweringStats lowerStats;
 };
 
 static double recordDeltaTime(double& timer)
@@ -213,10 +216,10 @@ static bool compileFile(const char* name, CompileFormat format, Luau::CodeGen::A
         case CompileFormat::CodegenAsm:
         case CompileFormat::CodegenIr:
         case CompileFormat::CodegenVerbose:
-            printf("%s", getCodegenAssembly(name, bcb.getBytecode(), options).c_str());
+            printf("%s", getCodegenAssembly(name, bcb.getBytecode(), options, &stats.lowerStats).c_str());
             break;
         case CompileFormat::CodegenNull:
-            stats.codegen += getCodegenAssembly(name, bcb.getBytecode(), options).size();
+            stats.codegen += getCodegenAssembly(name, bcb.getBytecode(), options, &stats.lowerStats).size();
             stats.codegenTime += recordDeltaTime(currts);
             break;
         case CompileFormat::Null:
@@ -355,13 +358,22 @@ int main(int argc, char** argv)
         failed += !compileFile(path.c_str(), compileFormat, assemblyTarget, stats);
 
     if (compileFormat == CompileFormat::Null)
+    {
         printf("Compiled %d KLOC into %d KB bytecode (read %.2fs, parse %.2fs, compile %.2fs)\n", int(stats.lines / 1000), int(stats.bytecode / 1024),
             stats.readTime, stats.parseTime, stats.compileTime);
+    }
     else if (compileFormat == CompileFormat::CodegenNull)
+    {
         printf("Compiled %d KLOC into %d KB bytecode => %d KB native code (%.2fx) (read %.2fs, parse %.2fs, compile %.2fs, codegen %.2fs)\n",
             int(stats.lines / 1000), int(stats.bytecode / 1024), int(stats.codegen / 1024),
             stats.bytecode == 0 ? 0.0 : double(stats.codegen) / double(stats.bytecode), stats.readTime, stats.parseTime, stats.compileTime,
             stats.codegenTime);
 
+        printf("Lowering stats:\n");
+        printf("- spills to stack: %d, spills to restore: %d, max spill slot %u\n", stats.lowerStats.spillsToSlot, stats.lowerStats.spillsToRestore,
+            stats.lowerStats.maxSpillSlotsUsed);
+        printf("- regalloc failed: %d, lowering failed %d\n", stats.lowerStats.regAllocErrors, stats.lowerStats.loweringErrors);
+    }
+
     return failed ? 1 : 0;
 }

CodeGen/include/Luau/CodeGen.h

@@ -3,6 +3,7 @@
 
 #include <string>
 
+#include <stddef.h>
 #include <stdint.h>
 
 struct lua_State;
@@ -74,8 +75,18 @@ struct AssemblyOptions
     void* annotatorContext = nullptr;
 };
 
+struct LoweringStats
+{
+    int spillsToSlot = 0;
+    int spillsToRestore = 0;
+    unsigned maxSpillSlotsUsed = 0;
+
+    int regAllocErrors = 0;
+    int loweringErrors = 0;
+};
+
 // Generates assembly for target function and all inner functions
-std::string getAssembly(lua_State* L, int idx, AssemblyOptions options = {});
+std::string getAssembly(lua_State* L, int idx, AssemblyOptions options = {}, LoweringStats* stats = nullptr);
 
 using PerfLogFn = void (*)(void* context, uintptr_t addr, unsigned size, const char* symbol);
 

CodeGen/include/Luau/IrRegAllocX64.h

@@ -12,6 +12,9 @@ namespace Luau
 {
 namespace CodeGen
 {
+
+struct LoweringStats;
+
 namespace X64
 {
 
@@ -33,7 +36,7 @@ struct IrSpillX64
 
 struct IrRegAllocX64
 {
-    IrRegAllocX64(AssemblyBuilderX64& build, IrFunction& function);
+    IrRegAllocX64(AssemblyBuilderX64& build, IrFunction& function, LoweringStats* stats);
 
     RegisterX64 allocReg(SizeX64 size, uint32_t instIdx);
     RegisterX64 allocRegOrReuse(SizeX64 size, uint32_t instIdx, std::initializer_list<IrOp> oprefs);
@@ -70,6 +73,7 @@ struct IrRegAllocX64
 
     AssemblyBuilderX64& build;
     IrFunction& function;
+    LoweringStats* stats = nullptr;
 
     uint32_t currInstIdx = ~0u;
 

CodeGen/include/Luau/IrUtils.h

@@ -264,5 +264,14 @@ uint32_t getNativeContextOffset(int bfid);
 // Cleans up blocks that were created with no users
 void killUnusedBlocks(IrFunction& function);
 
+// Get blocks in order that tries to maximize fallthrough between them during lowering
+// We want to mostly preserve build order with fallbacks outlined
+// But we also use hints from optimization passes that chain blocks together where there's only one out-in edge between them
+std::vector<uint32_t> getSortedBlockOrder(IrFunction& function);
+
+// Returns first non-dead block that comes after block at index 'i' in the sorted blocks array
+// 'dummy' block is returned if the end of array was reached
+IrBlock& getNextBlock(IrFunction& function, std::vector<uint32_t>& sortedBlocks, IrBlock& dummy, size_t i);
+
 } // namespace CodeGen
 } // namespace Luau

CodeGen/src/AssemblyBuilderA64.cpp

@@ -1091,7 +1091,7 @@ void AssemblyBuilderA64::placeER(const char* name, RegisterA64 dst, RegisterA64
     LUAU_ASSERT(shift >= 0 && shift <= 4);
 
     uint32_t sf = (dst.kind == KindA64::x) ? 0x80000000 : 0; // could be useful in the future for byte->word extends
-    int option = 0b010; // UXTW
+    int option = 0b010;                                      // UXTW
 
     place(dst.index | (src1.index << 5) | (shift << 10) | (option << 13) | (src2.index << 16) | (1 << 21) | (op << 24) | sf);
     commit();

CodeGen/src/CodeGen.cpp

@@ -106,7 +106,7 @@ static std::optional<NativeProto> createNativeFunction(AssemblyBuilder& build, M
     IrBuilder ir;
     ir.buildFunctionIr(proto);
 
-    if (!lowerFunction(ir, build, helpers, proto, {}))
+    if (!lowerFunction(ir, build, helpers, proto, {}, /* stats */ nullptr))
         return std::nullopt;
 
     return createNativeProto(proto, ir);

CodeGen/src/CodeGenAssembly.cpp

@@ -43,7 +43,7 @@ static void logFunctionHeader(AssemblyBuilder& build, Proto* proto)
 }
 
 template<typename AssemblyBuilder>
-static std::string getAssemblyImpl(AssemblyBuilder& build, const TValue* func, AssemblyOptions options)
+static std::string getAssemblyImpl(AssemblyBuilder& build, const TValue* func, AssemblyOptions options, LoweringStats* stats)
 {
     std::vector<Proto*> protos;
     gatherFunctions(protos, clvalue(func)->l.p);
@@ -66,7 +66,7 @@ static std::string getAssemblyImpl(AssemblyBuilder& build, const TValue* func, A
             if (options.includeAssembly || options.includeIr)
                 logFunctionHeader(build, p);
 
-            if (!lowerFunction(ir, build, helpers, p, options))
+            if (!lowerFunction(ir, build, helpers, p, options, stats))
             {
                 if (build.logText)
                     build.logAppend("; skipping (can't lower)\n");
@@ -90,7 +90,7 @@ static std::string getAssemblyImpl(AssemblyBuilder& build, const TValue* func, A
 unsigned int getCpuFeaturesA64();
 #endif
 
-std::string getAssembly(lua_State* L, int idx, AssemblyOptions options)
+std::string getAssembly(lua_State* L, int idx, AssemblyOptions options, LoweringStats* stats)
 {
     LUAU_ASSERT(lua_isLfunction(L, idx));
     const TValue* func = luaA_toobject(L, idx);
@@ -106,35 +106,35 @@ std::string getAssembly(lua_State* L, int idx, AssemblyOptions options)
         X64::AssemblyBuilderX64 build(/* logText= */ options.includeAssembly);
 #endif
 
-        return getAssemblyImpl(build, func, options);
+        return getAssemblyImpl(build, func, options, stats);
     }
 
     case AssemblyOptions::A64:
     {
         A64::AssemblyBuilderA64 build(/* logText= */ options.includeAssembly, /* features= */ A64::Feature_JSCVT);
 
-        return getAssemblyImpl(build, func, options);
+        return getAssemblyImpl(build, func, options, stats);
     }
 
     case AssemblyOptions::A64_NoFeatures:
     {
         A64::AssemblyBuilderA64 build(/* logText= */ options.includeAssembly, /* features= */ 0);
 
-        return getAssemblyImpl(build, func, options);
+        return getAssemblyImpl(build, func, options, stats);
     }
 
     case AssemblyOptions::X64_Windows:
     {
         X64::AssemblyBuilderX64 build(/* logText= */ options.includeAssembly, X64::ABIX64::Windows);
 
-        return getAssemblyImpl(build, func, options);
+        return getAssemblyImpl(build, func, options, stats);
     }
 
     case AssemblyOptions::X64_SystemV:
     {
         X64::AssemblyBuilderX64 build(/* logText= */ options.includeAssembly, X64::ABIX64::SystemV);
 
-        return getAssemblyImpl(build, func, options);
+        return getAssemblyImpl(build, func, options, stats);
     }
 
     default:

CodeGen/src/CodeGenLower.h

@@ -44,42 +44,10 @@ inline void gatherFunctions(std::vector<Proto*>& results, Proto* proto)
         gatherFunctions(results, proto->p[i]);
 }
 
