Sync to upstream/release/607 (#1131)

# What's changed?

* Fix up the `std::iterator_traits` definitions for some Luau data
structures.
* Replace some of the usages of `std::unordered_set` and
`std::unordered_map` with Luau-provided data structures to increase
performance and reduce overall number of heap allocations.
* Update some of the documentation links in comments throughout the
codebase to correctly point to the moved repository.
* Expanded JSON encoder for AST to support singleton types.
* Fixed a bug in `luau-analyze` where exceptions in the last module
being checked during multithreaded analysis would not be rethrown.

### New type solver

* Introduce a `refine` type family to handle deferred refinements during
type inference, replacing the old `RefineConstraint`.
* Continued work on the implementation of type states, fixing some known
bugs/blockers.
* Added support for variadic functions in new non-strict mode, enabling
broader support for builtins and the Roblox API.

### Internal Contributors

Co-authored-by: Aaron Weiss <aaronweiss@roblox.com>
Co-authored-by: Alexander McCord <amccord@roblox.com>
Co-authored-by: Andy Friesen <afriesen@roblox.com>
Co-authored-by: Vyacheslav Egorov <vegorov@roblox.com>

---------

Co-authored-by: Alexander McCord <amccord@roblox.com>
Co-authored-by: Andy Friesen <afriesen@roblox.com>
Co-authored-by: Vighnesh <vvijay@roblox.com>
Co-authored-by: Aviral Goel <agoel@roblox.com>
Co-authored-by: David Cope <dcope@roblox.com>
Co-authored-by: Lily Brown <lbrown@roblox.com>
Co-authored-by: Vyacheslav Egorov <vegorov@roblox.com>

Analysis/include/Luau/Constraint.h

@@ -197,24 +197,6 @@ struct UnpackConstraint
     bool resultIsLValue = false;
 };
 
-// resultType ~ refine type mode discriminant
-//
-// Compute type & discriminant (or type | discriminant) as soon as possible (but
-// no sooner), simplify, and bind resultType to that type.
-struct RefineConstraint
-{
-    enum
-    {
-        Intersection,
-        Union
-    } mode;
-
-    TypeId resultType;
-
-    TypeId type;
-    TypeId discriminant;
-};
-
 // resultType ~ T0 op T1 op ... op TN
 //
 // op is either union or intersection.  If any of the input types are blocked,
@@ -249,7 +231,7 @@ struct ReducePackConstraint
 
 using ConstraintV = Variant<SubtypeConstraint, PackSubtypeConstraint, GeneralizationConstraint, InstantiationConstraint, IterableConstraint,
     NameConstraint, TypeAliasExpansionConstraint, FunctionCallConstraint, PrimitiveTypeConstraint, HasPropConstraint, SetPropConstraint,
-    SetIndexerConstraint, SingletonOrTopTypeConstraint, UnpackConstraint, RefineConstraint, SetOpConstraint, ReduceConstraint, ReducePackConstraint>;
+    SetIndexerConstraint, SingletonOrTopTypeConstraint, UnpackConstraint, SetOpConstraint, ReduceConstraint, ReducePackConstraint>;
 
 struct Constraint
 {

Analysis/include/Luau/ConstraintSolver.h

@@ -132,7 +132,6 @@ struct ConstraintSolver
     bool tryDispatch(const SetIndexerConstraint& c, NotNull<const Constraint> constraint, bool force);
     bool tryDispatch(const SingletonOrTopTypeConstraint& c, NotNull<const Constraint> constraint);
     bool tryDispatch(const UnpackConstraint& c, NotNull<const Constraint> constraint);
-    bool tryDispatch(const RefineConstraint& c, NotNull<const Constraint> constraint, bool force);
     bool tryDispatch(const SetOpConstraint& c, NotNull<const Constraint> constraint, bool force);
     bool tryDispatch(const ReduceConstraint& c, NotNull<const Constraint> constraint, bool force);
     bool tryDispatch(const ReducePackConstraint& c, NotNull<const Constraint> constraint, bool force);

Analysis/include/Luau/DataFlowGraph.h

@@ -137,8 +137,8 @@ private:
     DfgScope* childScope(DfgScope* scope, DfgScope::ScopeType scopeType = DfgScope::Linear);
 
     void join(DfgScope* p, DfgScope* a, DfgScope* b);
-    void joinBindings(DfgScope::Bindings& p, const DfgScope::Bindings& a, const DfgScope::Bindings& b);
+    void joinBindings(DfgScope* p, const DfgScope& a, const DfgScope& b);
-    void joinProps(DfgScope::Props& p, const DfgScope::Props& a, const DfgScope::Props& b);
+    void joinProps(DfgScope* p, const DfgScope& a, const DfgScope& b);
 
     DefId lookup(DfgScope* scope, Symbol symbol);
     DefId lookup(DfgScope* scope, DefId def, const std::string& key);

Analysis/include/Luau/Set.h

@@ -118,6 +118,12 @@ public:
     class const_iterator
     {
     public:
+        using value_type = T;
+        using reference = T&;
+        using pointer = T*;
+        using difference_type = ptrdiff_t;
+        using iterator_category = std::forward_iterator_tag;
+
         const_iterator(typename Impl::const_iterator impl, typename Impl::const_iterator end)
             : impl(impl)
             , end(end)

Analysis/include/Luau/Type.h

@@ -176,7 +176,7 @@ struct PrimitiveType
     }
 };
 
-// Singleton types https://github.com/Roblox/luau/blob/master/rfcs/syntax-singleton-types.md
+// Singleton types https://github.com/luau-lang/rfcs/blob/master/docs/syntax-singleton-types.md
 // Types for true and false
 struct BooleanSingleton
 {

Analysis/include/Luau/TypeFamily.h

@@ -162,6 +162,8 @@ struct BuiltinTypeFamilies
     TypeFamily leFamily;
     TypeFamily eqFamily;
 
+    TypeFamily refineFamily;
+
     void addToScope(NotNull<TypeArena> arena, NotNull<Scope> scope) const;
 };
 

Analysis/src/AstJsonEncoder.cpp

@@ -200,6 +200,23 @@ struct AstJsonEncoder : public AstVisitor
     {
         writeString(name.value ? name.value : "");
     }
+    void write(std::optional<AstArgumentName> name)
+    {
+        if (name)
+            write(*name);
+        else
+            writeRaw("null");
+    }
+    void write(AstArgumentName name)
+    {
+        writeRaw("{");
+        bool c = pushComma();
+        writeType("AstArgumentName");
+        write("name", name.first);
+        write("location", name.second);
+        popComma(c);
+        writeRaw("}");
+    }
 
     void write(const Position& position)
     {
@@ -848,6 +865,7 @@ struct AstJsonEncoder : public AstVisitor
             PROP(generics);
             PROP(genericPacks);
             PROP(argTypes);
+            PROP(argNames);
             PROP(returnTypes);
         });
     }
@@ -902,6 +920,22 @@ struct AstJsonEncoder : public AstVisitor
         });
     }
 
+    bool visit(class AstTypeSingletonBool* node) override
+    {
+        writeNode(node, "AstTypeSingletonBool", [&]() {
+            write("value", node->value);
+        });
+        return false;
+    }
+
+    bool visit(class AstTypeSingletonString* node) override
+    {
+        writeNode(node, "AstTypeSingletonString", [&]() {
+            write("value", node->value);
+        });
+        return false;
+    }
+
     bool visit(class AstExprGroup* node) override
     {
         write(node);

Analysis/src/ConstraintGenerator.cpp

@@ -257,7 +257,7 @@ void ConstraintGenerator::unionRefinements(const RefinementContext& lhs, const R
             return types[0];
         else if (2 == types.size())
         {
-            // TODO: It may be advantageous to create a RefineConstraint here when there are blockedTypes.
+            // TODO: It may be advantageous to introduce a refine type family here when there are blockedTypes.
             SimplifyResult sr = simplifyIntersection(builtinTypes, arena, types[0], types[1]);
             if (sr.blockedTypes.empty())
                 return sr.result;
@@ -441,10 +441,14 @@ void ConstraintGenerator::applyRefinements(const ScopePtr& scope, Location locat
             {
                 if (mustDeferIntersection(ty) || mustDeferIntersection(dt))
                 {
-                    TypeId r = arena->addType(BlockedType{});
+                    TypeId resultType = arena->addType(TypeFamilyInstanceType{
-                    addConstraint(scope, location, RefineConstraint{RefineConstraint::Intersection, r, ty, dt});
+                        NotNull{&kBuiltinTypeFamilies.refineFamily},
+                        {ty, dt},
+                        {},
+                    });
+                    addConstraint(scope, location, ReduceConstraint{resultType});
 
-                    ty = r;
+                    ty = resultType;
                 }
                 else
                 {
@@ -1005,9 +1009,6 @@ ControlFlow ConstraintGenerator::visit(const ScopePtr& scope, AstStatAssign* ass
 
         checkLValue(scope, lvalue, assignee);
         assignees.push_back(assignee);
-
-        DefId def = dfg->getDef(lvalue);
-        scope->lvalueTypes[def] = assignee;
     }
 
     TypePackId resultPack = checkPack(scope, assign->values).tp;

Analysis/src/ConstraintSolver.cpp

@@ -545,8 +545,6 @@ bool ConstraintSolver::tryDispatch(NotNull<const Constraint> constraint, bool fo
         success = tryDispatch(*sottc, constraint);
     else if (auto uc = get<UnpackConstraint>(*constraint))
         success = tryDispatch(*uc, constraint);
-    else if (auto rc = get<RefineConstraint>(*constraint))
-        success = tryDispatch(*rc, constraint, force);
     else if (auto soc = get<SetOpConstraint>(*constraint))
         success = tryDispatch(*soc, constraint, force);
     else if (auto rc = get<ReduceConstraint>(*constraint))
@@ -887,9 +885,9 @@ bool ConstraintSolver::tryDispatch(const TypeAliasExpansionConstraint& c, NotNul
     // In order to prevent infinite types from being expanded and causing us to
     // cycle infinitely, we need to scan the type function for cases where we
     // expand the same alias with different type saturatedTypeArguments. See
-    // https://github.com/Roblox/luau/pull/68 for the RFC responsible for this.
+    // https://github.com/luau-lang/luau/pull/68 for the RFC responsible for
-    // This is a little nicer than using a recursion limit because we can catch
+    // this. This is a little nicer than using a recursion limit because we can
-    // the infinite expansion before actually trying to expand it.
+    // catch the infinite expansion before actually trying to expand it.
     InfiniteTypeFinder itf{this, signature, constraint->scope};
     itf.traverse(tf->type);
 
@@ -1505,151 +1503,6 @@ bool ConstraintSolver::tryDispatch(const UnpackConstraint& c, NotNull<const Cons
     return true;
 }
 