-inline IrBlock& getNextBlock(IrFunction& function, std::vector<uint32_t>& sortedBlocks, IrBlock& dummy, size_t i)
-{
-    for (size_t j = i + 1; j < sortedBlocks.size(); ++j)
-    {
-        IrBlock& block = function.blocks[sortedBlocks[j]];
-        if (block.kind != IrBlockKind::Dead)
-            return block;
-    }
-
-    return dummy;
-}
-
 template<typename AssemblyBuilder, typename IrLowering>
 inline bool lowerImpl(AssemblyBuilder& build, IrLowering& lowering, IrFunction& function, int bytecodeid, AssemblyOptions options)
 {
-    // While we will need a better block ordering in the future, right now we want to mostly preserve build order with fallbacks outlined
+    std::vector<uint32_t> sortedBlocks = getSortedBlockOrder(function);
-    std::vector<uint32_t> sortedBlocks;
-    sortedBlocks.reserve(function.blocks.size());
-    for (uint32_t i = 0; i < function.blocks.size(); i++)
-        sortedBlocks.push_back(i);
-
-    std::sort(sortedBlocks.begin(), sortedBlocks.end(), [&](uint32_t idxA, uint32_t idxB) {
-        const IrBlock& a = function.blocks[idxA];
-        const IrBlock& b = function.blocks[idxB];
-
-        // Place fallback blocks at the end
-        if ((a.kind == IrBlockKind::Fallback) != (b.kind == IrBlockKind::Fallback))
-            return (a.kind == IrBlockKind::Fallback) < (b.kind == IrBlockKind::Fallback);
-
-        // Try to order by instruction order
-        if (a.sortkey != b.sortkey)
-            return a.sortkey < b.sortkey;
-
-        // Chains of blocks are merged together by having the same sort key and consecutive chain key
-        return a.chainkey < b.chainkey;
-    });
 
     // For each IR instruction that begins a bytecode instruction, which bytecode instruction is it?
     std::vector<uint32_t> bcLocations(function.instructions.size() + 1, ~0u);
@@ -231,24 +199,26 @@ inline bool lowerImpl(AssemblyBuilder& build, IrLowering& lowering, IrFunction&
     return true;
 }
 
-inline bool lowerIr(X64::AssemblyBuilderX64& build, IrBuilder& ir, ModuleHelpers& helpers, Proto* proto, AssemblyOptions options)
+inline bool lowerIr(
+    X64::AssemblyBuilderX64& build, IrBuilder& ir, ModuleHelpers& helpers, Proto* proto, AssemblyOptions options, LoweringStats* stats)
 {
     optimizeMemoryOperandsX64(ir.function);
 
-    X64::IrLoweringX64 lowering(build, helpers, ir.function);
+    X64::IrLoweringX64 lowering(build, helpers, ir.function, stats);
 
     return lowerImpl(build, lowering, ir.function, proto->bytecodeid, options);
 }
 
-inline bool lowerIr(A64::AssemblyBuilderA64& build, IrBuilder& ir, ModuleHelpers& helpers, Proto* proto, AssemblyOptions options)
+inline bool lowerIr(
+    A64::AssemblyBuilderA64& build, IrBuilder& ir, ModuleHelpers& helpers, Proto* proto, AssemblyOptions options, LoweringStats* stats)
 {
-    A64::IrLoweringA64 lowering(build, helpers, ir.function);
+    A64::IrLoweringA64 lowering(build, helpers, ir.function, stats);
 
     return lowerImpl(build, lowering, ir.function, proto->bytecodeid, options);
 }
 
 template<typename AssemblyBuilder>
-inline bool lowerFunction(IrBuilder& ir, AssemblyBuilder& build, ModuleHelpers& helpers, Proto* proto, AssemblyOptions options)
+inline bool lowerFunction(IrBuilder& ir, AssemblyBuilder& build, ModuleHelpers& helpers, Proto* proto, AssemblyOptions options, LoweringStats* stats)
 {
     killUnusedBlocks(ir.function);
 
@@ -264,7 +234,7 @@ inline bool lowerFunction(IrBuilder& ir, AssemblyBuilder& build, ModuleHelpers&
             createLinearBlocks(ir, useValueNumbering);
     }
 
-    return lowerIr(build, ir, helpers, proto, options);
+    return lowerIr(build, ir, helpers, proto, options, stats);
 }
 
 } // namespace CodeGen

CodeGen/src/IrBuilder.cpp

@@ -385,7 +385,8 @@ void IrBuilder::translateInst(LuauOpcode op, const Instruction* pc, int i)
         translateInstDupTable(*this, pc, i);
         break;
     case LOP_SETLIST:
-        inst(IrCmd::SETLIST, constUint(i), vmReg(LUAU_INSN_A(*pc)), vmReg(LUAU_INSN_B(*pc)), constInt(LUAU_INSN_C(*pc) - 1), constUint(pc[1]), undef());
+        inst(IrCmd::SETLIST, constUint(i), vmReg(LUAU_INSN_A(*pc)), vmReg(LUAU_INSN_B(*pc)), constInt(LUAU_INSN_C(*pc) - 1), constUint(pc[1]),
+            undef());
         break;
     case LOP_GETUPVAL:
         translateInstGetUpval(*this, pc, i);

CodeGen/src/IrLoweringA64.cpp

@@ -240,11 +240,12 @@ static bool emitBuiltin(
     }
 }
 
-IrLoweringA64::IrLoweringA64(AssemblyBuilderA64& build, ModuleHelpers& helpers, IrFunction& function)
+IrLoweringA64::IrLoweringA64(AssemblyBuilderA64& build, ModuleHelpers& helpers, IrFunction& function, LoweringStats* stats)
     : build(build)
     , helpers(helpers)
     , function(function)
-    , regs(function, {{x0, x15}, {x16, x17}, {q0, q7}, {q16, q31}})
+    , stats(stats)
+    , regs(function, stats, {{x0, x15}, {x16, x17}, {q0, q7}, {q16, q31}})
     , valueTracker(function)
     , exitHandlerMap(~0u)
 {
@@ -2016,6 +2017,15 @@ void IrLoweringA64::finishFunction()
         build.mov(x0, handler.pcpos * sizeof(Instruction));
         build.b(helpers.updatePcAndContinueInVm);
     }
+
+    if (stats)
+    {
+        if (error)
+            stats->loweringErrors++;
+
+        if (regs.error)
+            stats->regAllocErrors++;
+    }
 }
 
 bool IrLoweringA64::hasError() const

CodeGen/src/IrLoweringA64.h

@@ -17,13 +17,14 @@ namespace CodeGen
 
 struct ModuleHelpers;
 struct AssemblyOptions;
+struct LoweringStats;
 
 namespace A64
 {
 
 struct IrLoweringA64
 {
-    IrLoweringA64(AssemblyBuilderA64& build, ModuleHelpers& helpers, IrFunction& function);
+    IrLoweringA64(AssemblyBuilderA64& build, ModuleHelpers& helpers, IrFunction& function, LoweringStats* stats);
 
     void lowerInst(IrInst& inst, uint32_t index, const IrBlock& next);
     void finishBlock(const IrBlock& curr, const IrBlock& next);
@@ -74,6 +75,7 @@ struct IrLoweringA64
     ModuleHelpers& helpers;
 
     IrFunction& function;
+    LoweringStats* stats = nullptr;
 
     IrRegAllocA64 regs;
 

CodeGen/src/IrLoweringX64.cpp

@@ -22,11 +22,12 @@ namespace CodeGen
 namespace X64
 {
 
-IrLoweringX64::IrLoweringX64(AssemblyBuilderX64& build, ModuleHelpers& helpers, IrFunction& function)
+IrLoweringX64::IrLoweringX64(AssemblyBuilderX64& build, ModuleHelpers& helpers, IrFunction& function, LoweringStats* stats)
     : build(build)
     , helpers(helpers)
     , function(function)
-    , regs(build, function)
+    , stats(stats)
+    , regs(build, function, stats)
     , valueTracker(function)
     , exitHandlerMap(~0u)
 {
@@ -1646,6 +1647,15 @@ void IrLoweringX64::finishFunction()
         build.mov(edx, handler.pcpos * sizeof(Instruction));
         build.jmp(helpers.updatePcAndContinueInVm);
     }
+
+    if (stats)
+    {
+        if (regs.maxUsedSlot > kSpillSlots)
+            stats->regAllocErrors++;
+
+        if (regs.maxUsedSlot > stats->maxSpillSlotsUsed)
+            stats->maxSpillSlotsUsed = regs.maxUsedSlot;
+    }
 }
 
 bool IrLoweringX64::hasError() const

CodeGen/src/IrLoweringX64.h

@@ -19,13 +19,14 @@ namespace CodeGen
 
 struct ModuleHelpers;
 struct AssemblyOptions;
+struct LoweringStats;
 
 namespace X64
 {
 
 struct IrLoweringX64
 {
-    IrLoweringX64(AssemblyBuilderX64& build, ModuleHelpers& helpers, IrFunction& function);
+    IrLoweringX64(AssemblyBuilderX64& build, ModuleHelpers& helpers, IrFunction& function, LoweringStats* stats);
 
     void lowerInst(IrInst& inst, uint32_t index, const IrBlock& next);
     void finishBlock(const IrBlock& curr, const IrBlock& next);
@@ -76,6 +77,7 @@ struct IrLoweringX64
     ModuleHelpers& helpers;
 
     IrFunction& function;
+    LoweringStats* stats = nullptr;
 
     IrRegAllocX64 regs;
 

CodeGen/src/IrRegAllocA64.cpp

@@ -2,6 +2,7 @@
 #include "IrRegAllocA64.h"
 
 #include "Luau/AssemblyBuilderA64.h"
+#include "Luau/CodeGen.h"
 #include "Luau/IrUtils.h"
 
 #include "BitUtils.h"
@@ -109,8 +110,9 @@ static void restoreInst(AssemblyBuilderA64& build, uint32_t& freeSpillSlots, IrF
     inst.regA64 = reg;
 }
 
-IrRegAllocA64::IrRegAllocA64(IrFunction& function, std::initializer_list<std::pair<RegisterA64, RegisterA64>> regs)
+IrRegAllocA64::IrRegAllocA64(IrFunction& function, LoweringStats* stats, std::initializer_list<std::pair<RegisterA64, RegisterA64>> regs)
     : function(function)
+    , stats(stats)
 {
     for (auto& p : regs)
     {
@@ -329,6 +331,9 @@ size_t IrRegAllocA64::spill(AssemblyBuilderA64& build, uint32_t index, std::init
                 spills.push_back(s);
 
                 def.needsReload = true;
+
+                if (stats)
+                    stats->spillsToRestore++;
             }
             else
             {
@@ -345,6 +350,14 @@ size_t IrRegAllocA64::spill(AssemblyBuilderA64& build, uint32_t index, std::init
                 spills.push_back(s);
 
                 def.spilled = true;
+
+                if (stats)
+                {
+                    stats->spillsToSlot++;
+
+                    if (slot != kInvalidSpill && unsigned(slot + 1) > stats->maxSpillSlotsUsed)
+                        stats->maxSpillSlotsUsed = slot + 1;
+                }
             }
 
             def.regA64 = noreg;