-namespace
-{
-
-/*
- * Search for types that prevent us from being ready to dispatch a particular
- * RefineConstraint.
- */
-struct FindRefineConstraintBlockers : TypeOnceVisitor
-{
-    DenseHashSet<TypeId> found{nullptr};
-    bool visit(TypeId ty, const BlockedType&) override
-    {
-        found.insert(ty);
-        return false;
-    }
-
-    bool visit(TypeId ty, const PendingExpansionType&) override
-    {
-        found.insert(ty);
-        return false;
-    }
-
-    bool visit(TypeId ty, const ClassType&) override
-    {
-        return false;
-    }
-};
-
-} // namespace
-
-static bool isNegatedAny(TypeId ty)
-{
-    ty = follow(ty);
-    const NegationType* nt = get<NegationType>(ty);
-    if (!nt)
-        return false;
-    TypeId negatedTy = follow(nt->ty);
-    return bool(get<AnyType>(negatedTy));
-}
-
-bool ConstraintSolver::tryDispatch(const RefineConstraint& c, NotNull<const Constraint> constraint, bool force)
-{
-    if (isBlocked(c.discriminant))
-        return block(c.discriminant, constraint);
-
-    FindRefineConstraintBlockers fbt;
-    fbt.traverse(c.discriminant);
-
-    if (!fbt.found.empty())
-    {
-        bool foundOne = false;
-
-        for (TypeId blocked : fbt.found)
-        {
-            if (blocked == c.type)
-                continue;
-
-            block(blocked, constraint);
-            foundOne = true;
-        }
-
-        if (foundOne)
-            return false;
-    }
-
-    /* HACK: Refinements sometimes produce a type T & ~any under the assumption
-     * that ~any is the same as any.  This is so so weird, but refinements needs
-     * some way to say "I may refine this, but I'm not sure."
-     *
-     * It does this by refining on a blocked type and deferring the decision
-     * until it is unblocked.
-     *
-     * Refinements also get negated, so we wind up with types like T & ~*blocked*
-     *
-     * We need to treat T & ~any as T in this case.
-     */
-
-    if (c.mode == RefineConstraint::Intersection && isNegatedAny(c.discriminant))
-    {
-        asMutable(c.resultType)->ty.emplace<BoundType>(c.type);
-        unblock(c.resultType, constraint->location);
-        return true;
-    }
-
-    const TypeId type = follow(c.type);
-
-    if (hasUnresolvedConstraints(type))
-        return block(type, constraint);
-
-    LUAU_ASSERT(get<BlockedType>(c.resultType));
-
-    if (type == c.resultType)
-    {
-        /*
-         * Sometimes, we get a constraint of the form
-         *
-         * *blocked-N* ~ refine *blocked-N* & U
-         *
-         * The constraint essentially states that a particular type is a
-         * refinement of itself. This is weird and I think vacuous.
-         *
-         * I *believe* it is safe to replace the result with a fresh type that
-         * is constrained by U.  We effect this by minting a fresh type for the
-         * result when U = any, else we bind the result to whatever discriminant
-         * was offered.
-         */
-        if (get<AnyType>(follow(c.discriminant)))
-        {
-            TypeId f = freshType(arena, builtinTypes, constraint->scope);
-            asMutable(c.resultType)->ty.emplace<BoundType>(f);
-        }
-        else
-            asMutable(c.resultType)->ty.emplace<BoundType>(c.discriminant);
-
-        unblock(c.resultType, constraint->location);
-        return true;
-    }
-
-    auto [result, blockedTypes] = c.mode == RefineConstraint::Intersection ? simplifyIntersection(builtinTypes, NotNull{arena}, type, c.discriminant)
-                                                                           : simplifyUnion(builtinTypes, NotNull{arena}, type, c.discriminant);
-
-    if (!force && !blockedTypes.empty())
-        return block(blockedTypes, constraint);
-
-    switch (shouldSuppressErrors(normalizer, c.type))
-    {
-    case ErrorSuppression::Suppress:
-    {
-        auto resultOrError = simplifyUnion(builtinTypes, arena, result, builtinTypes->errorType).result;
-        asMutable(c.resultType)->ty.emplace<BoundType>(resultOrError);
-        break;
-    }
-    case ErrorSuppression::DoNotSuppress:
-        asMutable(c.resultType)->ty.emplace<BoundType>(result);
-        break;
-    case ErrorSuppression::NormalizationFailed:
-        reportError(NormalizationTooComplex{}, constraint->location);
-        break;
-    }
-
-    unblock(c.resultType, constraint->location);
-
-    return true;
-}
-
 bool ConstraintSolver::tryDispatch(const SetOpConstraint& c, NotNull<const Constraint> constraint, bool force)
 {
     bool blocked = false;

Analysis/src/DataFlowGraph.cpp

@@ -180,36 +180,39 @@ DfgScope* DataFlowGraphBuilder::childScope(DfgScope* scope, DfgScope::ScopeType
 
 void DataFlowGraphBuilder::join(DfgScope* p, DfgScope* a, DfgScope* b)
 {
-    joinBindings(p->bindings, a->bindings, b->bindings);
+    joinBindings(p, *a, *b);
-    joinProps(p->props, a->props, b->props);
+    joinProps(p, *a, *b);
 }
 
-void DataFlowGraphBuilder::joinBindings(DfgScope::Bindings& p, const DfgScope::Bindings& a, const DfgScope::Bindings& b)
+void DataFlowGraphBuilder::joinBindings(DfgScope* p, const DfgScope& a, const DfgScope& b)
 {
-    for (const auto& [sym, def1] : a)
+    for (const auto& [sym, def1] : a.bindings)
     {
-        if (auto def2 = b.find(sym))
+        if (auto def2 = b.bindings.find(sym))
-            p[sym] = defArena->phi(NotNull{def1}, NotNull{*def2});
+            p->bindings[sym] = defArena->phi(NotNull{def1}, NotNull{*def2});
-        else if (auto def2 = p.find(sym))
+        else if (auto def2 = p->lookup(sym))
-            p[sym] = defArena->phi(NotNull{def1}, NotNull{*def2});
+            p->bindings[sym] = defArena->phi(NotNull{def1}, NotNull{*def2});
     }
 
-    for (const auto& [sym, def1] : b)
+    for (const auto& [sym, def1] : b.bindings)
     {
-        if (auto def2 = p.find(sym))
+        if (auto def2 = p->lookup(sym))
-            p[sym] = defArena->phi(NotNull{def1}, NotNull{*def2});
+            p->bindings[sym] = defArena->phi(NotNull{def1}, NotNull{*def2});
     }
 }
 
-void DataFlowGraphBuilder::joinProps(DfgScope::Props& p, const DfgScope::Props& a, const DfgScope::Props& b)
+void DataFlowGraphBuilder::joinProps(DfgScope* result, const DfgScope& a, const DfgScope& b)
 {
-    auto phinodify = [this](auto& p, const auto& a, const auto& b) mutable {
+    auto phinodify = [this](DfgScope* scope, const auto& a, const auto& b, DefId parent) mutable {
+        auto& p = scope->props[parent];
         for (const auto& [k, defA] : a)
         {
             if (auto it = b.find(k); it != b.end())
                 p[k] = defArena->phi(NotNull{it->second}, NotNull{defA});
             else if (auto it = p.find(k); it != p.end())
                 p[k] = defArena->phi(NotNull{it->second}, NotNull{defA});
+            else if (auto def2 = scope->lookup(parent, k))
+                p[k] = defArena->phi(*def2, NotNull{defA});
             else
                 p[k] = defA;
         }
@@ -220,27 +223,29 @@ void DataFlowGraphBuilder::joinProps(DfgScope::Props& p, const DfgScope::Props&
                 continue;
             else if (auto it = p.find(k); it != p.end())
                 p[k] = defArena->phi(NotNull{it->second}, NotNull{defB});
+            else if (auto def2 = scope->lookup(parent, k))
+                p[k] = defArena->phi(*def2, NotNull{defB});
             else
                 p[k] = defB;
         }
     };
 
-    for (const auto& [def, a1] : a)
+    for (const auto& [def, a1] : a.props)
     {
-        p.try_insert(def, {});
+        result->props.try_insert(def, {});
-        if (auto a2 = b.find(def))
+        if (auto a2 = b.props.find(def))
-            phinodify(p[def], a1, *a2);
+            phinodify(result, a1, *a2, NotNull{def});
-        else if (auto a2 = p.find(def))
+        else if (auto a2 = result->props.find(def))
-            phinodify(p[def], a1, *a2);
+            phinodify(result, a1, *a2, NotNull{def});
     }
 
-    for (const auto& [def, a1] : b)
+    for (const auto& [def, a1] : b.props)
     {
-        p.try_insert(def, {});
+        result->props.try_insert(def, {});
-        if (a.find(def))
+        if (a.props.find(def))
             continue;
-        else if (auto a2 = p.find(def))
+        else if (auto a2 = result->props.find(def))
-            phinodify(p[def], a1, *a2);
+            phinodify(result, a1, *a2, NotNull{def});
     }
 }
 
@@ -466,6 +471,14 @@ ControlFlow DataFlowGraphBuilder::visit(DfgScope* scope, AstStatLocal* l)
         // make sure that the non-aliased defs are also marked as a subscript for refinements.
         bool subscripted = i < defs.size() && containsSubscriptedDefinition(defs[i]);
         DefId def = defArena->freshCell(subscripted);
+        if (i < l->values.size)
+        {
+            AstExpr* e = l->values.data[i];
+            if (const AstExprTable* tbl = e->as<AstExprTable>())
+            {
+                def = defs[i];
+            }
+        }
         graph.localDefs[local] = def;
         scope->bindings[local] = def;
         captures[local].allVersions.push_back(def);
@@ -769,7 +782,7 @@ DataFlowResult DataFlowGraphBuilder::visitExpr(DfgScope* scope, AstExprIndexExpr
         return {def, keyArena->node(parentKey, def, index)};
     }
 
-    return {defArena->freshCell(/* subscripted= */true), nullptr};
+    return {defArena->freshCell(/* subscripted= */ true), nullptr};
 }
 
 DataFlowResult DataFlowGraphBuilder::visitExpr(DfgScope* scope, AstExprFunction* f)
@@ -819,14 +832,20 @@ DataFlowResult DataFlowGraphBuilder::visitExpr(DfgScope* scope, AstExprFunction*
 
 DataFlowResult DataFlowGraphBuilder::visitExpr(DfgScope* scope, AstExprTable* t)
 {
+    DefId tableCell = defArena->freshCell();
+    scope->props[tableCell] = {};
     for (AstExprTable::Item item : t->items)
     {
+        DataFlowResult result = visitExpr(scope, item.value);
         if (item.key)
+        {
             visitExpr(scope, item.key);
-        visitExpr(scope, item.value);
+            if (auto string = item.key->as<AstExprConstantString>())
+                scope->props[tableCell][string->value.data] = result.def;
+        }
     }
 
-    return {defArena->freshCell(), nullptr};
+    return {tableCell, nullptr};
 }
 
 DataFlowResult DataFlowGraphBuilder::visitExpr(DfgScope* scope, AstExprUnary* u)

Analysis/src/Frontend.cpp

@@ -37,6 +37,7 @@ LUAU_FASTFLAGVARIABLE(DebugLuauLogSolverToJson, false)
 LUAU_FASTFLAGVARIABLE(DebugLuauReadWriteProperties, false)
 LUAU_FASTFLAGVARIABLE(CorrectEarlyReturnInMarkDirty, false)
 LUAU_FASTFLAGVARIABLE(LuauDefinitionFileSetModuleName, false)
+LUAU_FASTFLAGVARIABLE(LuauRethrowSingleModuleIce, false)
 
 namespace Luau
 {
@@ -679,8 +680,7 @@ std::vector<ModuleName> Frontend::checkQueuedModules(std::optional<FrontendOptio
             sendItemTask(i);
         nextItems.clear();
 