CodeGen/src/IrRegAllocA64.h

@@ -12,6 +12,9 @@ namespace Luau
 {
 namespace CodeGen
 {
+
+struct LoweringStats;
+
 namespace A64
 {
 
@@ -19,7 +22,7 @@ class AssemblyBuilderA64;
 
 struct IrRegAllocA64
 {
-    IrRegAllocA64(IrFunction& function, std::initializer_list<std::pair<RegisterA64, RegisterA64>> regs);
+    IrRegAllocA64(IrFunction& function, LoweringStats* stats, std::initializer_list<std::pair<RegisterA64, RegisterA64>> regs);
 
     RegisterA64 allocReg(KindA64 kind, uint32_t index);
     RegisterA64 allocTemp(KindA64 kind);
@@ -69,6 +72,7 @@ struct IrRegAllocA64
     Set& getSet(KindA64 kind);
 
     IrFunction& function;
+    LoweringStats* stats = nullptr;
     Set gpr, simd;
 
     std::vector<Spill> spills;

CodeGen/src/IrRegAllocX64.cpp

@@ -1,6 +1,7 @@
 // This file is part of the Luau programming language and is licensed under MIT License; see LICENSE.txt for details
 #include "Luau/IrRegAllocX64.h"
 
+#include "Luau/CodeGen.h"
 #include "Luau/IrUtils.h"
 
 #include "EmitCommonX64.h"
@@ -14,9 +15,10 @@ namespace X64
 
 static const RegisterX64 kGprAllocOrder[] = {rax, rdx, rcx, rbx, rsi, rdi, r8, r9, r10, r11};
 
-IrRegAllocX64::IrRegAllocX64(AssemblyBuilderX64& build, IrFunction& function)
+IrRegAllocX64::IrRegAllocX64(AssemblyBuilderX64& build, IrFunction& function, LoweringStats* stats)
     : build(build)
     , function(function)
+    , stats(stats)
     , usableXmmRegCount(getXmmRegisterCount(build.abi))
 {
     freeGprMap.fill(true);
@@ -225,10 +227,16 @@ void IrRegAllocX64::preserve(IrInst& inst)
 
         spill.stackSlot = uint8_t(i);
         inst.spilled = true;
+
+        if (stats)
+            stats->spillsToSlot++;
     }
     else
     {
         inst.needsReload = true;
+
+        if (stats)
+            stats->spillsToRestore++;
     }
 
     spills.push_back(spill);

CodeGen/src/IrTranslation.cpp

@@ -615,7 +615,7 @@ static IrOp getLoopStepK(IrBuilder& build, int ra)
 {
     IrBlock& active = build.function.blocks[build.activeBlockIdx];
 
-	if (active.start + 2 < build.function.instructions.size())
+    if (active.start + 2 < build.function.instructions.size())
     {
         IrInst& sv = build.function.instructions[build.function.instructions.size() - 2];
         IrInst& st = build.function.instructions[build.function.instructions.size() - 1];
@@ -665,7 +665,7 @@ void translateInstForNPrep(IrBuilder& build, const Instruction* pc, int pcpos)
             IrOp step = build.inst(IrCmd::LOAD_DOUBLE, build.vmReg(ra + 1));
 
             // step > 0
-			// note: equivalent to 0 < step, but lowers into one instruction on both X64 and A64
+            // note: equivalent to 0 < step, but lowers into one instruction on both X64 and A64
             build.inst(IrCmd::JUMP_CMP_NUM, step, zero, build.cond(IrCondition::Greater), direct, reverse);
 
             // Condition to start the loop: step > 0 ? idx <= limit : limit <= idx
@@ -763,7 +763,7 @@ void translateInstForNLoop(IrBuilder& build, const Instruction* pc, int pcpos)
             IrOp reverse = build.block(IrBlockKind::Internal);
 
             // step > 0
-			// note: equivalent to 0 < step, but lowers into one instruction on both X64 and A64
+            // note: equivalent to 0 < step, but lowers into one instruction on both X64 and A64
             build.inst(IrCmd::JUMP_CMP_NUM, step, zero, build.cond(IrCondition::Greater), direct, reverse);
 
             // Condition to continue the loop: step > 0 ? idx <= limit : limit <= idx
@@ -776,8 +776,8 @@ void translateInstForNLoop(IrBuilder& build, const Instruction* pc, int pcpos)
             build.beginBlock(direct);
             build.inst(IrCmd::JUMP_CMP_NUM, idx, limit, build.cond(IrCondition::LessEqual), loopRepeat, loopExit);
         }
-		else
+        else
-		{
+        {
             double stepN = build.function.doubleOp(stepK);
 
             // Condition to continue the loop: step > 0 ? idx <= limit : limit <= idx
@@ -785,9 +785,9 @@ void translateInstForNLoop(IrBuilder& build, const Instruction* pc, int pcpos)
                 build.inst(IrCmd::JUMP_CMP_NUM, idx, limit, build.cond(IrCondition::LessEqual), loopRepeat, loopExit);
             else
                 build.inst(IrCmd::JUMP_CMP_NUM, limit, idx, build.cond(IrCondition::LessEqual), loopRepeat, loopExit);
-		}
+        }
     }
-	else
+    else
     {
         build.inst(IrCmd::INTERRUPT, build.constUint(pcpos));
 

CodeGen/src/IrUtils.cpp

@@ -860,5 +860,43 @@ void killUnusedBlocks(IrFunction& function)
     }
 }
 
+std::vector<uint32_t> getSortedBlockOrder(IrFunction& function)
+{
+    std::vector<uint32_t> sortedBlocks;
+    sortedBlocks.reserve(function.blocks.size());
+    for (uint32_t i = 0; i < function.blocks.size(); i++)
+        sortedBlocks.push_back(i);
+
+    std::sort(sortedBlocks.begin(), sortedBlocks.end(), [&](uint32_t idxA, uint32_t idxB) {
+        const IrBlock& a = function.blocks[idxA];
+        const IrBlock& b = function.blocks[idxB];
+
+        // Place fallback blocks at the end
+        if ((a.kind == IrBlockKind::Fallback) != (b.kind == IrBlockKind::Fallback))
+            return (a.kind == IrBlockKind::Fallback) < (b.kind == IrBlockKind::Fallback);
+
+        // Try to order by instruction order
+        if (a.sortkey != b.sortkey)
+            return a.sortkey < b.sortkey;
+
+        // Chains of blocks are merged together by having the same sort key and consecutive chain key
+        return a.chainkey < b.chainkey;
+    });
+
+    return sortedBlocks;
+}
+
+IrBlock& getNextBlock(IrFunction& function, std::vector<uint32_t>& sortedBlocks, IrBlock& dummy, size_t i)
+{
+    for (size_t j = i + 1; j < sortedBlocks.size(); ++j)
+    {
+        IrBlock& block = function.blocks[sortedBlocks[j]];
+        if (block.kind != IrBlockKind::Dead)
+            return block;
+    }
+
+    return dummy;
+}
+
 } // namespace CodeGen
 } // namespace Luau

CodeGen/src/NativeState.cpp

@@ -14,21 +14,21 @@
 #include <math.h>
 #include <string.h>
 
+LUAU_FASTINTVARIABLE(LuauCodeGenBlockSize, 4 * 1024 * 1024)
+LUAU_FASTINTVARIABLE(LuauCodeGenMaxTotalSize, 256 * 1024 * 1024)
+
 namespace Luau
 {
 namespace CodeGen
 {
 
-constexpr unsigned kBlockSize = 4 * 1024 * 1024;
-constexpr unsigned kMaxTotalSize = 256 * 1024 * 1024;
-
 NativeState::NativeState()
     : NativeState(nullptr, nullptr)
 {
 }
 
 NativeState::NativeState(AllocationCallback* allocationCallback, void* allocationCallbackContext)
-    : codeAllocator{kBlockSize, kMaxTotalSize, allocationCallback, allocationCallbackContext}
+    : codeAllocator{size_t(FInt::LuauCodeGenBlockSize), size_t(FInt::LuauCodeGenMaxTotalSize), allocationCallback, allocationCallbackContext}
 {
 }
 

Common/include/Luau/ExperimentalFlags.h

@@ -13,6 +13,7 @@ inline bool isFlagExperimental(const char* flag)
     static const char* const kList[] = {
         "LuauInstantiateInSubtyping",  // requires some fixes to lua-apps code
         "LuauTinyControlFlowAnalysis", // waiting for updates to packages depended by internal builtin plugins
+        "LuauFixIndexerSubtypingOrdering", // requires some small fixes to lua-apps code since this fixes a false negative
         // makes sure we always have at least one entry
         nullptr,
     };

VM/src/lmathlib.cpp

@@ -275,15 +275,15 @@ static int math_randomseed(lua_State* L)
     return 0;
 }
 
-static const unsigned char kPerlinHash[257] = {151, 160, 137, 91, 90, 15, 131, 13, 201, 95, 96, 53, 194, 233, 7, 225, 140, 36, 103, 30, 69, 142, 8, 99,
+static const unsigned char kPerlinHash[257] = {151, 160, 137, 91, 90, 15, 131, 13, 201, 95, 96, 53, 194, 233, 7, 225, 140, 36, 103, 30, 69, 142, 8,
-    37, 240, 21, 10, 23, 190, 6, 148, 247, 120, 234, 75, 0, 26, 197, 62, 94, 252, 219, 203, 117, 35, 11, 32, 57, 177, 33, 88, 237, 149, 56, 87, 174,
+    99, 37, 240, 21, 10, 23, 190, 6, 148, 247, 120, 234, 75, 0, 26, 197, 62, 94, 252, 219, 203, 117, 35, 11, 32, 57, 177, 33, 88, 237, 149, 56, 87,
-    20, 125, 136, 171, 168, 68, 175, 74, 165, 71, 134, 139, 48, 27, 166, 77, 146, 158, 231, 83, 111, 229, 122, 60, 211, 133, 230, 220, 105, 92, 41,
+    174, 20, 125, 136, 171, 168, 68, 175, 74, 165, 71, 134, 139, 48, 27, 166, 77, 146, 158, 231, 83, 111, 229, 122, 60, 211, 133, 230, 220, 105, 92,
-    55, 46, 245, 40, 244, 102, 143, 54, 65, 25, 63, 161, 1, 216, 80, 73, 209, 76, 132, 187, 208, 89, 18, 169, 200, 196, 135, 130, 116, 188, 159, 86,
+    41, 55, 46, 245, 40, 244, 102, 143, 54, 65, 25, 63, 161, 1, 216, 80, 73, 209, 76, 132, 187, 208, 89, 18, 169, 200, 196, 135, 130, 116, 188, 159,
-    164, 100, 109, 198, 173, 186, 3, 64, 52, 217, 226, 250, 124, 123, 5, 202, 38, 147, 118, 126, 255, 82, 85, 212, 207, 206, 59, 227, 47, 16, 58, 17,
+    86, 164, 100, 109, 198, 173, 186, 3, 64, 52, 217, 226, 250, 124, 123, 5, 202, 38, 147, 118, 126, 255, 82, 85, 212, 207, 206, 59, 227, 47, 16, 58,
-    182, 189, 28, 42, 223, 183, 170, 213, 119, 248, 152, 2, 44, 154, 163, 70, 221, 153, 101, 155, 167, 43, 172, 9, 129, 22, 39, 253, 19, 98, 108, 110,
+    17, 182, 189, 28, 42, 223, 183, 170, 213, 119, 248, 152, 2, 44, 154, 163, 70, 221, 153, 101, 155, 167, 43, 172, 9, 129, 22, 39, 253, 19, 98, 108,
-    79, 113, 224, 232, 178, 185, 112, 104, 218, 246, 97, 228, 251, 34, 242, 193, 238, 210, 144, 12, 191, 179, 162, 241, 81, 51, 145, 235, 249, 14,
+    110, 79, 113, 224, 232, 178, 185, 112, 104, 218, 246, 97, 228, 251, 34, 242, 193, 238, 210, 144, 12, 191, 179, 162, 241, 81, 51, 145, 235, 249,
-    239, 107, 49, 192, 214, 31, 181, 199, 106, 157, 184, 84, 204, 176, 115, 121, 50, 45, 127, 4, 150, 254, 138, 236, 205, 93, 222, 114, 67, 29, 24,
+    14, 239, 107, 49, 192, 214, 31, 181, 199, 106, 157, 184, 84, 204, 176, 115, 121, 50, 45, 127, 4, 150, 254, 138, 236, 205, 93, 222, 114, 67, 29,
-    72, 243, 141, 128, 195, 78, 66, 215, 61, 156, 180, 151};
+    24, 72, 243, 141, 128, 195, 78, 66, 215, 61, 156, 180, 151};
 
 const float kPerlinGrad[16][3] = {{1, 1, 0}, {-1, 1, 0}, {1, -1, 0}, {-1, -1, 0}, {1, 0, 1}, {-1, 0, 1}, {1, 0, -1}, {-1, 0, -1}, {0, 1, 1},
     {0, -1, 1}, {0, 1, -1}, {0, -1, -1}, {1, 1, 0}, {0, -1, 1}, {-1, 1, 0}, {0, -1, -1}};

VM/src/lvmexecute.cpp

@@ -2291,7 +2291,7 @@ reentry:
                     {
                         // table or userdata with __call, will be called during FORGLOOP
                         // TODO: we might be able to stop supporting this depending on whether it's used in practice
-                        void (*telemetrycb)(lua_State* L, int gtt, int stt, int itt) = lua_iter_call_telemetry;
+                        void (*telemetrycb)(lua_State * L, int gtt, int stt, int itt) = lua_iter_call_telemetry;
 
                         if (telemetrycb)
                             telemetrycb(L, ttype(ra), ttype(ra + 1), ttype(ra + 2));

tests/Autocomplete.test.cpp

@@ -3667,8 +3667,7 @@ TEST_CASE_FIXTURE(ACFixture, "string_completion_outside_quotes")
     )");
 
     StringCompletionCallback callback = [](std::string, std::optional<const ClassType*>,
-                                            std::optional<std::string> contents) -> std::optional<AutocompleteEntryMap>
+                                            std::optional<std::string> contents) -> std::optional<AutocompleteEntryMap> {
-    {
         Luau::AutocompleteEntryMap results = {{"test", Luau::AutocompleteEntry{Luau::AutocompleteEntryKind::String, std::nullopt, false, false}}};
         return results;
     };

tests/CodeAllocator.test.cpp

@@ -57,8 +57,7 @@ TEST_CASE("CodeAllocationCallbacks")
 
     AllocationData allocationData{};
 
-    const auto allocationCallback = [](void* context, void* oldPointer, size_t oldSize, void* newPointer, size_t newSize)
+    const auto allocationCallback = [](void* context, void* oldPointer, size_t oldSize, void* newPointer, size_t newSize) {
-    {
         AllocationData& allocationData = *static_cast<AllocationData*>(context);
         if (oldPointer != nullptr)
         {

tests/Compiler.test.cpp

@@ -74,7 +74,7 @@ TEST_CASE("BytecodeIsStable")
 
     // Bytecode ops (serialized & in-memory)
     CHECK(LOP_FASTCALL2K == 75); // bytecode v1
-    CHECK(LOP_JUMPXEQKS == 80); // bytecode v3
+    CHECK(LOP_JUMPXEQKS == 80);  // bytecode v3
 
     // Bytecode fastcall ids (serialized & in-memory)
     // Note: these aren't strictly bound to specific bytecode versions, but must monotonically increase to keep backwards compat
@@ -7371,7 +7371,8 @@ TEST_CASE("BuiltinFoldMathK")
 function test()
     return math.pi * 2
 end
-)", 0, 2),
+)",
+                        0, 2),
         R"(
 LOADK R0 K0 [6.2831853071795862]
 RETURN R0 1
@@ -7382,7 +7383,8 @@ RETURN R0 1
 function test()
     return math.pi * 2
 end
-)", 0, 1),
+)",
+                        0, 1),
         R"(
 GETIMPORT R1 3 [math.pi]
 MULK R0 R1 K0 [2]
@@ -7396,7 +7398,8 @@ function test()
 end
 
 math = { pi = 4 }
-)", 0, 2),
+)",
+                        0, 2),
         R"(
 GETGLOBAL R2 K1 ['math']
 GETTABLEKS R1 R2 K2 ['pi']

tests/IrCallWrapperX64.test.cpp

@@ -12,7 +12,7 @@ class IrCallWrapperX64Fixture
 public:
     IrCallWrapperX64Fixture(ABIX64 abi = ABIX64::Windows)
         : build(/* logText */ true, abi)
-        , regs(build, function)
+        , regs(build, function, nullptr)
         , callWrap(regs, build, ~0u)
     {
     }

tests/IrRegAllocX64.test.cpp

@@ -11,7 +11,7 @@ class IrRegAllocX64Fixture
 public:
     IrRegAllocX64Fixture()
         : build(/* logText */ true, ABIX64::Windows)
-        , regs(build, function)
+        , regs(build, function, nullptr)
     {
     }
 

tests/Normalize.test.cpp

@@ -516,6 +516,71 @@ struct NormalizeFixture : Fixture
 
 TEST_SUITE_BEGIN("Normalize");
 
+TEST_CASE_FIXTURE(NormalizeFixture, "string_intersection_is_commutative")
+{
+    auto c4 = toString(normal(R"(
+        string & (string & Not<"a"> & Not<"b">)
+)"));
+    auto c4Reverse = toString(normal(R"(
+        (string & Not<"a"> & Not<"b">) & string
+)"));
+    CHECK(c4 == c4Reverse);
+    CHECK_EQ("string & ~\"a\" & ~\"b\"", c4);
+
+    auto c5 = toString(normal(R"(
+        (string & Not<"a"> & Not<"b">) & (string & Not<"b"> & Not<"c">)
+)"));
+    auto c5Reverse = toString(normal(R"(
+        (string & Not<"b"> & Not<"c">) & (string & Not<"a"> & Not<"c">)
+)"));
+    CHECK(c5 == c5Reverse);
+    CHECK_EQ("string & ~\"a\" & ~\"b\" & ~\"c\"", c5);
+
+    auto c6 = toString(normal(R"(
+        ("a" | "b") & (string & Not<"b"> & Not<"c">)
+)"));
+    auto c6Reverse = toString(normal(R"(
+        (string & Not<"b"> & Not<"c">) & ("a" | "b")
+)"));
+    CHECK(c6 == c6Reverse);
+    CHECK_EQ("\"a\"", c6);
+
+    auto c7 = toString(normal(R"(
+        string & ("b" | "c")
+)"));
+    auto c7Reverse = toString(normal(R"(
+        ("b" | "c") & string
+)"));
+    CHECK(c7 == c7Reverse);
+    CHECK_EQ("\"b\" | \"c\"", c7);
+
+    auto c8 = toString(normal(R"(
+(string & Not<"a"> & Not<"b">) & ("b" | "c")
+)"));
+    auto c8Reverse = toString(normal(R"(
+        ("b" | "c") & (string & Not<"a"> & Not<"b">)
+)"));
+    CHECK(c8 == c8Reverse);
+    CHECK_EQ("\"c\"", c8);
+    auto c9 = toString(normal(R"(
+            ("a" | "b") & ("b" | "c")
+    )"));
+    auto c9Reverse = toString(normal(R"(
+            ("b" | "c") & ("a" | "b")
+    )"));
+    CHECK(c9 == c9Reverse);
+    CHECK_EQ("\"b\"", c9);
+
+    auto l = toString(normal(R"(
+         (string | number) & ("a" | true)
+    )"));
+    auto r = toString(normal(R"(
+         ("a" | true) & (string | number)
+    )"));
+    CHECK(l == r);
+    CHECK_EQ("\"a\"", l);
+}
+
 TEST_CASE_FIXTURE(NormalizeFixture, "negate_string")
 {
     CHECK("number" == toString(normal(R"(

tests/Subtyping.test.cpp

@@ -6,6 +6,7 @@
 
 #include "Luau/Normalize.h"
 #include "Luau/Subtyping.h"
+#include "Luau/Type.h"
 #include "Luau/TypePack.h"
 
 using namespace Luau;
@@ -17,7 +18,15 @@ struct SubtypeFixture : Fixture
     UnifierSharedState sharedState{&ice};
     Normalizer normalizer{&arena, builtinTypes, NotNull{&sharedState}};
 
-    Subtyping subtyping{builtinTypes, NotNull{&arena}, NotNull{&normalizer}, NotNull{&iceReporter}};
+    ScopePtr rootScope{new Scope(builtinTypes->emptyTypePack)};
+    ScopePtr moduleScope{new Scope(rootScope)};
+
+    Subtyping subtyping = mkSubtyping(rootScope);
+
+    Subtyping mkSubtyping(const ScopePtr& scope)
+    {
+        return Subtyping{builtinTypes, NotNull{&arena}, NotNull{&normalizer}, NotNull{&iceReporter}, NotNull{scope.get()}};
+    }
 