-        // If we aren't done, but don't have anything processing, we hit a cycle
+        if (FFlag::LuauRethrowSingleModuleIce && processing == 0)
-        if (remaining != 0 && processing == 0)
         {
             // Typechecking might have been cancelled by user, don't return partial results
             if (cancelled)
@@ -688,9 +688,24 @@ std::vector<ModuleName> Frontend::checkQueuedModules(std::optional<FrontendOptio
 
             // We might have stopped because of a pending exception
             if (itemWithException)
-            {
                 recordItemResult(buildQueueItems[*itemWithException]);
-                break;
+        }
+
+        // If we aren't done, but don't have anything processing, we hit a cycle
+        if (remaining != 0 && processing == 0)
+        {
+            if (!FFlag::LuauRethrowSingleModuleIce)
+            {
+                // Typechecking might have been cancelled by user, don't return partial results
+                if (cancelled)
+                    return {};
+
+                // We might have stopped because of a pending exception
+                if (itemWithException)
+                {
+                    recordItemResult(buildQueueItems[*itemWithException]);
+                    break;
+                }
             }
 
             sendCycleItemTask();

Analysis/src/NonStrictTypeChecker.cpp

@@ -12,6 +12,7 @@
 #include "Luau/TypeArena.h"
 #include "Luau/TypeFamily.h"
 #include "Luau/Def.h"
+#include "Luau/TypeFwd.h"
 
 #include <iostream>
 #include <iterator>
@@ -57,8 +58,6 @@ struct StackPusher
 
 struct NonStrictContext
 {
-    std::unordered_map<const Def*, TypeId> context;
-
     NonStrictContext() = default;
 
     NonStrictContext(const NonStrictContext&) = delete;
@@ -109,7 +108,12 @@ struct NonStrictContext
     // Returns true if the removal was successful
     bool remove(const DefId& def)
     {
-        return context.erase(def.get()) == 1;
+        std::vector<DefId> defs;
+        collectOperands(def, &defs);
+        bool result = true;
+        for (DefId def : defs)
+            result = result && context.erase(def.get()) == 1;
+        return result;
     }
 
     std::optional<TypeId> find(const DefId& def) const
@@ -118,6 +122,14 @@ struct NonStrictContext
         return find(d);
     }
 
+    void addContext(const DefId& def, TypeId ty)
+    {
+        std::vector<DefId> defs;
+        collectOperands(def, &defs);
+        for (DefId def : defs)
+            context[def.get()] = ty;
+    }
+
 private:
     std::optional<TypeId> find(const Def* d) const
     {
@@ -126,6 +138,9 @@ private:
             return {it->second};
         return {};
     }
+
+    std::unordered_map<const Def*, TypeId> context;
+
 };
 
 struct NonStrictTypeChecker
@@ -508,8 +523,25 @@ struct NonStrictTypeChecker
                 // ...
                 // (unknown^N-1, ~S_N) -> error
                 std::vector<TypeId> argTypes;
-                for (TypeId ty : fn->argTypes)
+                argTypes.reserve(call->args.size);
-                    argTypes.push_back(ty);
+                // Pad out the arg types array with the types you would expect to see
+                TypePackIterator curr = begin(fn->argTypes);
+                TypePackIterator fin = end(fn->argTypes);
+                while (curr != fin)
+                {
+                    argTypes.push_back(*curr);
+                    ++curr;
+                }
+                if (auto argTail = curr.tail())
+                {
+                    if (const VariadicTypePack* vtp = get<VariadicTypePack>(follow(*argTail)))
+                    {
+                        while (argTypes.size() < call->args.size)
+                        {
+                            argTypes.push_back(vtp->ty);
+                        }
+                    }
+                }
                 // For a checked function, these gotta be the same size
                 LUAU_ASSERT(call->args.size == argTypes.size());
                 for (size_t i = 0; i < call->args.size; i++)
@@ -523,7 +555,7 @@ struct NonStrictTypeChecker
                     TypeId expectedArgType = argTypes[i];
                     DefId def = dfg->getDef(arg);
                     TypeId runTimeErrorTy = getOrCreateNegation(expectedArgType);
-                    fresh.context[def.get()] = runTimeErrorTy;
+                    fresh.addContext(def, runTimeErrorTy);
                 }
 
                 // Populate the context and now iterate through each of the arguments to the call to find out if we satisfy the types
@@ -613,15 +645,20 @@ struct NonStrictTypeChecker
     std::optional<TypeId> willRunTimeError(AstExpr* fragment, const NonStrictContext& context)
     {
         DefId def = dfg->getDef(fragment);
-        if (std::optional<TypeId> contextTy = context.find(def))
+        std::vector<DefId> defs;
+        collectOperands(def, &defs);
+        for (DefId def : defs)
         {
+            if (std::optional<TypeId> contextTy = context.find(def))
+            {
 
-            TypeId actualType = lookupType(fragment);
+                TypeId actualType = lookupType(fragment);
-            SubtypingResult r = subtyping.isSubtype(actualType, *contextTy);
+                SubtypingResult r = subtyping.isSubtype(actualType, *contextTy);
-            if (r.normalizationTooComplex)
+                if (r.normalizationTooComplex)
-                reportError(NormalizationTooComplex{}, fragment->location);
+                    reportError(NormalizationTooComplex{}, fragment->location);
-            if (r.isSubtype)
+                if (r.isSubtype)
-                return {actualType};
+                    return {actualType};
+            }
         }
 
         return {};
@@ -630,15 +667,20 @@ struct NonStrictTypeChecker
     std::optional<TypeId> willRunTimeErrorFunctionDefinition(AstLocal* fragment, const NonStrictContext& context)
     {
         DefId def = dfg->getDef(fragment);
-        if (std::optional<TypeId> contextTy = context.find(def))
+        std::vector<DefId> defs;
+        collectOperands(def, &defs);
+        for (DefId def : defs)
         {
-            SubtypingResult r1 = subtyping.isSubtype(builtinTypes->unknownType, *contextTy);
+            if (std::optional<TypeId> contextTy = context.find(def))
-            SubtypingResult r2 = subtyping.isSubtype(*contextTy, builtinTypes->unknownType);
+            {
-            if (r1.normalizationTooComplex || r2.normalizationTooComplex)
+                SubtypingResult r1 = subtyping.isSubtype(builtinTypes->unknownType, *contextTy);
-                reportError(NormalizationTooComplex{}, fragment->location);
+                SubtypingResult r2 = subtyping.isSubtype(*contextTy, builtinTypes->unknownType);
-            bool isUnknown = r1.isSubtype && r2.isSubtype;
+                if (r1.normalizationTooComplex || r2.normalizationTooComplex)
-            if (isUnknown)
+                    reportError(NormalizationTooComplex{}, fragment->location);
-                return {builtinTypes->unknownType};
+                bool isUnknown = r1.isSubtype && r2.isSubtype;
+                if (isUnknown)
+                    return {builtinTypes->unknownType};
+            }
         }
         return {};
     }

Analysis/src/ToString.cpp

@@ -3,8 +3,10 @@
 
 #include "Luau/Common.h"
 #include "Luau/Constraint.h"
+#include "Luau/DenseHash.h"
 #include "Luau/Location.h"
 #include "Luau/Scope.h"
+#include "Luau/Set.h"
 #include "Luau/TxnLog.h"
 #include "Luau/TypeInfer.h"
 #include "Luau/TypePack.h"
@@ -53,8 +55,8 @@ struct FindCyclicTypes final : TypeVisitor
     FindCyclicTypes& operator=(const FindCyclicTypes&) = delete;
 
     bool exhaustive = false;
-    std::unordered_set<TypeId> visited;
+    Luau::Set<TypeId> visited{{}};
-    std::unordered_set<TypePackId> visitedPacks;
+    Luau::Set<TypePackId> visitedPacks{{}};
     std::set<TypeId> cycles;
     std::set<TypePackId> cycleTPs;
 
@@ -70,17 +72,17 @@ struct FindCyclicTypes final : TypeVisitor
 
     bool visit(TypeId ty) override
     {
-        return visited.insert(ty).second;
+        return visited.insert(ty);
     }
 
     bool visit(TypePackId tp) override
     {
-        return visitedPacks.insert(tp).second;
+        return visitedPacks.insert(tp);
     }
 
     bool visit(TypeId ty, const FreeType& ft) override
     {
-        if (!visited.insert(ty).second)
+        if (!visited.insert(ty))
             return false;
 
         if (FFlag::DebugLuauDeferredConstraintResolution)
@@ -102,7 +104,7 @@ struct FindCyclicTypes final : TypeVisitor
 
     bool visit(TypeId ty, const LocalType& lt) override
     {
-        if (!visited.insert(ty).second)
+        if (!visited.insert(ty))
             return false;
 
         traverse(lt.domain);
@@ -112,7 +114,7 @@ struct FindCyclicTypes final : TypeVisitor
 
     bool visit(TypeId ty, const TableType& ttv) override
     {
-        if (!visited.insert(ty).second)
+        if (!visited.insert(ty))
             return false;
 
         if (ttv.name || ttv.syntheticName)
@@ -175,10 +177,11 @@ struct StringifierState
     ToStringOptions& opts;
     ToStringResult& result;
 
-    std::unordered_map<TypeId, std::string> cycleNames;
+    DenseHashMap<TypeId, std::string> cycleNames{{}};
-    std::unordered_map<TypePackId, std::string> cycleTpNames;
+    DenseHashMap<TypePackId, std::string> cycleTpNames{{}};
-    std::unordered_set<void*> seen;
+    Set<void*> seen{{}};
-    std::unordered_set<std::string> usedNames;
+    // `$$$` was chosen as the tombstone for `usedNames` since it is not a valid name syntactically and is relatively short for string comparison reasons.
+    DenseHashSet<std::string> usedNames{"$$$"};
     size_t indentation = 0;
 
     bool exhaustive;
@@ -197,7 +200,7 @@ struct StringifierState
     bool hasSeen(const void* tv)
     {
         void* ttv = const_cast<void*>(tv);
-        if (seen.find(ttv) != seen.end())
+        if (seen.contains(ttv))
             return true;
 
         seen.insert(ttv);
@@ -207,9 +210,9 @@ struct StringifierState
     void unsee(const void* tv)
     {
         void* ttv = const_cast<void*>(tv);
-        auto iter = seen.find(ttv);
+
-        if (iter != seen.end())
+        if (seen.contains(ttv))
-            seen.erase(iter);
+            seen.erase(ttv);
     }
 
     std::string getName(TypeId ty)
@@ -222,7 +225,7 @@ struct StringifierState
         for (int count = 0; count < 256; ++count)
         {
             std::string candidate = generateName(usedNames.size() + count);
-            if (!usedNames.count(candidate))
+            if (!usedNames.contains(candidate))
             {
                 usedNames.insert(candidate);
                 n = candidate;
@@ -245,7 +248,7 @@ struct StringifierState
         for (int count = 0; count < 256; ++count)
         {
             std::string candidate = generateName(previousNameIndex + count);
-            if (!usedNames.count(candidate))
+            if (!usedNames.contains(candidate))
             {
                 previousNameIndex += count;
                 usedNames.insert(candidate);
@@ -358,10 +361,9 @@ struct TypeStringifier
             return;
         }
 
-        auto it = state.cycleNames.find(tv);
+        if (auto p = state.cycleNames.find(tv))
-        if (it != state.cycleNames.end())
         {
-            state.emit(it->second);
+            state.emit(*p);
             return;
         }
 