     TypePackId pack(std::initializer_list<TypeId> tys)
     {
@@ -66,11 +75,18 @@ struct SubtypeFixture : Fixture
         return arena.addType(UnionType{{a, b}});
     }
 
+    // `~`
     TypeId negate(TypeId ty)
     {
         return arena.addType(NegationType{ty});
     }
 
+    // "literal"
+    TypeId str(const char* literal)
+    {
+        return arena.addType(SingletonType{StringSingleton{literal}});
+    }
+
     TypeId cls(const std::string& name, std::optional<TypeId> parent = std::nullopt)
     {
         return arena.addType(ClassType{name, {}, parent.value_or(builtinTypes->classType), {}, {}, nullptr, ""});
@@ -97,8 +113,8 @@ struct SubtypeFixture : Fixture
         return arena.addType(MetatableType{tbl(std::move(tableProps)), tbl(std::move(metaProps))});
     }
 
-    TypeId genericT = arena.addType(GenericType{"T"});
+    TypeId genericT = arena.addType(GenericType{moduleScope.get(), "T"});
-    TypeId genericU = arena.addType(GenericType{"U"});
+    TypeId genericU = arena.addType(GenericType{moduleScope.get(), "U"});
 
     TypePackId genericAs = arena.addTypePack(GenericTypePack{"A"});
     TypePackId genericBs = arena.addTypePack(GenericTypePack{"B"});
@@ -113,6 +129,10 @@ struct SubtypeFixture : Fixture
     TypeId helloType2 = arena.addType(SingletonType{StringSingleton{"hello"}});
     TypeId worldType = arena.addType(SingletonType{StringSingleton{"world"}});
 
+    TypeId aType = arena.addType(SingletonType{StringSingleton{"a"}});
+    TypeId bType = arena.addType(SingletonType{StringSingleton{"b"}});
+    TypeId trueSingleton = arena.addType(SingletonType{BooleanSingleton{true}});
+    TypeId falseSingleton = arena.addType(SingletonType{BooleanSingleton{false}});
     TypeId helloOrWorldType = join(helloType, worldType);
     TypeId trueOrFalseType = join(builtinTypes->trueType, builtinTypes->falseType);
 
@@ -128,7 +148,7 @@ struct SubtypeFixture : Fixture
      *    \- AnotherChild
      *       |- AnotherGrandchildOne
      *       \- AnotherGrandchildTwo
-    */
+     */
     TypeId rootClass = cls("Root");
     TypeId childClass = cls("Child", rootClass);
     TypeId grandchildOneClass = cls("GrandchildOne", childClass);
@@ -138,9 +158,9 @@ struct SubtypeFixture : Fixture
     TypeId anotherGrandchildTwoClass = cls("AnotherGrandchildTwo", anotherChildClass);
 
     TypeId vec2Class = cls("Vec2", {
-        {"X", builtinTypes->numberType},
+                                       {"X", builtinTypes->numberType},
-        {"Y", builtinTypes->numberType},
+                                       {"Y", builtinTypes->numberType},
-    });
+                                   });
 
     // "hello" | "hello"
     TypeId helloOrHelloType = arena.addType(UnionType{{helloType, helloType}});
@@ -149,160 +169,76 @@ struct SubtypeFixture : Fixture
     const TypeId nothingToNothingType = fn({}, {});
 
     // (number) -> string
-    const TypeId numberToStringType = fn(
+    const TypeId numberToStringType = fn({builtinTypes->numberType}, {builtinTypes->stringType});
-        {builtinTypes->numberType},
-        {builtinTypes->stringType}
-    );
 
     // (unknown) -> string
-    const TypeId unknownToStringType = fn(
+    const TypeId unknownToStringType = fn({builtinTypes->unknownType}, {builtinTypes->stringType});
-        {builtinTypes->unknownType},
-        {builtinTypes->stringType}
-    );
 
     // (number) -> ()
-    const TypeId numberToNothingType = fn(
+    const TypeId numberToNothingType = fn({builtinTypes->numberType}, {});
-        {builtinTypes->numberType},
-        {}
-    );
 
     // () -> number
-    const TypeId nothingToNumberType = fn(
+    const TypeId nothingToNumberType = fn({}, {builtinTypes->numberType});
-        {},
-        {builtinTypes->numberType}
-    );
 
     // (number) -> number
-    const TypeId numberToNumberType = fn(
+    const TypeId numberToNumberType = fn({builtinTypes->numberType}, {builtinTypes->numberType});
-        {builtinTypes->numberType},
-        {builtinTypes->numberType}
-    );
 
     // (number) -> unknown
-    const TypeId numberToUnknownType = fn(
+    const TypeId numberToUnknownType = fn({builtinTypes->numberType}, {builtinTypes->unknownType});
-        {builtinTypes->numberType},
-        {builtinTypes->unknownType}
-    );
 
     // (number) -> (string, string)
-    const TypeId numberToTwoStringsType = fn(
+    const TypeId numberToTwoStringsType = fn({builtinTypes->numberType}, {builtinTypes->stringType, builtinTypes->stringType});
-        {builtinTypes->numberType},
-        {builtinTypes->stringType, builtinTypes->stringType}
-    );
 
     // (number) -> (string, unknown)
-    const TypeId numberToStringAndUnknownType = fn(
+    const TypeId numberToStringAndUnknownType = fn({builtinTypes->numberType}, {builtinTypes->stringType, builtinTypes->unknownType});
-        {builtinTypes->numberType},
-        {builtinTypes->stringType, builtinTypes->unknownType}
-    );
 
     // (number, number) -> string
-    const TypeId numberNumberToStringType = fn(
+    const TypeId numberNumberToStringType = fn({builtinTypes->numberType, builtinTypes->numberType}, {builtinTypes->stringType});
-        {builtinTypes->numberType, builtinTypes->numberType},
-        {builtinTypes->stringType}
-    );
 
     // (unknown, number) -> string
-    const TypeId unknownNumberToStringType = fn(
+    const TypeId unknownNumberToStringType = fn({builtinTypes->unknownType, builtinTypes->numberType}, {builtinTypes->stringType});
-        {builtinTypes->unknownType, builtinTypes->numberType},
-        {builtinTypes->stringType}
-    );
 
     // (number, string) -> string
-    const TypeId numberAndStringToStringType = fn(
+    const TypeId numberAndStringToStringType = fn({builtinTypes->numberType, builtinTypes->stringType}, {builtinTypes->stringType});
-        {builtinTypes->numberType, builtinTypes->stringType},
-        {builtinTypes->stringType}
-    );
 
     // (number, ...string) -> string
-    const TypeId numberAndStringsToStringType = fn(
+    const TypeId numberAndStringsToStringType =
-        {builtinTypes->numberType}, VariadicTypePack{builtinTypes->stringType},
+        fn({builtinTypes->numberType}, VariadicTypePack{builtinTypes->stringType}, {builtinTypes->stringType});
-        {builtinTypes->stringType}
-    );
 
     // (number, ...string?) -> string
-    const TypeId numberAndOptionalStringsToStringType = fn(
+    const TypeId numberAndOptionalStringsToStringType =
-        {builtinTypes->numberType}, VariadicTypePack{builtinTypes->optionalStringType},
+        fn({builtinTypes->numberType}, VariadicTypePack{builtinTypes->optionalStringType}, {builtinTypes->stringType});
-        {builtinTypes->stringType}
-    );
 
     // (...number) -> number
-    const TypeId numbersToNumberType = arena.addType(FunctionType{
+    const TypeId numbersToNumberType =
-        arena.addTypePack(VariadicTypePack{builtinTypes->numberType}),
+        arena.addType(FunctionType{arena.addTypePack(VariadicTypePack{builtinTypes->numberType}), arena.addTypePack({builtinTypes->numberType})});
-        arena.addTypePack({builtinTypes->numberType})
-    });
 
     // <T>(T) -> ()
-    const TypeId genericTToNothingType = arena.addType(FunctionType{
+    const TypeId genericTToNothingType = arena.addType(FunctionType{{genericT}, {}, arena.addTypePack({genericT}), builtinTypes->emptyTypePack});
-        {genericT},
-        {},
-        arena.addTypePack({genericT}),
-        builtinTypes->emptyTypePack
-    });
 
     // <T>(T) -> T
-    const TypeId genericTToTType = arena.addType(FunctionType{
+    const TypeId genericTToTType = arena.addType(FunctionType{{genericT}, {}, arena.addTypePack({genericT}), arena.addTypePack({genericT})});
-        {genericT},
-        {},
-        arena.addTypePack({genericT}),
-        arena.addTypePack({genericT})
-    });
 
     // <U>(U) -> ()
-    const TypeId genericUToNothingType = arena.addType(FunctionType{
+    const TypeId genericUToNothingType = arena.addType(FunctionType{{genericU}, {}, arena.addTypePack({genericU}), builtinTypes->emptyTypePack});
-        {genericU},
-        {},
-        arena.addTypePack({genericU}),
-        builtinTypes->emptyTypePack
-    });
 
     // <T>() -> T
-    const TypeId genericNothingToTType = arena.addType(FunctionType{
+    const TypeId genericNothingToTType = arena.addType(FunctionType{{genericT}, {}, builtinTypes->emptyTypePack, arena.addTypePack({genericT})});
-        {genericT},
-        {},
-        builtinTypes->emptyTypePack,
-        arena.addTypePack({genericT})
-    });
 
     // <A...>(A...) -> A...
-    const TypeId genericAsToAsType = arena.addType(FunctionType{
+    const TypeId genericAsToAsType = arena.addType(FunctionType{{}, {genericAs}, genericAs, genericAs});
-        {},
-        {genericAs},
-        genericAs,
-        genericAs
-    });
 
     // <A...>(A...) -> number
-    const TypeId genericAsToNumberType = arena.addType(FunctionType{
+    const TypeId genericAsToNumberType = arena.addType(FunctionType{{}, {genericAs}, genericAs, arena.addTypePack({builtinTypes->numberType})});
-        {},
-        {genericAs},
-        genericAs,
-        arena.addTypePack({builtinTypes->numberType})
-    });
 
     // <B...>(B...) -> B...
-    const TypeId genericBsToBsType = arena.addType(FunctionType{
+    const TypeId genericBsToBsType = arena.addType(FunctionType{{}, {genericBs}, genericBs, genericBs});
-        {},
-        {genericBs},
-        genericBs,
-        genericBs
-    });
 