@@ -886,7 +888,7 @@ struct TypeStringifier
 
             std::string saved = std::move(state.result.name);
 
-            bool needParens = !state.cycleNames.count(el) && (get<IntersectionType>(el) || get<FunctionType>(el));
+            bool needParens = !state.cycleNames.contains(el) && (get<IntersectionType>(el) || get<FunctionType>(el));
 
             if (needParens)
                 state.emit("(");
@@ -953,7 +955,7 @@ struct TypeStringifier
 
             std::string saved = std::move(state.result.name);
 
-            bool needParens = !state.cycleNames.count(el) && (get<UnionType>(el) || get<FunctionType>(el));
+            bool needParens = !state.cycleNames.contains(el) && (get<UnionType>(el) || get<FunctionType>(el));
 
             if (needParens)
                 state.emit("(");
@@ -1101,10 +1103,9 @@ struct TypePackStringifier
             return;
         }
 
-        auto it = state.cycleTpNames.find(tp);
+        if (auto p = state.cycleTpNames.find(tp))
-        if (it != state.cycleTpNames.end())
         {
-            state.emit(it->second);
+            state.emit(*p);
             return;
         }
 
@@ -1278,7 +1279,7 @@ void TypeStringifier::stringify(TypePackId tpid, const std::vector<std::optional
 }
 
 static void assignCycleNames(const std::set<TypeId>& cycles, const std::set<TypePackId>& cycleTPs,
-    std::unordered_map<TypeId, std::string>& cycleNames, std::unordered_map<TypePackId, std::string>& cycleTpNames, bool exhaustive)
+    DenseHashMap<TypeId, std::string>& cycleNames, DenseHashMap<TypePackId, std::string>& cycleTpNames, bool exhaustive)
 {
     int nextIndex = 1;
 
@@ -1372,9 +1373,8 @@ ToStringResult toStringDetailed(TypeId ty, ToStringOptions& opts)
      *
      * t1 where t1 = the_whole_root_type
      */
-    auto it = state.cycleNames.find(ty);
+    if (auto p = state.cycleNames.find(ty))
-    if (it != state.cycleNames.end())
+        state.emit(*p);
-        state.emit(it->second);
     else
         tvs.stringify(ty);
 
@@ -1466,9 +1466,8 @@ ToStringResult toStringDetailed(TypePackId tp, ToStringOptions& opts)
      *
      * t1 where t1 = the_whole_root_type
      */
-    auto it = state.cycleTpNames.find(tp);
+    if (auto p = state.cycleTpNames.find(tp))
-    if (it != state.cycleTpNames.end())
+        state.emit(*p);
-        state.emit(it->second);
     else
         tvs.stringify(tp);
 
@@ -1766,11 +1765,6 @@ std::string toString(const Constraint& constraint, ToStringOptions& opts)
         }
         else if constexpr (std::is_same_v<T, UnpackConstraint>)
             return tos(c.resultPack) + " ~ unpack " + tos(c.sourcePack);
-        else if constexpr (std::is_same_v<T, RefineConstraint>)
-        {
-            const char* op = c.mode == RefineConstraint::Union ? "union" : "intersect";
-            return tos(c.resultType) + " ~ refine " + tos(c.type) + " " + op + " " + tos(c.discriminant);
-        }
         else if constexpr (std::is_same_v<T, SetOpConstraint>)
         {
             const char* op = c.mode == SetOpConstraint::Union ? " | " : " & ";

Analysis/src/TypeAttach.cpp

@@ -106,7 +106,12 @@ public:
             return allocator->alloc<AstTypeReference>(Location(), std::nullopt, AstName("thread"), std::nullopt, Location());
         case PrimitiveType::Buffer:
             return allocator->alloc<AstTypeReference>(Location(), std::nullopt, AstName("buffer"), std::nullopt, Location());
+        case PrimitiveType::Function:
+            return allocator->alloc<AstTypeReference>(Location(), std::nullopt, AstName("function"), std::nullopt, Location());
+        case PrimitiveType::Table:
+            return allocator->alloc<AstTypeReference>(Location(), std::nullopt, AstName("table"), std::nullopt, Location());
         default:
+            LUAU_ASSERT(false); // this should be unreachable.
             return nullptr;
         }
     }

Analysis/src/TypeChecker2.cpp

@@ -1603,8 +1603,8 @@ struct TypeChecker2
         visit(indexExpr->expr, ValueContext::RValue);
         visit(indexExpr->index, ValueContext::RValue);
 
-        TypeId exprType = lookupType(indexExpr->expr);
+        TypeId exprType = follow(lookupType(indexExpr->expr));
-        TypeId indexType = lookupType(indexExpr->index);
+        TypeId indexType = follow(lookupType(indexExpr->index));
 
         if (auto tt = get<TableType>(exprType))
         {

Analysis/src/TypeFamily.cpp

@@ -349,7 +349,8 @@ TypeFamilyReductionResult<TypeId> lenFamilyFn(const std::vector<TypeId>& typePar
     TypeId operandTy = follow(typeParams.at(0));
 
     // check to see if the operand type is resolved enough, and wait to reduce if not
-    if (isPending(operandTy, ctx->solver))
+    // the use of `typeFromNormal` later necessitates blocking on local types.
+    if (isPending(operandTy, ctx->solver) || get<LocalType>(operandTy))
         return {std::nullopt, false, {operandTy}, {}};
 
     const NormalizedType* normTy = ctx->normalizer->normalize(operandTy);
@@ -964,6 +965,92 @@ TypeFamilyReductionResult<TypeId> eqFamilyFn(const std::vector<TypeId>& typePara
     return {ctx->builtins->booleanType, false, {}, {}};
 }
 
+// Collect types that prevent us from reducing a particular refinement.
+struct FindRefinementBlockers : TypeOnceVisitor
+{
+    DenseHashSet<TypeId> found{nullptr};
+    bool visit(TypeId ty, const BlockedType&) override
+    {
+        found.insert(ty);
+        return false;
+    }
+
+    bool visit(TypeId ty, const PendingExpansionType&) override
+    {
+        found.insert(ty);
+        return false;
+    }
+
+    bool visit(TypeId ty, const LocalType&) override
+    {
+        found.insert(ty);
+        return false;
+    }
+
+    bool visit(TypeId ty, const ClassType&) override
+    {
+        return false;
+    }
+};
+
+
+TypeFamilyReductionResult<TypeId> refineFamilyFn(const std::vector<TypeId>& typeParams, const std::vector<TypePackId>& packParams, NotNull<TypeFamilyContext> ctx)
+{
+    if (typeParams.size() != 2 || !packParams.empty())
+    {
+        ctx->ice->ice("refine type family: encountered a type family instance without the required argument structure");
+        LUAU_ASSERT(false);
+    }
+
+    TypeId targetTy = follow(typeParams.at(0));
+    TypeId discriminantTy = follow(typeParams.at(1));
+
+    // check to see if both operand types are resolved enough, and wait to reduce if not
+    if (isPending(targetTy, ctx->solver))
+        return {std::nullopt, false, {targetTy}, {}};
+    else if (isPending(discriminantTy, ctx->solver))
+        return {std::nullopt, false, {discriminantTy}, {}};
+
+    // we need a more complex check for blocking on the discriminant in particular
+    FindRefinementBlockers frb;
+    frb.traverse(discriminantTy);
+
+    if (!frb.found.empty())
+        return {std::nullopt, false, {frb.found.begin(), frb.found.end()}, {}};
+
+    /* HACK: Refinements sometimes produce a type T & ~any under the assumption
+     * that ~any is the same as any.  This is so so weird, but refinements needs
+     * some way to say "I may refine this, but I'm not sure."
+     *
+     * It does this by refining on a blocked type and deferring the decision
+     * until it is unblocked.
+     *
+     * Refinements also get negated, so we wind up with types like T & ~*blocked*
+     *
+     * We need to treat T & ~any as T in this case.
+     */
+
+    if (auto nt = get<NegationType>(discriminantTy))
+        if (get<AnyType>(follow(nt->ty)))
+            return {targetTy, false, {}, {}};
+
+    TypeId intersection = ctx->arena->addType(IntersectionType{{targetTy, discriminantTy}});
+    const NormalizedType* normIntersection = ctx->normalizer->normalize(intersection);
+    const NormalizedType* normType = ctx->normalizer->normalize(targetTy);
+
+    // if the intersection failed to normalize, we can't reduce, but know nothing about inhabitance.
+    if (!normIntersection || !normType)
+        return {std::nullopt, false, {}, {}};
+
+    TypeId resultTy = ctx->normalizer->typeFromNormal(*normIntersection);
+
+    // include the error type if the target type is error-suppressing and the intersection we computed is not
+    if (normType->shouldSuppressErrors() && !normIntersection->shouldSuppressErrors())
+        resultTy = ctx->arena->addType(UnionType{{resultTy, ctx->builtins->errorType}});
+
+    return {resultTy, false, {}, {}};
+}
+
 BuiltinTypeFamilies::BuiltinTypeFamilies()
     : notFamily{"not", notFamilyFn}
     , lenFamily{"len", lenFamilyFn}
@@ -981,6 +1068,7 @@ BuiltinTypeFamilies::BuiltinTypeFamilies()
     , ltFamily{"lt", ltFamilyFn}
     , leFamily{"le", leFamilyFn}
     , eqFamily{"eq", eqFamilyFn}
+    , refineFamily{"refine", refineFamilyFn}
 {
 }
 

CLI/Analyze.cpp

@@ -325,7 +325,7 @@ int main(int argc, char** argv)
         else if (strncmp(argv[i], "--fflags=", 9) == 0)
             setLuauFlags(argv[i] + 9);
         else if (strncmp(argv[i], "-j", 2) == 0)
-            threadCount = strtol(argv[i] + 2, nullptr, 10);
+            threadCount = int(strtol(argv[i] + 2, nullptr, 10));
     }
 
 #if !defined(LUAU_ENABLE_TIME_TRACE)
@@ -363,6 +363,7 @@ int main(int argc, char** argv)
     if (threadCount <= 0)
         threadCount = std::min(TaskScheduler::getThreadCount(), 8u);
 
+    try
     {
         TaskScheduler scheduler(threadCount);
 
@@ -370,6 +371,19 @@ int main(int argc, char** argv)
             scheduler.push(std::move(f));
         });
     }
+    catch (const Luau::InternalCompilerError& ice)
+    {
+        Luau::Location location = ice.location ? *ice.location : Luau::Location();
+
+        std::string moduleName = ice.moduleName ? *ice.moduleName : "<unknown module>";
+        std::string humanReadableName = frontend.fileResolver->getHumanReadableModuleName(moduleName);
+
+        Luau::TypeError error(location, moduleName, Luau::InternalError{ice.message});
+
+        report(format, humanReadableName.c_str(), location, "InternalCompilerError",
+            Luau::toString(error, Luau::TypeErrorToStringOptions{frontend.fileResolver}).c_str());
+        return 1;
+    }
 
     int failed = 0;
 

CLI/Require.cpp

@@ -22,16 +22,9 @@ RequireResolver::RequireResolver(lua_State* L, std::string path)
     if (isAbsolutePath(pathToResolve))
         luaL_argerrorL(L, 1, "cannot require an absolute path");
 
-    bool isAlias = !pathToResolve.empty() && pathToResolve[0] == '@';
-    if (!isAlias && !isExplicitlyRelative(pathToResolve))
-        luaL_argerrorL(L, 1, "must require an alias prepended with '@' or an explicitly relative path");
-
     std::replace(pathToResolve.begin(), pathToResolve.end(), '\\', '/');
-    if (isAlias)
+
-    {
+    substituteAliasIfPresent(pathToResolve);
-        pathToResolve = pathToResolve.substr(1);
-        substituteAliasIfPresent(pathToResolve);
-    }
 }
 
 [[nodiscard]] RequireResolver::ResolvedRequire RequireResolver::resolveRequire(lua_State* L, std::string path)
@@ -209,16 +202,22 @@ std::string RequireResolver::getRequiringContextRelative()
 
 void RequireResolver::substituteAliasIfPresent(std::string& path)
 {
-    std::string potentialAlias = path.substr(0, path.find_first_of("\\/"));
+    if (path.size() < 1 || path[0] != '@')
+        return;
+    std::string potentialAlias = path.substr(1, path.find_first_of("\\/"));
 
     // Not worth searching when potentialAlias cannot be an alias
     if (!Luau::isValidAlias(potentialAlias))
-        return;
+        luaL_errorL(L, "@%s is not a valid alias", potentialAlias.c_str());
 
     std::optional<std::string> alias = getAlias(potentialAlias);
     if (alias)
     {
-        path = *alias + path.substr(potentialAlias.size());
+        path = *alias + path.substr(potentialAlias.size() + 1);
+    }
+    else
+    {
+        luaL_errorL(L, "@%s is not a valid alias", potentialAlias.c_str());
     }
 }
 

Common/include/Luau/DenseHash.h

@@ -3,6 +3,7 @@
 
 #include "Luau/Common.h"
 
+#include <cstddef>
 #include <functional>
 #include <utility>
 #include <type_traits>
@@ -285,9 +286,8 @@ public:
         using value_type = Item;
         using reference = Item&;
         using pointer = Item*;
-        using iterator = pointer;
+        using difference_type = ptrdiff_t;
-        using difference_type = size_t;
+        using iterator_category = std::forward_iterator_tag;
-        using iterator_category = std::input_iterator_tag;
 
         const_iterator()
             : set(0)
@@ -348,6 +348,12 @@ public:
     class iterator
     {
     public:
+        using value_type = MutableItem;
+        using reference = MutableItem&;
+        using pointer = MutableItem*;
+        using difference_type = ptrdiff_t;
+        using iterator_category = std::forward_iterator_tag;
+
         iterator()
             : set(0)
             , index(0)

tests/AstJsonEncoder.test.cpp

@@ -438,7 +438,7 @@ TEST_CASE_FIXTURE(JsonEncoderFixture, "encode_AstStatDeclareClass")
     REQUIRE(2 == root->body.size);
 
     std::string_view expected1 =
-        R"({"type":"AstStatDeclareClass","location":"1,22 - 4,11","name":"Foo","props":[{"name":"prop","type":"AstDeclaredClassProp","luauType":{"type":"AstTypeReference","location":"2,18 - 2,24","name":"number","nameLocation":"2,18 - 2,24","parameters":[]}},{"name":"method","type":"AstDeclaredClassProp","luauType":{"type":"AstTypeFunction","location":"3,21 - 4,11","generics":[],"genericPacks":[],"argTypes":{"type":"AstTypeList","types":[{"type":"AstTypeReference","location":"3,39 - 3,45","name":"number","nameLocation":"3,39 - 3,45","parameters":[]}]},"returnTypes":{"type":"AstTypeList","types":[{"type":"AstTypeReference","location":"3,48 - 3,54","name":"string","nameLocation":"3,48 - 3,54","parameters":[]}]}}}],"indexer":null})";
+        R"({"type":"AstStatDeclareClass","location":"1,22 - 4,11","name":"Foo","props":[{"name":"prop","type":"AstDeclaredClassProp","luauType":{"type":"AstTypeReference","location":"2,18 - 2,24","name":"number","nameLocation":"2,18 - 2,24","parameters":[]}},{"name":"method","type":"AstDeclaredClassProp","luauType":{"type":"AstTypeFunction","location":"3,21 - 4,11","generics":[],"genericPacks":[],"argTypes":{"type":"AstTypeList","types":[{"type":"AstTypeReference","location":"3,39 - 3,45","name":"number","nameLocation":"3,39 - 3,45","parameters":[]}]},"argNames":[{"type":"AstArgumentName","name":"foo","location":"3,34 - 3,37"}],"returnTypes":{"type":"AstTypeList","types":[{"type":"AstTypeReference","location":"3,48 - 3,54","name":"string","nameLocation":"3,48 - 3,54","parameters":[]}]}}}],"indexer":null})";
     CHECK(toJson(root->body.data[0]) == expected1);
 
     std::string_view expected2 =
@@ -451,7 +451,39 @@ TEST_CASE_FIXTURE(JsonEncoderFixture, "encode_annotation")
     AstStat* statement = expectParseStatement("type T = ((number) -> (string | nil)) & ((string) -> ())");
 
     std::string_view expected =
-        R"({"type":"AstStatTypeAlias","location":"0,0 - 0,55","name":"T","generics":[],"genericPacks":[],"type":{"type":"AstTypeIntersection","location":"0,9 - 0,55","types":[{"type":"AstTypeFunction","location":"0,10 - 0,36","generics":[],"genericPacks":[],"argTypes":{"type":"AstTypeList","types":[{"type":"AstTypeReference","location":"0,11 - 0,17","name":"number","nameLocation":"0,11 - 0,17","parameters":[]}]},"returnTypes":{"type":"AstTypeList","types":[{"type":"AstTypeUnion","location":"0,23 - 0,35","types":[{"type":"AstTypeReference","location":"0,23 - 0,29","name":"string","nameLocation":"0,23 - 0,29","parameters":[]},{"type":"AstTypeReference","location":"0,32 - 0,35","name":"nil","nameLocation":"0,32 - 0,35","parameters":[]}]}]}},{"type":"AstTypeFunction","location":"0,41 - 0,55","generics":[],"genericPacks":[],"argTypes":{"type":"AstTypeList","types":[{"type":"AstTypeReference","location":"0,42 - 0,48","name":"string","nameLocation":"0,42 - 0,48","parameters":[]}]},"returnTypes":{"type":"AstTypeList","types":[]}}]},"exported":false})";
+        R"({"type":"AstStatTypeAlias","location":"0,0 - 0,55","name":"T","generics":[],"genericPacks":[],"type":{"type":"AstTypeIntersection","location":"0,9 - 0,55","types":[{"type":"AstTypeFunction","location":"0,10 - 0,36","generics":[],"genericPacks":[],"argTypes":{"type":"AstTypeList","types":[{"type":"AstTypeReference","location":"0,11 - 0,17","name":"number","nameLocation":"0,11 - 0,17","parameters":[]}]},"argNames":[],"returnTypes":{"type":"AstTypeList","types":[{"type":"AstTypeUnion","location":"0,23 - 0,35","types":[{"type":"AstTypeReference","location":"0,23 - 0,29","name":"string","nameLocation":"0,23 - 0,29","parameters":[]},{"type":"AstTypeReference","location":"0,32 - 0,35","name":"nil","nameLocation":"0,32 - 0,35","parameters":[]}]}]}},{"type":"AstTypeFunction","location":"0,41 - 0,55","generics":[],"genericPacks":[],"argTypes":{"type":"AstTypeList","types":[{"type":"AstTypeReference","location":"0,42 - 0,48","name":"string","nameLocation":"0,42 - 0,48","parameters":[]}]},"argNames":[],"returnTypes":{"type":"AstTypeList","types":[]}}]},"exported":false})";
+
+    CHECK(toJson(statement) == expected);
+}
+
+TEST_CASE_FIXTURE(JsonEncoderFixture, "encode_type_literal")
+{
+    AstStat* statement = expectParseStatement(R"(type Action = { strings: "A" | "B" | "C", mixed: "This" | "That" | true })");
+
+    auto json = toJson(statement);
+
+    std::string_view expected =
+        R"({"type":"AstStatTypeAlias","location":"0,0 - 0,73","name":"Action","generics":[],"genericPacks":[],"type":{"type":"AstTypeTable","location":"0,14 - 0,73","props":[{"name":"strings","type":"AstTableProp","location":"0,16 - 0,23","propType":{"type":"AstTypeUnion","location":"0,25 - 0,40","types":[{"type":"AstTypeSingletonString","location":"0,25 - 0,28","value":"A"},{"type":"AstTypeSingletonString","location":"0,31 - 0,34","value":"B"},{"type":"AstTypeSingletonString","location":"0,37 - 0,40","value":"C"}]}},{"name":"mixed","type":"AstTableProp","location":"0,42 - 0,47","propType":{"type":"AstTypeUnion","location":"0,49 - 0,71","types":[{"type":"AstTypeSingletonString","location":"0,49 - 0,55","value":"This"},{"type":"AstTypeSingletonString","location":"0,58 - 0,64","value":"That"},{"type":"AstTypeSingletonBool","location":"0,67 - 0,71","value":true}]}}],"indexer":null},"exported":false})";
+
+    CHECK(toJson(statement) == expected);
+}
+
+TEST_CASE_FIXTURE(JsonEncoderFixture, "encode_indexed_type_literal")
+{
+    AstStat* statement = expectParseStatement(R"(type StringSet = { [string]: true })");
+
+    std::string_view expected =
+        R"({"type":"AstStatTypeAlias","location":"0,0 - 0,35","name":"StringSet","generics":[],"genericPacks":[],"type":{"type":"AstTypeTable","location":"0,17 - 0,35","props":[],"indexer":{"location":"0,19 - 0,33","indexType":{"type":"AstTypeReference","location":"0,20 - 0,26","name":"string","nameLocation":"0,20 - 0,26","parameters":[]},"resultType":{"type":"AstTypeSingletonBool","location":"0,29 - 0,33","value":true}}},"exported":false})";
+
+    CHECK(toJson(statement) == expected);
+}
+
+TEST_CASE_FIXTURE(JsonEncoderFixture, "encode_AstTypeFunction")
+{
+    AstStat* statement = expectParseStatement(R"(type fun = (string, bool, named: number) -> ())");
+
+    std::string_view expected =
+            R"({"type":"AstStatTypeAlias","location":"0,0 - 0,46","name":"fun","generics":[],"genericPacks":[],"type":{"type":"AstTypeFunction","location":"0,11 - 0,46","generics":[],"genericPacks":[],"argTypes":{"type":"AstTypeList","types":[{"type":"AstTypeReference","location":"0,12 - 0,18","name":"string","nameLocation":"0,12 - 0,18","parameters":[]},{"type":"AstTypeReference","location":"0,20 - 0,24","name":"bool","nameLocation":"0,20 - 0,24","parameters":[]},{"type":"AstTypeReference","location":"0,33 - 0,39","name":"number","nameLocation":"0,33 - 0,39","parameters":[]}]},"argNames":[null,null,{"type":"AstArgumentName","name":"named","location":"0,26 - 0,31"}],"returnTypes":{"type":"AstTypeList","types":[]}},"exported":false})";
 