     // <B..., C...>(B...) -> C...
-    const TypeId genericBsToCsType = arena.addType(FunctionType{
+    const TypeId genericBsToCsType = arena.addType(FunctionType{{}, {genericBs, genericCs}, genericBs, genericCs});
-        {},
-        {genericBs, genericCs},
-        genericBs,
-        genericCs
-    });
 
     // <A...>() -> A...
-    const TypeId genericNothingToAsType = arena.addType(FunctionType{
+    const TypeId genericNothingToAsType = arena.addType(FunctionType{{}, {genericAs}, builtinTypes->emptyTypePack, genericAs});
-        {},
-        {genericAs},
-        builtinTypes->emptyTypePack,
-        genericAs
-    });
 
     // { lower : string -> string }
     TypeId tableWithLower = tbl(TableType::Props{{"lower", fn({builtinTypes->stringType}, {builtinTypes->stringType})}});
@@ -728,60 +664,98 @@ TEST_CASE_FIXTURE(SubtypeFixture, "{x: number?} <!: {x: number}")
 
 TEST_CASE_FIXTURE(SubtypeFixture, "{x: <T>(T) -> ()} <: {x: <U>(U) -> ()}")
 {
-    CHECK_IS_SUBTYPE(
+    CHECK_IS_SUBTYPE(tbl({{"x", genericTToNothingType}}), tbl({{"x", genericUToNothingType}}));
-        tbl({{"x", genericTToNothingType}}),
-        tbl({{"x", genericUToNothingType}})
-    );
 }
 
 TEST_CASE_FIXTURE(SubtypeFixture, "{ @metatable { x: number } } <: { @metatable {} }")
 {
-    CHECK_IS_SUBTYPE(
+    CHECK_IS_SUBTYPE(meta({{"x", builtinTypes->numberType}}), meta({}));
-        meta({{"x", builtinTypes->numberType}}),
-        meta({})
-    );
 }
 
 TEST_CASE_FIXTURE(SubtypeFixture, "{ @metatable { x: number } } <!: { @metatable { x: boolean } }")
 {
-    CHECK_IS_NOT_SUBTYPE(
+    CHECK_IS_NOT_SUBTYPE(meta({{"x", builtinTypes->numberType}}), meta({{"x", builtinTypes->booleanType}}));
-        meta({{"x", builtinTypes->numberType}}),
-        meta({{"x", builtinTypes->booleanType}})
-    );
 }
 
 TEST_CASE_FIXTURE(SubtypeFixture, "{ @metatable {} } <!: { @metatable { x: boolean } }")
 {
-    CHECK_IS_NOT_SUBTYPE(
+    CHECK_IS_NOT_SUBTYPE(meta({}), meta({{"x", builtinTypes->booleanType}}));
-        meta({}),
-        meta({{"x", builtinTypes->booleanType}})
-    );
 }
 
 TEST_CASE_FIXTURE(SubtypeFixture, "{ @metatable {} } <: {}")
 {
-    CHECK_IS_SUBTYPE(
+    CHECK_IS_SUBTYPE(meta({}), tbl({}));
-        meta({}),
-        tbl({})
-    );
 }
 
 TEST_CASE_FIXTURE(SubtypeFixture, "{ @metatable { u: boolean }, x: number } <: { x: number }")
 {
-    CHECK_IS_SUBTYPE(
+    CHECK_IS_SUBTYPE(meta({{"u", builtinTypes->booleanType}}, {{"x", builtinTypes->numberType}}), tbl({{"x", builtinTypes->numberType}}));
-        meta({{"u", builtinTypes->booleanType}}, {{"x", builtinTypes->numberType}}),
-        tbl({{"x", builtinTypes->numberType}})
-    );
 }
 
 TEST_CASE_FIXTURE(SubtypeFixture, "{ @metatable { x: number } } <!: { x: number }")
 {
-    CHECK_IS_NOT_SUBTYPE(
+    CHECK_IS_NOT_SUBTYPE(meta({{"x", builtinTypes->numberType}}), tbl({{"x", builtinTypes->numberType}}));
-        meta({{"x", builtinTypes->numberType}}),
-        tbl({{"x", builtinTypes->numberType}})
-    );
 }
 
+// Negated subtypes
+TEST_IS_NOT_SUBTYPE(negate(builtinTypes->neverType), builtinTypes->stringType);
+TEST_IS_SUBTYPE(negate(builtinTypes->unknownType), builtinTypes->stringType);
+TEST_IS_NOT_SUBTYPE(negate(builtinTypes->anyType), builtinTypes->stringType);
+TEST_IS_SUBTYPE(negate(meet(builtinTypes->neverType, builtinTypes->unknownType)), builtinTypes->stringType);
+TEST_IS_NOT_SUBTYPE(negate(join(builtinTypes->neverType, builtinTypes->unknownType)), builtinTypes->stringType);
+
+// Negated supertypes: never/unknown/any/error
+TEST_IS_SUBTYPE(builtinTypes->stringType, negate(builtinTypes->neverType));
+TEST_IS_SUBTYPE(builtinTypes->neverType, negate(builtinTypes->unknownType));
+TEST_IS_NOT_SUBTYPE(builtinTypes->stringType, negate(builtinTypes->unknownType));
+TEST_IS_SUBTYPE(builtinTypes->numberType, negate(builtinTypes->anyType));
+TEST_IS_SUBTYPE(builtinTypes->unknownType, negate(builtinTypes->anyType));
+
+// Negated supertypes: unions
+TEST_IS_SUBTYPE(builtinTypes->booleanType, negate(join(builtinTypes->stringType, builtinTypes->numberType)));
+TEST_IS_SUBTYPE(rootClass, negate(join(childClass, builtinTypes->numberType)));
+TEST_IS_SUBTYPE(str("foo"), negate(join(builtinTypes->numberType, builtinTypes->booleanType)));
+TEST_IS_NOT_SUBTYPE(str("foo"), negate(join(builtinTypes->stringType, builtinTypes->numberType)));
+TEST_IS_NOT_SUBTYPE(childClass, negate(join(rootClass, builtinTypes->numberType)));
+TEST_IS_NOT_SUBTYPE(numbersToNumberType, negate(join(builtinTypes->functionType, rootClass)));
+
+// Negated supertypes: intersections
+TEST_IS_SUBTYPE(builtinTypes->booleanType, negate(meet(builtinTypes->stringType, str("foo"))));
+TEST_IS_SUBTYPE(builtinTypes->trueType, negate(meet(builtinTypes->booleanType, builtinTypes->numberType)));
+TEST_IS_SUBTYPE(rootClass, negate(meet(builtinTypes->classType, childClass)));
+TEST_IS_SUBTYPE(childClass, negate(meet(builtinTypes->classType, builtinTypes->numberType)));
+TEST_IS_NOT_SUBTYPE(builtinTypes->unknownType, negate(meet(builtinTypes->classType, builtinTypes->numberType)));
+TEST_IS_NOT_SUBTYPE(str("foo"), negate(meet(builtinTypes->stringType, negate(str("bar")))));
+
+// Negated supertypes: tables and metatables
+TEST_IS_SUBTYPE(tbl({}), negate(builtinTypes->numberType));
+TEST_IS_NOT_SUBTYPE(tbl({}), negate(builtinTypes->tableType));
+TEST_IS_SUBTYPE(meta({}), negate(builtinTypes->numberType));
+TEST_IS_NOT_SUBTYPE(meta({}), negate(builtinTypes->tableType));
+
+// Negated supertypes: Functions
+TEST_IS_SUBTYPE(numberToNumberType, negate(builtinTypes->classType));
+TEST_IS_NOT_SUBTYPE(numberToNumberType, negate(builtinTypes->functionType));
+
+// Negated supertypes: Primitives and singletons
+TEST_IS_SUBTYPE(builtinTypes->stringType, negate(builtinTypes->numberType));
+TEST_IS_SUBTYPE(str("foo"), meet(builtinTypes->stringType, negate(str("bar"))));
+TEST_IS_NOT_SUBTYPE(builtinTypes->trueType, negate(builtinTypes->booleanType));
+TEST_IS_NOT_SUBTYPE(str("foo"), negate(str("foo")));
+TEST_IS_NOT_SUBTYPE(str("foo"), negate(builtinTypes->stringType));
+TEST_IS_SUBTYPE(builtinTypes->falseType, negate(builtinTypes->trueType));
+TEST_IS_SUBTYPE(builtinTypes->falseType, meet(builtinTypes->booleanType, negate(builtinTypes->trueType)));
+TEST_IS_NOT_SUBTYPE(builtinTypes->stringType, meet(builtinTypes->booleanType, negate(builtinTypes->trueType)));
+TEST_IS_NOT_SUBTYPE(builtinTypes->stringType, negate(str("foo")));
+TEST_IS_NOT_SUBTYPE(builtinTypes->booleanType, negate(builtinTypes->falseType));
+
+// Negated supertypes: Classes
+TEST_IS_SUBTYPE(rootClass, negate(builtinTypes->tableType));
+TEST_IS_NOT_SUBTYPE(rootClass, negate(builtinTypes->classType));
+TEST_IS_NOT_SUBTYPE(childClass, negate(rootClass));
+TEST_IS_NOT_SUBTYPE(childClass, meet(builtinTypes->classType, negate(rootClass)));
+TEST_IS_SUBTYPE(anotherChildClass, meet(builtinTypes->classType, negate(childClass)));
+
 TEST_CASE_FIXTURE(SubtypeFixture, "Root <: class")
 {
     CHECK_IS_SUBTYPE(rootClass, builtinTypes->classType);
@@ -829,13 +803,11 @@ TEST_CASE_FIXTURE(SubtypeFixture, "Child & ~GrandchildOne <!: number")
 
 TEST_CASE_FIXTURE(SubtypeFixture, "t1 where t1 = {trim: (t1) -> string} <: t2 where t2 = {trim: (t2) -> string}")
 {
-    TypeId t1 = cyclicTable([&](TypeId ty, TableType* tt)
+    TypeId t1 = cyclicTable([&](TypeId ty, TableType* tt) {
-    {
         tt->props["trim"] = fn({ty}, {builtinTypes->stringType});
     });
 
-    TypeId t2 = cyclicTable([&](TypeId ty, TableType* tt)
+    TypeId t2 = cyclicTable([&](TypeId ty, TableType* tt) {
-    {
         tt->props["trim"] = fn({ty}, {builtinTypes->stringType});
     });
 