     CHECK(toJson(statement) == expected);
 }

tests/AstQuery.test.cpp

@@ -78,10 +78,14 @@ TEST_CASE_FIXTURE(DocumentationSymbolFixture, "class_method")
         declare class Foo
             function bar(self, x: string): number
         end
+
+        declare Foo: {
+            new: () -> Foo
+        }
     )");
 
     std::optional<DocumentationSymbol> symbol = getDocSymbol(R"(
-        local x: Foo
+        local x: Foo = Foo.new()
         x:bar("asdf")
     )",
         Position(2, 11));
@@ -96,10 +100,14 @@ TEST_CASE_FIXTURE(DocumentationSymbolFixture, "overloaded_class_method")
             function bar(self, x: string): number
             function bar(self, x: number): string
         end
+
+        declare Foo: {
+            new: () -> Foo
+        }
     )");
 
     std::optional<DocumentationSymbol> symbol = getDocSymbol(R"(
-        local x: Foo
+        local x: Foo = Foo.new()
         x:bar("asdf")
     )",
         Position(2, 11));

tests/Autocomplete.test.cpp

@@ -3265,8 +3265,9 @@ end
 
     {
         check(R"(
-local t: Foo
+local function f(t: Foo)
-t:@1
+    t:@1
+end
         )");
 
         auto ac = autocomplete('1');
@@ -3281,8 +3282,9 @@ t:@1
 
     {
         check(R"(
-local t: Foo
+local function f(t: Foo)
-t.@1
+    t.@1
+end
         )");
 
         auto ac = autocomplete('1');

tests/DataFlowGraph.test.cpp

@@ -1,5 +1,6 @@
 // This file is part of the Luau programming language and is licensed under MIT License; see LICENSE.txt for details
 #include "Luau/DataFlowGraph.h"
+#include "Fixture.h"
 #include "Luau/Error.h"
 #include "Luau/Parser.h"
 
@@ -560,4 +561,74 @@ TEST_CASE_FIXTURE(DataFlowGraphFixture, "local_f_which_is_prototyped_enclosed_by
     CHECK(phi->operands.at(1) == f4);
 }
 
+TEST_CASE_FIXTURE(DataFlowGraphFixture, "phi_node_if_case_binding")
+{
+    dfg(R"(
+local x = nil
+if true then
+    if true then
+        x = 5
+    end
+    print(x)
+else
+    print(x)
+end
+)");
+    DefId x1 = graph->getDef(query<AstStatLocal>(module)->vars.data[0]);
+    DefId x2 = getDef<AstExprLocal, 1>(); // x = 5
+    DefId x3 = getDef<AstExprLocal, 2>(); // print(x)
+
+    const Phi* phi = get<Phi>(x3);
+    REQUIRE(phi);
+    CHECK(phi->operands.at(0) == x2);
+    CHECK(phi->operands.at(1) == x1);
+}
+
+TEST_CASE_FIXTURE(DataFlowGraphFixture, "phi_node_if_case_table_prop")
+{
+    dfg(R"(
+local t = {}
+t.x = true
+if true then
+    if true then
+        t.x = 5
+    end
+    print(t.x)
+else
+    print(t.x)
+end
+)");
+
+    DefId x1 = getDef<AstExprIndexName, 1>(); // t.x = true
+    DefId x2 = getDef<AstExprIndexName, 2>(); // t.x = 5
+
+    DefId x3 = getDef<AstExprIndexName, 3>(); // print(t.x)
+    const Phi* phi = get<Phi>(x3);
+    REQUIRE(phi);
+    CHECK(phi->operands.size() == 2);
+    CHECK(phi->operands.at(0) == x1);
+    CHECK(phi->operands.at(1) == x2);
+}
+
+TEST_CASE_FIXTURE(DataFlowGraphFixture, "phi_node_if_case_table_prop_literal")
+{
+    dfg(R"(
+local t = { x = true }
+if true then
+    t.x = 5
+end
+print(t.x)
+
+)");
+
+    DefId x1 = getDef<AstExprConstantBool, 1>(); // {x = true <- }
+    DefId x2 = getDef<AstExprIndexName, 1>();    // t.x = 5
+    DefId x3 = getDef<AstExprIndexName, 2>();    // print(t.x)
+    const Phi* phi = get<Phi>(x3);
+    REQUIRE(phi);
+    CHECK(phi->operands.size() == 2);
+    CHECK(phi->operands.at(0) == x1);
+    CHECK(phi->operands.at(1) == x2);
+}
+
 TEST_SUITE_END();

tests/NonStrictTypeChecker.test.cpp

@@ -81,12 +81,41 @@ declare function @checked abs(n: number): number
 declare function @checked lower(s: string): string
 declare function cond() : boolean
 declare function @checked contrived(n : Not<number>) : number
+
+-- interesting types of things that we would like to mark as checked
+declare function @checked onlyNums(...: number) : number
+declare function @checked mixedArgs(x: string, ...: number) : number
+declare function @checked optionalArg(x: string?) : number
 )BUILTIN_SRC";
 };
 
-
 TEST_SUITE_BEGIN("NonStrictTypeCheckerTest");
 
+TEST_CASE_FIXTURE(NonStrictTypeCheckerFixture, "interesting_checked_functions")
+{
+    CheckResult result = checkNonStrict(R"(
+onlyNums(1,1,1)
+onlyNums(1, "a")
+
+mixedArgs("a", 1, 2)
+mixedArgs(1, 1, 1)
+mixedArgs("a", true)
+
+optionalArg(nil)
+optionalArg("a")
+optionalArg(3)
+)");
+
+    LUAU_REQUIRE_ERROR_COUNT(4, result);
+    NONSTRICT_REQUIRE_CHECKED_ERR(Position(2, 12), "onlyNums", result); // onlyNums(1, "a")
+
+    NONSTRICT_REQUIRE_CHECKED_ERR(Position(5, 10), "mixedArgs", result); // mixedArgs(1, 1, 1)
+    NONSTRICT_REQUIRE_CHECKED_ERR(Position(6, 15), "mixedArgs", result); // mixedArgs("a", true)
+
+    NONSTRICT_REQUIRE_CHECKED_ERR(Position(10, 12), "optionalArg", result); // optionalArg(3)
+}
+
+
 TEST_CASE_FIXTURE(NonStrictTypeCheckerFixture, "simple_negation_caching_example")
 {
     CheckResult result = checkNonStrict(R"(
@@ -387,4 +416,35 @@ lower(x)
     NONSTRICT_REQUIRE_CHECKED_ERR(Position(2, 6), "lower", result);
 }
 
+TEST_CASE_FIXTURE(NonStrictTypeCheckerFixture, "phi_node_assignment")
+{
+    CheckResult result = checkNonStrict(R"(
+local x = "a" -- x1
+if cond() then
+    x = 3 -- x2
+end
+lower(x) -- phi {x1, x2}
+)");
+
+    LUAU_REQUIRE_NO_ERRORS(result);
+}
+
+TEST_CASE_FIXTURE(NonStrictTypeCheckerFixture, "phi_node_assignment_err")
+{
+    CheckResult result = checkNonStrict(R"(
+local x = nil
+if cond() then
+    if cond() then
+        x = 5
+    end
+    abs(x)
+else
+    lower(x)
+end
+)");
+
+    LUAU_REQUIRE_ERROR_COUNT(1, result);
+    NONSTRICT_REQUIRE_CHECKED_ERR(Position(8, 10), "lower", result);
+}
+
 TEST_SUITE_END();

tests/RequireByString.test.cpp

@@ -394,4 +394,39 @@ TEST_CASE_FIXTURE(ReplWithPathFixture, "RequirePathWithParentAlias")
     assertOutputContainsAll({"true", "result from other_dependency"});
 }
 
+
+TEST_CASE_FIXTURE(ReplWithPathFixture, "RequireAliasThatDoesNotExist")
+{
+    ScopedFastFlag sff{FFlag::LuauUpdatedRequireByStringSemantics, true};
+    std::string nonExistentAlias = "@this.alias.does.not.exist";
+
+    runProtectedRequire(nonExistentAlias);
+    assertOutputContainsAll({"false", "@this.alias.does.not.exist is not a valid alias"});
+}
+
+TEST_CASE_FIXTURE(ReplWithPathFixture, "AliasHasIllegalFormat")
+{
+    ScopedFastFlag sff{FFlag::LuauUpdatedRequireByStringSemantics, true};
+    std::string illegalCharacter = "@@";
+
+    runProtectedRequire(illegalCharacter);
+    assertOutputContainsAll({"false", "@@ is not a valid alias"});
+
+    std::string pathAlias1 = "@.";
+
+    runProtectedRequire(pathAlias1);
+    assertOutputContainsAll({"false", ". is not a valid alias"});
+
+
+    std::string pathAlias2 = "@..";
+
+    runProtectedRequire(pathAlias2);
+    assertOutputContainsAll({"false", ".. is not a valid alias"});
+
+    std::string emptyAlias = "@";
+
+    runProtectedRequire(emptyAlias);
+    assertOutputContainsAll({"false", " is not a valid alias"});
+}
+
 TEST_SUITE_END();

tests/TypeInfer.builtins.test.cpp

@@ -938,6 +938,11 @@ TEST_CASE_FIXTURE(BuiltinsFixture, "tonumber_returns_optional_number_type2")
 
 TEST_CASE_FIXTURE(BuiltinsFixture, "dont_add_definitions_to_persistent_types")
 {
+    // This test makes no sense with type states and I think it generally makes no sense under the new solver.
+    // TODO: clip.
+    if (FFlag::DebugLuauDeferredConstraintResolution)
+        return;
+
     CheckResult result = check(R"(
         local f = math.sin
         local function g(x) return math.sin(x) end
@@ -1093,7 +1098,7 @@ end
 TEST_CASE_FIXTURE(Fixture, "string_match")
 {
     CheckResult result = check(R"(
-        local s:string
+        local s: string = "hello"
         local p = s:match("foo")
     )");
 

tests/TypeInfer.definitions.test.cpp

@@ -230,10 +230,14 @@ TEST_CASE_FIXTURE(Fixture, "class_definition_function_prop")
         declare class Foo
             X: (number) -> string
         end
+
+        declare Foo: {
+            new: () -> Foo
+        }
     )");
 
     CheckResult result = check(R"(
-        local x: Foo
+        local x: Foo = Foo.new()
         local prop = x.X
     )");
 
@@ -250,10 +254,14 @@ TEST_CASE_FIXTURE(Fixture, "definition_file_class_function_args")
 
             y: (a: number, b: string) -> string
         end
+
+        declare Foo: {
+            new: () -> Foo
+        }
     )");
 
     CheckResult result = check(R"(
-        local x: Foo
+        local x: Foo = Foo.new()
         local methodRef1 = x.foo1
         local methodRef2 = x.foo2
         local prop = x.y

tests/TypeInfer.functions.test.cpp

@@ -2183,4 +2183,19 @@ TEST_CASE_FIXTURE(BuiltinsFixture, "apply_of_lambda_with_inferred_and_explicit_t
     LUAU_REQUIRE_NO_ERRORS(result);
 }
 
+TEST_CASE_FIXTURE(BuiltinsFixture, "regex_benchmark_string_format_minimization")
+{
+    CheckResult result = check(R"(
+        (nil :: any)(function(n)
+            if tonumber(n) then
+                n = tonumber(n)
+            elseif n ~= nil then
+                string.format("invalid argument #4 to 'sub': number expected, got %s", typeof(n))
+            end
+        end);
+    )");
+
+    LUAU_REQUIRE_NO_ERRORS(result);
+}
+
 TEST_SUITE_END();

tests/TypeInfer.generics.test.cpp

@@ -304,8 +304,15 @@ TEST_CASE_FIXTURE(Fixture, "calling_self_generic_methods")
         end
     )");
 
-    // TODO: Should typecheck but currently errors CLI-54277
+    if (FFlag::DebugLuauDeferredConstraintResolution)
-    LUAU_REQUIRE_ERRORS(result);
+    {
+        LUAU_REQUIRE_NO_ERRORS(result);
+
+        CHECK_EQ("{ f: (t1) -> (), id: <a>(unknown, a) -> a } where t1 = { id: ((t1, number) -> number) & ((t1, string) -> string) }",
+            toString(requireType("x"), {true}));
+    }
+    else
+        LUAU_REQUIRE_ERRORS(result);
 }
 
 TEST_CASE_FIXTURE(Fixture, "infer_generic_property")
@@ -692,7 +699,7 @@ local d: D = c
 TEST_CASE_FIXTURE(BuiltinsFixture, "generic_functions_dont_cache_type_parameters")
 {
     CheckResult result = check(R"(
--- See https://github.com/Roblox/luau/issues/332
+-- See https://github.com/luau-lang/luau/issues/332
 -- This function has a type parameter with the same name as clones,
 -- so if we cache type parameter names for functions these get confused.
 -- function id<Z>(x : Z) : Z
@@ -1147,7 +1154,7 @@ TEST_CASE_FIXTURE(Fixture, "substitution_with_bound_table")
 
 TEST_CASE_FIXTURE(Fixture, "apply_type_function_nested_generics1")
 {
-    // https://github.com/Roblox/luau/issues/484
+    // https://github.com/luau-lang/luau/issues/484
     CheckResult result = check(R"(
 --!strict
 type MyObject = {
@@ -1175,7 +1182,7 @@ local complex: ComplexObject<string> = {
 
 TEST_CASE_FIXTURE(Fixture, "apply_type_function_nested_generics2")
 {
-    // https://github.com/Roblox/luau/issues/484
+    // https://github.com/luau-lang/luau/issues/484
     CheckResult result = check(R"(
 --!strict
 type MyObject = {
@@ -1186,15 +1193,15 @@ type ComplexObject<T> = {
 	nested: MyObject
 }
 
-local complex2: ComplexObject<string> = nil
+function f(complex: ComplexObject<string>)
+    local x = complex.nested.getReturnValue(function(): string
+        return ""
+    end)
 
-local x = complex2.nested.getReturnValue(function(): string
+    local y = complex.nested.getReturnValue(function()
-	return ""
+        return 3
-end)
+    end)
-
+end
-local y = complex2.nested.getReturnValue(function()
-	return 3
-end)
     )");
 
     LUAU_REQUIRE_NO_ERRORS(result);

tests/TypeInfer.refinements.test.cpp

@@ -1855,7 +1855,9 @@ TEST_CASE_FIXTURE(BuiltinsFixture, "type_annotations_arent_relevant_when_doing_d
 
 TEST_CASE_FIXTURE(BuiltinsFixture, "function_call_with_colon_after_refining_not_to_be_nil")
 {
-    ScopedFastFlag sff{FFlag::DebugLuauDeferredConstraintResolution, true};
+    // don't run this test at all without DCR
+    if (!FFlag::DebugLuauDeferredConstraintResolution)
+        return;
 
     CheckResult result = check(R"(
         --!strict

tests/TypeInfer.singletons.test.cpp

@@ -3,7 +3,6 @@
 #include "Fixture.h"
 
 #include "doctest.h"
-#include "Luau/BuiltinDefinitions.h"
 
 using namespace Luau;
 
@@ -141,9 +140,9 @@ TEST_CASE_FIXTURE(Fixture, "overloaded_function_call_with_singletons")
 TEST_CASE_FIXTURE(Fixture, "overloaded_function_call_with_singletons_mismatch")
 {
     CheckResult result = check(R"(
-        function f(a, b) end
+        function f(g: ((true, string) -> ()) & ((false, number) -> ()))
-        local g : ((true, string) -> ()) & ((false, number) -> ()) = (f::any)
+            g(true, 37)
-        g(true, 37)
+        end
     )");
 
     LUAU_REQUIRE_ERROR_COUNT(2, result);
@@ -429,7 +428,11 @@ TEST_CASE_FIXTURE(Fixture, "widening_happens_almost_everywhere")
     )");
 
     LUAU_REQUIRE_NO_ERRORS(result);
-    CHECK_EQ("string", toString(requireType("copy")));
+
+    if (FFlag::DebugLuauDeferredConstraintResolution)
+        CHECK_EQ(R"("foo")", toString(requireType("copy")));
+    else
+        CHECK_EQ("string", toString(requireType("copy")));
 }
 
 TEST_CASE_FIXTURE(Fixture, "widening_happens_almost_everywhere_except_for_tables")

tests/TypeInfer.typestates.test.cpp

@@ -292,6 +292,7 @@ TEST_CASE_FIXTURE(TypeStateFixture, "invalidate_type_refinements_upon_assignment
     LUAU_REQUIRE_NO_ERRORS(result);
 }
 
+#if 0
 TEST_CASE_FIXTURE(TypeStateFixture, "local_t_is_assigned_a_fresh_table_with_x_assigned_a_union_and_then_assert_restricts_actual_outflow_of_types")
 {
     CheckResult result = check(R"(
@@ -314,6 +315,7 @@ TEST_CASE_FIXTURE(TypeStateFixture, "local_t_is_assigned_a_fresh_table_with_x_as
     // CHECK("boolean | string" == toString(requireType("x")));
     CHECK("boolean | number | number | string" == toString(requireType("x")));
 }
+#endif
 
 TEST_CASE_FIXTURE(TypeStateFixture, "captured_locals_are_unions_of_all_assignments")
 {
@@ -399,4 +401,21 @@ TEST_CASE_FIXTURE(TypeStateFixture, "prototyped_recursive_functions_but_has_prev
     CHECK("((() -> ()) | number)?" == toString(requireType("f")));
 }
 
+TEST_CASE_FIXTURE(TypeStateFixture, "multiple_assignments_in_loops")
+{
+    CheckResult result = check(R"(
+        local x = nil
+
+        for i = 1, 10 do
+            x = 5
+            x = "hello"
+        end
+
+        print(x)
+    )");
+
+    LUAU_REQUIRE_NO_ERRORS(result);
+    CHECK("(number | string)?" == toString(requireType("x")));
+}
+
 TEST_SUITE_END();

tests/TypeInfer.unionTypes.test.cpp

@@ -45,8 +45,9 @@ TEST_CASE_FIXTURE(Fixture, "allow_specific_assign")
 TEST_CASE_FIXTURE(Fixture, "allow_more_specific_assign")
 {
     CheckResult result = check(R"(
-        local a:number|string = 22
+        function f(a: number | string, b: (number | string)?)
-        local b:number|string|nil = a
+            b = a
+        end
     )");
 
     LUAU_REQUIRE_NO_ERRORS(result);
@@ -55,25 +56,14 @@ TEST_CASE_FIXTURE(Fixture, "allow_more_specific_assign")
 TEST_CASE_FIXTURE(Fixture, "disallow_less_specific_assign")
 {
     CheckResult result = check(R"(
-        local a:number = 10
+        function f(a: number, b: number | string)
-        local b:number|string = 20
+            a = b
-        a = b
+        end
     )");
 
     LUAU_REQUIRE_ERROR_COUNT(1, result);
 }
 
-TEST_CASE_FIXTURE(Fixture, "disallow_less_specific_assign2")
-{
-    CheckResult result = check(R"(
-        local a:number? = 10
-        local b:number|string? = 20
-        a = b
-    )");
-
-    REQUIRE_EQ(1, result.errors.size());
-}
-
 TEST_CASE_FIXTURE(Fixture, "optional_arguments")
 {
     CheckResult result = check(R"(
@@ -128,13 +118,14 @@ TEST_CASE_FIXTURE(Fixture, "index_on_a_union_type_with_property_guaranteed_to_ex
     CheckResult result = check(R"(
         type A = {x: number}
         type B = {x: number}
-        local t: A | B
 
-        local r = t.x
+        function f(t: A | B)
+            return t.x
+        end
     )");
 
     LUAU_REQUIRE_NO_ERRORS(result);
-    CHECK_EQ(*builtinTypes->numberType, *requireType("r"));
+    CHECK_EQ("(A | B) -> number", toString(requireType("f")));
 }
 
 TEST_CASE_FIXTURE(Fixture, "index_on_a_union_type_with_mixed_types")
@@ -142,13 +133,14 @@ TEST_CASE_FIXTURE(Fixture, "index_on_a_union_type_with_mixed_types")
     CheckResult result = check(R"(
         type A = {x: number}
         type B = {x: string}
-        local t: A | B
 
-        local r = t.x
+        function f(t: A | B)
+            return t.x
+        end
     )");
 
     LUAU_REQUIRE_NO_ERRORS(result);
-    CHECK_EQ("number | string", toString(requireType("r")));
+    CHECK_EQ("(A | B) -> number | string", toString(requireType("f")));
 }
 
 TEST_CASE_FIXTURE(Fixture, "index_on_a_union_type_works_at_arbitrary_depth")
@@ -156,13 +148,14 @@ TEST_CASE_FIXTURE(Fixture, "index_on_a_union_type_works_at_arbitrary_depth")
     CheckResult result = check(R"(
         type A = {x: {y: {z: {thing: number}}}}
         type B = {x: {y: {z: {thing: string}}}}
-        local t: A | B
 
-        local r = t.x.y.z.thing
+        function f(t: A | B)
+            return t.x.y.z.thing
+        end
     )");
 
     LUAU_REQUIRE_NO_ERRORS(result);
-    CHECK_EQ("number | string", toString(requireType("r")));
+    CHECK_EQ("(A | B) -> number | string", toString(requireType("f")));
 }
 
 TEST_CASE_FIXTURE(Fixture, "index_on_a_union_type_with_one_optional_property")
@@ -170,13 +163,14 @@ TEST_CASE_FIXTURE(Fixture, "index_on_a_union_type_with_one_optional_property")
     CheckResult result = check(R"(
         type A = {x: number}
         type B = {x: number?}
-        local t: A | B
 
-        local r = t.x
+        function f(t: A | B)
+            return t.x
+        end
     )");
 
     LUAU_REQUIRE_NO_ERRORS(result);
-    CHECK_EQ("number?", toString(requireType("r")));
+    CHECK_EQ("(A | B) -> number?", toString(requireType("f")));
 }
 
 TEST_CASE_FIXTURE(Fixture, "index_on_a_union_type_with_missing_property")
@@ -184,22 +178,22 @@ TEST_CASE_FIXTURE(Fixture, "index_on_a_union_type_with_missing_property")
     CheckResult result = check(R"(
         type A = {x: number}
         type B = {}
-        local t: A | B
 
-        local r = t.x
+        function f(t: A | B)
+            return t.x
+        end
     )");
 
     LUAU_REQUIRE_ERROR_COUNT(1, result);
 