@@ -844,13 +816,11 @@ TEST_CASE_FIXTURE(SubtypeFixture, "t1 where t1 = {trim: (t1) -> string} <: t2 wh
 
 TEST_CASE_FIXTURE(SubtypeFixture, "t1 where t1 = {trim: (t1) -> string} <!: t2 where t2 = {trim: (t2) -> t2}")
 {
-    TypeId t1 = cyclicTable([&](TypeId ty, TableType* tt)
+    TypeId t1 = cyclicTable([&](TypeId ty, TableType* tt) {
-    {
         tt->props["trim"] = fn({ty}, {builtinTypes->stringType});
     });
 
-    TypeId t2 = cyclicTable([&](TypeId ty, TableType* tt)
+    TypeId t2 = cyclicTable([&](TypeId ty, TableType* tt) {
-    {
         tt->props["trim"] = fn({ty}, {ty});
     });
 
@@ -859,13 +829,11 @@ TEST_CASE_FIXTURE(SubtypeFixture, "t1 where t1 = {trim: (t1) -> string} <!: t2 w
 
 TEST_CASE_FIXTURE(SubtypeFixture, "t1 where t1 = {trim: (t1) -> t1} <!: t2 where t2 = {trim: (t2) -> string}")
 {
-    TypeId t1 = cyclicTable([&](TypeId ty, TableType* tt)
+    TypeId t1 = cyclicTable([&](TypeId ty, TableType* tt) {
-    {
         tt->props["trim"] = fn({ty}, {ty});
     });
 
-    TypeId t2 = cyclicTable([&](TypeId ty, TableType* tt)
+    TypeId t2 = cyclicTable([&](TypeId ty, TableType* tt) {
-    {
         tt->props["trim"] = fn({ty}, {builtinTypes->stringType});
     });
 
@@ -960,6 +928,50 @@ TEST_CASE_FIXTURE(SubtypeFixture, "(string) -> number <: ~fun & (string) -> numb
     CHECK_IS_NOT_SUBTYPE(numberToStringType, meet(negate(builtinTypes->functionType), numberToStringType));
 }
 
+TEST_CASE_FIXTURE(SubtypeFixture, "~\"a\" & ~\"b\" & string <: { lower : (string) -> ()}")
+{
+    CHECK_IS_SUBTYPE(meet(meet(negate(aType), negate(bType)), builtinTypes->stringType), tableWithLower);
+}
+
+TEST_CASE_FIXTURE(SubtypeFixture, "\"a\" | (~\"b\" & string) <: { lower : (string) -> ()}")
+{
+    CHECK_IS_SUBTYPE(join(aType, meet(negate(bType), builtinTypes->stringType)), tableWithLower);
+}
+
+TEST_CASE_FIXTURE(SubtypeFixture, "(string | number) & (\"a\" | true) <: { lower: (string) -> string }")
+{
+    auto base = meet(join(builtinTypes->stringType, builtinTypes->numberType), join(aType, trueSingleton));
+    CHECK_IS_SUBTYPE(base, tableWithLower);
+}
+
+/*
+ * Within the scope to which a generic belongs, that generic ought to be treated
+ * as its bounds.
+ *
+ * We do not yet support bounded generics, so all generics are considered to be
+ * bounded by unknown.
+ */
+TEST_CASE_FIXTURE(SubtypeFixture, "unknown <: X")
+{
+    ScopePtr childScope{new Scope(rootScope)};
+    ScopePtr grandChildScope{new Scope(childScope)};
+
+    TypeId genericX = arena.addType(GenericType(childScope.get(), "X"));
+
+    SubtypingResult usingGlobalScope = subtyping.isSubtype(builtinTypes->unknownType, genericX);
+    CHECK_MESSAGE(!usingGlobalScope.isSubtype, "Expected " << builtinTypes->unknownType << " </: " << genericX);
+
+    Subtyping childSubtyping{mkSubtyping(childScope)};
+
+    SubtypingResult usingChildScope = childSubtyping.isSubtype(builtinTypes->unknownType, genericX);
+    CHECK_MESSAGE(usingChildScope.isSubtype, "Expected " << builtinTypes->unknownType << " <: " << genericX);
+
+    Subtyping grandChildSubtyping{mkSubtyping(grandChildScope)};
+
+    SubtypingResult usingGrandChildScope = grandChildSubtyping.isSubtype(builtinTypes->unknownType, genericX);
+    CHECK_MESSAGE(usingGrandChildScope.isSubtype, "Expected " << builtinTypes->unknownType << " <: " << genericX);
+}
+
 /*
  * <A>(A) -> A <: <X>(X) -> X
  *      A can be bound to X.

tests/TypeInfer.operators.test.cpp

@@ -1067,8 +1067,9 @@ local w = c and 1
         CHECK("false | number" == toString(requireType("z")));
     else
         CHECK("boolean | number" == toString(requireType("z"))); // 'false' widened to boolean
+
     if (FFlag::DebugLuauDeferredConstraintResolution)
-        CHECK("((false?) & a) | number" == toString(requireType("w")));
+        CHECK("((false?) & unknown) | number" == toString(requireType("w")));
     else
         CHECK("(boolean | number)?" == toString(requireType("w")));
 }

tests/TypeInfer.refinements.test.cpp

@@ -308,7 +308,6 @@ TEST_CASE_FIXTURE(BuiltinsFixture, "typeguard_in_if_condition_position")
     else
         CHECK_EQ("number", toString(requireTypeAtPosition({3, 26})));
     CHECK_EQ("number", toString(requireTypeAtPosition({6, 26})));
-
 }
 
 TEST_CASE_FIXTURE(BuiltinsFixture, "typeguard_in_assert_position")

tests/TypeInfer.tables.test.cpp

@@ -3788,4 +3788,40 @@ TEST_CASE_FIXTURE(Fixture, "cyclic_shifted_tables")
     LUAU_REQUIRE_NO_ERRORS(result);
 }
 
+
+TEST_CASE_FIXTURE(Fixture, "cli_84607_missing_prop_in_array_or_dict")
+{
+    ScopedFastFlag sff{"LuauFixIndexerSubtypingOrdering", true};
+
+    CheckResult result = check(R"(
+        type Thing = { name: string, prop: boolean }
+
+        local arrayOfThings : {Thing} = {
+            { name = "a" }
+        }
+
+        local dictOfThings : {[string]: Thing} = {
+            a = { name = "a" }
+        }
+    )");
+
+    LUAU_REQUIRE_ERROR_COUNT(2, result);
+
+    TypeError& err1 = result.errors[0];
+    MissingProperties* error1 = get<MissingProperties>(err1);
+    REQUIRE(error1);
+    REQUIRE(error1->properties.size() == 1);
+
+    CHECK_EQ("prop", error1->properties[0]);
+
+    TypeError& err2 = result.errors[1];
+    TypeMismatch* mismatch = get<TypeMismatch>(err2);
+    REQUIRE(mismatch);
+    MissingProperties* error2 = get<MissingProperties>(*mismatch->error);
+    REQUIRE(error2);
+    REQUIRE(error2->properties.size() == 1);
+
+    CHECK_EQ("prop", error2->properties[0]);
+}
+
 TEST_SUITE_END();

tests/TypeInfer.test.cpp

@@ -1428,7 +1428,7 @@ TEST_CASE_FIXTURE(BuiltinsFixture, "be_sure_to_use_active_txnlog_when_evaluating
 
     LUAU_REQUIRE_ERRORS(result);
 
-    for (const auto& e: result.errors)
+    for (const auto& e : result.errors)
         CHECK(5 == e.location.begin.line);
 }
 

tests/Unifier2.test.cpp

@@ -81,18 +81,12 @@ TEST_CASE_FIXTURE(Unifier2Fixture, "T <: U")
 
 TEST_CASE_FIXTURE(Unifier2Fixture, "(string) -> () <: (X) -> Y...")
 {
-    TypeId stringToUnit = arena.addType(FunctionType{
+    TypeId stringToUnit = arena.addType(FunctionType{arena.addTypePack({builtinTypes.stringType}), arena.addTypePack({})});
-        arena.addTypePack({builtinTypes.stringType}),
-        arena.addTypePack({})
-    });
 
     auto [x, xFree] = freshType();
     TypePackId y = arena.freshTypePack(&scope);
 
-    TypeId xToY = arena.addType(FunctionType{
+    TypeId xToY = arena.addType(FunctionType{arena.addTypePack({x}), y});
-        arena.addTypePack({x}),
-        y
-    });
 
     u2.unify(stringToUnit, xToY);
 
@@ -105,4 +99,54 @@ TEST_CASE_FIXTURE(Unifier2Fixture, "(string) -> () <: (X) -> Y...")
     CHECK(!yPack->tail);
 }
 
+TEST_CASE_FIXTURE(Unifier2Fixture, "generalize_a_type_that_is_bounded_by_another_generalizable_type")
+{
+    auto [t1, ft1] = freshType();
+    auto [t2, ft2] = freshType();
+
+    // t2 <: t1 <: unknown
+    // unknown <: t2 <: t1
+
+    ft1->lowerBound = t2;
+    ft2->upperBound = t1;
+    ft2->lowerBound = builtinTypes.unknownType;
+
+    auto t2generalized = u2.generalize(NotNull{&scope}, t2);
+    REQUIRE(t2generalized);
+
+    CHECK(follow(t1) == follow(t2));
+
+    auto t1generalized = u2.generalize(NotNull{&scope}, t1);
+    REQUIRE(t1generalized);
+
+    CHECK(builtinTypes.unknownType == follow(t1));
+    CHECK(builtinTypes.unknownType == follow(t2));
+}
+
+// Same as generalize_a_type_that_is_bounded_by_another_generalizable_type
+// except that we generalize the types in the opposite order
+TEST_CASE_FIXTURE(Unifier2Fixture, "generalize_a_type_that_is_bounded_by_another_generalizable_type_in_reverse_order")
+{
+    auto [t1, ft1] = freshType();
+    auto [t2, ft2] = freshType();
+
+    // t2 <: t1 <: unknown
+    // unknown <: t2 <: t1
+
+    ft1->lowerBound = t2;
+    ft2->upperBound = t1;
+    ft2->lowerBound = builtinTypes.unknownType;
+
+    auto t1generalized = u2.generalize(NotNull{&scope}, t1);
+    REQUIRE(t1generalized);
+
+    CHECK(follow(t1) == follow(t2));
+
+    auto t2generalized = u2.generalize(NotNull{&scope}, t2);
+    REQUIRE(t2generalized);
+
+    CHECK(builtinTypes.unknownType == follow(t1));
+    CHECK(builtinTypes.unknownType == follow(t2));
+}
+
 TEST_SUITE_END();

tools/faillist.txt

@@ -3,7 +3,6 @@ AnnotationTests.two_type_params
 AstQuery.last_argument_function_call_type
 AutocompleteTest.anonymous_autofilled_generic_on_argument_type_pack_vararg
 AutocompleteTest.anonymous_autofilled_generic_type_pack_vararg
-AutocompleteTest.autocomplete_if_else_regression
 AutocompleteTest.autocomplete_interpolated_string_as_singleton
 AutocompleteTest.autocomplete_oop_implicit_self
 AutocompleteTest.autocomplete_response_perf1
@@ -92,8 +91,6 @@ IntersectionTypes.table_intersection_write_sealed_indirect
 IntersectionTypes.table_write_sealed_indirect
 Normalize.negations_of_tables
 Normalize.specific_functions_cannot_be_negated
-ParserTests.parse_nesting_based_end_detection
-ParserTests.parse_nesting_based_end_detection_single_line
 ProvisionalTests.assign_table_with_refined_property_with_a_similar_type_is_illegal
 ProvisionalTests.do_not_ice_when_trying_to_pick_first_of_generic_type_pack
 ProvisionalTests.error_on_eq_metamethod_returning_a_type_other_than_boolean
@@ -124,13 +121,13 @@ RefinementTest.x_as_any_if_x_is_instance_elseif_x_is_table
 TableTests.a_free_shape_can_turn_into_a_scalar_if_it_is_compatible
 TableTests.a_free_shape_cannot_turn_into_a_scalar_if_it_is_not_compatible
 TableTests.call_method
-TableTests.call_method_with_explicit_self_argument
 TableTests.cannot_augment_sealed_table
 TableTests.cannot_change_type_of_unsealed_table_prop
 TableTests.casting_sealed_tables_with_props_into_table_with_indexer
 TableTests.casting_tables_with_props_into_table_with_indexer4
 TableTests.casting_unsealed_tables_with_props_into_table_with_indexer
 TableTests.checked_prop_too_early
+TableTests.cli_84607_missing_prop_in_array_or_dict
 TableTests.cyclic_shifted_tables
 TableTests.defining_a_method_for_a_local_sealed_table_must_fail
 TableTests.defining_a_self_method_for_a_local_sealed_table_must_fail
@@ -139,6 +136,7 @@ TableTests.dont_crash_when_setmetatable_does_not_produce_a_metatabletypevar
 TableTests.dont_extend_unsealed_tables_in_rvalue_position
 TableTests.dont_hang_when_trying_to_look_up_in_cyclic_metatable_index
 TableTests.dont_leak_free_table_props
+TableTests.dont_quantify_table_that_belongs_to_outer_scope
 TableTests.dont_suggest_exact_match_keys
 TableTests.error_detailed_metatable_prop
 TableTests.explicitly_typed_table
@@ -154,19 +152,21 @@ TableTests.inequality_operators_imply_exactly_matching_types
 TableTests.infer_array_2
 TableTests.infer_indexer_from_value_property_in_literal
 TableTests.infer_type_when_indexing_from_a_table_indexer
-TableTests.inferred_properties_of_a_table_should_start_with_the_same_TypeLevel_of_that_table
 TableTests.inferred_return_type_of_free_table
 TableTests.instantiate_table_cloning_3
 TableTests.leaking_bad_metatable_errors
 TableTests.less_exponential_blowup_please
 TableTests.missing_metatable_for_sealed_tables_do_not_get_inferred
 TableTests.mixed_tables_with_implicit_numbered_keys
+TableTests.ok_to_add_property_to_free_table
 TableTests.ok_to_provide_a_subtype_during_construction
 TableTests.okay_to_add_property_to_unsealed_tables_by_assignment
-TableTests.okay_to_add_property_to_unsealed_tables_by_function_call
-TableTests.only_ascribe_synthetic_names_at_module_scope
 TableTests.oop_indexer_works
 TableTests.oop_polymorphic
+TableTests.pass_a_union_of_tables_to_a_function_that_requires_a_table
+TableTests.pass_a_union_of_tables_to_a_function_that_requires_a_table_2
+TableTests.pass_incompatible_union_to_a_generic_table_without_crashing
+TableTests.passing_compatible_unions_to_a_generic_table_without_crashing
 TableTests.quantify_even_that_table_was_never_exported_at_all
 TableTests.quantify_metatables_of_metatables_of_table
 TableTests.quantifying_a_bound_var_works
@@ -185,7 +185,6 @@ TableTests.table_unification_4
 TableTests.type_mismatch_on_massive_table_is_cut_short
 TableTests.used_colon_instead_of_dot
 TableTests.used_dot_instead_of_colon
-TableTests.used_dot_instead_of_colon_but_correctly
 TableTests.when_augmenting_an_unsealed_table_with_an_indexer_apply_the_correct_scope_to_the_indexer_type
 TableTests.wrong_assign_does_hit_indexer
 ToDot.function
@@ -215,6 +214,7 @@ TypeAliases.type_alias_of_an_imported_recursive_generic_type
 TypeFamilyTests.family_as_fn_arg
 TypeFamilyTests.table_internal_families
 TypeFamilyTests.unsolvable_family
+TypeInfer.be_sure_to_use_active_txnlog_when_evaluating_a_variadic_overload
 TypeInfer.bidirectional_checking_of_higher_order_function
 TypeInfer.check_type_infer_recursion_count
 TypeInfer.cli_39932_use_unifier_in_ensure_methods
@@ -226,21 +226,24 @@ TypeInfer.fuzz_free_table_type_change_during_index_check
 TypeInfer.infer_assignment_value_types_mutable_lval
 TypeInfer.infer_locals_via_assignment_from_its_call_site
 TypeInfer.no_stack_overflow_from_isoptional
-TypeInfer.recursive_function_that_invokes_itself_with_a_refinement_of_its_parameter
 TypeInfer.recursive_function_that_invokes_itself_with_a_refinement_of_its_parameter_2
 TypeInfer.tc_after_error_recovery_no_replacement_name_in_error
 TypeInfer.type_infer_cache_limit_normalizer
 TypeInfer.type_infer_recursion_limit_no_ice
 TypeInfer.type_infer_recursion_limit_normalizer
 TypeInferAnyError.can_subscript_any
+TypeInferAnyError.for_in_loop_iterator_is_any
 TypeInferAnyError.for_in_loop_iterator_is_any2
+TypeInferAnyError.for_in_loop_iterator_is_error
+TypeInferAnyError.for_in_loop_iterator_is_error2
 TypeInferAnyError.for_in_loop_iterator_returns_any
+TypeInferAnyError.for_in_loop_iterator_returns_any2
 TypeInferAnyError.intersection_of_any_can_have_props
 TypeInferAnyError.replace_every_free_type_when_unifying_a_complex_function_with_any
 TypeInferAnyError.union_of_types_regression_test
-TypeInferClasses.can_read_prop_of_base_class_using_string
 TypeInferClasses.class_type_mismatch_with_name_conflict
 TypeInferClasses.index_instance_property
+TypeInferClasses.table_class_unification_reports_sane_errors_for_missing_properties
 TypeInferFunctions.cannot_hoist_interior_defns_into_signature
 TypeInferFunctions.dont_assert_when_the_tarjan_limit_is_exceeded_during_generalization
 TypeInferFunctions.dont_infer_parameter_types_for_functions_from_their_call_site
@@ -254,10 +257,10 @@ TypeInferFunctions.higher_order_function_2
 TypeInferFunctions.higher_order_function_4
 TypeInferFunctions.improved_function_arg_mismatch_errors
 TypeInferFunctions.infer_anonymous_function_arguments
+TypeInferFunctions.infer_anonymous_function_arguments_outside_call
 TypeInferFunctions.infer_generic_function_function_argument
 TypeInferFunctions.infer_generic_function_function_argument_overloaded
 TypeInferFunctions.infer_generic_lib_function_function_argument
-TypeInferFunctions.infer_anonymous_function_arguments_outside_call
 TypeInferFunctions.infer_that_function_does_not_return_a_table
 TypeInferFunctions.luau_subtyping_is_np_hard
 TypeInferFunctions.no_lossy_function_type
@@ -269,21 +272,22 @@ TypeInferFunctions.too_few_arguments_variadic_generic2
 TypeInferFunctions.too_many_arguments_error_location
 TypeInferFunctions.too_many_return_values_in_parentheses
 TypeInferFunctions.too_many_return_values_no_function
-TypeInferFunctions.toposort_doesnt_break_mutual_recursion
-TypeInferFunctions.vararg_function_is_quantified
 TypeInferLoops.cli_68448_iterators_need_not_accept_nil
 TypeInferLoops.dcr_iteration_on_never_gives_never
 TypeInferLoops.for_in_loop
 TypeInferLoops.for_in_loop_error_on_factory_not_returning_the_right_amount_of_values
+TypeInferLoops.for_in_loop_on_error
 TypeInferLoops.for_in_loop_with_custom_iterator
 TypeInferLoops.for_in_loop_with_incompatible_args_to_iterator
 TypeInferLoops.for_in_loop_with_next
 TypeInferLoops.ipairs_produces_integral_indices
 TypeInferLoops.iteration_regression_issue_69967_alt
+TypeInferLoops.loop_iter_basic
 TypeInferLoops.loop_iter_metamethod_nil
 TypeInferLoops.loop_iter_metamethod_ok_with_inference
 TypeInferLoops.loop_iter_trailing_nil
 TypeInferLoops.unreachable_code_after_infinite_loop
+TypeInferLoops.varlist_declared_by_for_in_loop_should_be_free
 TypeInferModules.do_not_modify_imported_types_5
 TypeInferModules.module_type_conflict
 TypeInferModules.module_type_conflict_instantiated
@@ -294,7 +298,6 @@ TypeInferOOP.inferring_hundreds_of_self_calls_should_not_suffocate_memory
 TypeInferOOP.methods_are_topologically_sorted
 TypeInferOperators.and_binexps_dont_unify
 TypeInferOperators.cli_38355_recursive_union
-TypeInferOperators.compound_assign_mismatch_metatable
 TypeInferOperators.concat_op_on_string_lhs_and_free_rhs
 TypeInferOperators.disallow_string_and_types_without_metatables_from_arithmetic_binary_ops
 TypeInferOperators.luau_polyfill_is_array