     MissingUnionProperty* mup = get<MissingUnionProperty>(result.errors[0]);
     REQUIRE(mup);
-    CHECK_EQ(mup->type, requireType("t"));
+    CHECK_EQ("Key 'x' is missing from 'B' in the type 'A | B'", toString(result.errors[0]));
-    REQUIRE(mup->missing.size() == 1);
+
-    std::optional<TypeId> bTy = lookupType("B");
+    if (FFlag::DebugLuauDeferredConstraintResolution)
-    REQUIRE(bTy);
+        CHECK_EQ("(A | B) -> number | *error-type*", toString(requireType("f")));
-    CHECK_EQ(mup->missing[0], *bTy);
+    else
-    CHECK_EQ(mup->key, "x");
+        CHECK_EQ("(A | B) -> *error-type*", toString(requireType("f")));
-    CHECK_EQ("*error-type*", toString(requireType("r")));
 }
 
 TEST_CASE_FIXTURE(Fixture, "index_on_a_union_type_with_one_property_of_type_any")
@@ -207,13 +201,14 @@ TEST_CASE_FIXTURE(Fixture, "index_on_a_union_type_with_one_property_of_type_any"
     CheckResult result = check(R"(
         type A = {x: number}
         type B = {x: any}
-        local t: A | B
 
-        local r = t.x
+        function f(t: A | B)
+            return t.x
+        end
     )");
 
     LUAU_REQUIRE_NO_ERRORS(result);
-    CHECK_EQ(*builtinTypes->anyType, *requireType("r"));
+    CHECK_EQ("(A | B) -> any", toString(requireType("f")));
 }
 
 TEST_CASE_FIXTURE(Fixture, "union_equality_comparisons")
@@ -223,14 +218,13 @@ TEST_CASE_FIXTURE(Fixture, "union_equality_comparisons")
         type B = number | nil
         type C = number | boolean
 
-        local a = 1 :: A
+        function f(a: A, b: B, c: C)
-        local b = nil :: B
+            local n = 1
-        local c = true :: C
-        local n = 1
 
-        local x = a == b
+            local x = a == b
-        local y = a == n
+            local y = a == n
-        local z = a == c
+            local z = a == c
+        end
     )");
 
     LUAU_REQUIRE_NO_ERRORS(result);
@@ -239,16 +233,21 @@ TEST_CASE_FIXTURE(Fixture, "union_equality_comparisons")
 TEST_CASE_FIXTURE(Fixture, "optional_union_members")
 {
     CheckResult result = check(R"(
-local a = { a = { x = 1, y = 2 }, b = 3 }
+        local a = { a = { x = 1, y = 2 }, b = 3 }
-type A = typeof(a)
+        type A = typeof(a)
-local b: A? = a
+        function f(b: A?)
-local bf = b
+            return b.a.y
-local c = bf.a.y
+        end
     )");
 
     LUAU_REQUIRE_ERROR_COUNT(1, result);
-    CHECK_EQ(*builtinTypes->numberType, *requireType("c"));
+
     CHECK_EQ("Value of type 'A?' could be nil", toString(result.errors[0]));
+
+    if (FFlag::DebugLuauDeferredConstraintResolution)
+        CHECK_EQ("(A?) -> number | *error-type*", toString(requireType("f")));
+    else
+        CHECK_EQ("(A?) -> number", toString(requireType("f")));
 }
 
 TEST_CASE_FIXTURE(Fixture, "optional_union_functions")
@@ -257,37 +256,49 @@ TEST_CASE_FIXTURE(Fixture, "optional_union_functions")
         local a = {}
         function a.foo(x:number, y:number) return x + y end
         type A = typeof(a)
-        local b: A? = a
+        function f(b: A?)
-        local c = b.foo(1, 2)
+            return b.foo(1, 2)
+        end
     )");
 
     LUAU_REQUIRE_ERROR_COUNT(1, result);
-    CHECK_EQ(*builtinTypes->numberType, *requireType("c"));
+
     CHECK_EQ("Value of type 'A?' could be nil", toString(result.errors[0]));
+
+    if (FFlag::DebugLuauDeferredConstraintResolution)
+        CHECK_EQ("(A?) -> number | *error-type*", toString(requireType("f")));
+    else
+        CHECK_EQ("(A?) -> number", toString(requireType("f")));
 }
 
 TEST_CASE_FIXTURE(Fixture, "optional_union_methods")
 {
     CheckResult result = check(R"(
-local a = {}
+        local a = {}
-function a:foo(x:number, y:number) return x + y end
+        function a:foo(x:number, y:number) return x + y end
-type A = typeof(a)
+        type A = typeof(a)
-local b: A? = a
+        function f(b: A?)
-local c = b:foo(1, 2)
+            return b:foo(1, 2)
+        end
     )");
 
     LUAU_REQUIRE_ERROR_COUNT(1, result);
-    CHECK_EQ(*builtinTypes->numberType, *requireType("c"));
+
     CHECK_EQ("Value of type 'A?' could be nil", toString(result.errors[0]));
+
+    if (FFlag::DebugLuauDeferredConstraintResolution)
+        CHECK_EQ("(A?) -> number | *error-type*", toString(requireType("f")));
+    else
+        CHECK_EQ("(A?) -> number", toString(requireType("f")));
 }
 
 TEST_CASE_FIXTURE(Fixture, "optional_union_follow")
 {
     CheckResult result = check(R"(
-local y: number? = 2
+        local y: number? = 2
-local x = y
+        local x = y
-local function f(a: number, b: typeof(x), c: typeof(x)) return -a end
+        function f(a: number, b: number?, c: number?) return -a end
-return f()
+        return f()
     )");
 
     LUAU_REQUIRE_ERROR_COUNT(1, result);
@@ -302,10 +313,11 @@ return f()
 TEST_CASE_FIXTURE(Fixture, "optional_field_access_error")
 {
     CheckResult result = check(R"(
-type A = { x: number }
+        type A = { x: number }
-local b: A? = { x = 2 }
+        function f(b: A?)
-local c = b.x
+            local c = b.x
-local d = b.y
+            local d = b.y
+        end
     )");
 
     LUAU_REQUIRE_ERROR_COUNT(3, result);
@@ -317,9 +329,10 @@ local d = b.y
 TEST_CASE_FIXTURE(Fixture, "optional_index_error")
 {
     CheckResult result = check(R"(
-type A = {number}
+        type A = {number}
-local a: A? = {1, 2, 3}
+        function f(a: A?)
-local b = a[1]
+            local b = a[1]
+        end
     )");
 
     LUAU_REQUIRE_ERROR_COUNT(1, result);
@@ -329,9 +342,10 @@ local b = a[1]
 TEST_CASE_FIXTURE(Fixture, "optional_call_error")
 {
     CheckResult result = check(R"(
-type A = (number) -> number
+        type A = (number) -> number
-local a: A? = function(a) return -a end
+        function f(a: A?)
-local b = a(4)
+            local b = a(4)
+        end
     )");
 
     LUAU_REQUIRE_ERROR_COUNT(1, result);
@@ -341,18 +355,23 @@ local b = a(4)
 TEST_CASE_FIXTURE(Fixture, "optional_assignment_errors")
 {
     CheckResult result = check(R"(
-type A = { x: number }
+        type A = { x: number }
-local a: A? = { x = 2 }
+        function f(a: A?)
-a.x = 2
+            a.x = 2
+        end
     )");
 
     LUAU_REQUIRE_ERROR_COUNT(1, result);
     CHECK_EQ("Value of type 'A?' could be nil", toString(result.errors[0]));
+}
 
-    result = check(R"(
+TEST_CASE_FIXTURE(Fixture, "optional_assignment_errors_2")
-type A = { x: number } & { y: number }
+{
-local a: A? = { x = 2, y = 3 }
+    CheckResult result = check(R"(
-a.x = 2
+        type A = { x: number } & { y: number }
+        function f(a: A?)
+            a.x = 2
+        end
     )");
 
     LUAU_REQUIRE_ERROR_COUNT(1, result);
@@ -366,9 +385,10 @@ a.x = 2
 TEST_CASE_FIXTURE(Fixture, "optional_length_error")
 {
     CheckResult result = check(R"(
-type A = {number}
+        type A = {number}
-local a: A? = {1, 2, 3}
+        function f(a: A?)
-local b = #a
+            local b = #a
+        end
     )");
 
     LUAU_REQUIRE_ERROR_COUNT(1, result);
@@ -378,27 +398,27 @@ local b = #a
 TEST_CASE_FIXTURE(Fixture, "optional_missing_key_error_details")
 {
     CheckResult result = check(R"(
-type A = { x: number, y: number }
+        type A = { x: number, y: number }
-type B = { x: number, y: number }
+        type B = { x: number, y: number }
-type C = { x: number }
+        type C = { x: number }
-type D = { x: number }
+        type D = { x: number }
 
-local a: A|B|C|D
+        function f(a: A | B | C | D)
-local b = a.y
+            local y = a.y
+            local z = a.z
+        end
 
-local c: A|(B|C)?|D
+        function g(c: A | B | C | D | nil)
-local d = c.y
+            local d = c.y
-
+        end
-local e = a.z
     )");
 
     LUAU_REQUIRE_ERROR_COUNT(4, result);
     CHECK_EQ("Key 'y' is missing from 'C', 'D' in the type 'A | B | C | D'", toString(result.errors[0]));
+    CHECK_EQ("Type 'A | B | C | D' does not have key 'z'", toString(result.errors[1]));
 
-    CHECK_EQ("Value of type '(A | B | C | D)?' could be nil", toString(result.errors[1]));
+    CHECK_EQ("Value of type '(A | B | C | D)?' could be nil", toString(result.errors[2]));
-    CHECK_EQ("Key 'y' is missing from 'C', 'D' in the type 'A | B | C | D'", toString(result.errors[2]));
+    CHECK_EQ("Key 'y' is missing from 'C', 'D' in the type 'A | B | C | D'", toString(result.errors[3]));
-
-    CHECK_EQ("Type 'A | B | C | D' does not have key 'z'", toString(result.errors[3]));
 }
 
 TEST_CASE_FIXTURE(Fixture, "optional_iteration")
@@ -470,25 +490,27 @@ type Z = { z: number }
 
 type XYZ = X | Y | Z
 
-local a: XYZ
+function f(a: XYZ)
-local b: { w: number } = a
+    local b: { w: number } = a
+end
     )");
 
     LUAU_REQUIRE_ERROR_COUNT(1, result);
-    TypeMismatch* tm = get<TypeMismatch>(result.errors[0]);
-    REQUIRE(tm);
 
-    CHECK_EQ(tm->reason, "Not all union options are compatible.");
+    if (FFlag::DebugLuauDeferredConstraintResolution)
-
+    {
-    CHECK_EQ("X | Y | Z", toString(tm->givenType));
+        CHECK_EQ(toString(result.errors[0]),
-
+            "Type 'X | Y | Z' could not be converted into '{ w: number }'; type X | Y | Z[0] (X) is not a subtype of { w: number } ({ w: number })\n\t"
-    const TableType* expected = get<TableType>(tm->wantedType);
+            "type X | Y | Z[1] (Y) is not a subtype of { w: number } ({ w: number })\n\t"
-    REQUIRE(expected);
+            "type X | Y | Z[2] (Z) is not a subtype of { w: number } ({ w: number })");
-    CHECK_EQ(TableState::Sealed, expected->state);
+    }
-    CHECK_EQ(1, expected->props.size());
+    else
-    auto propW = expected->props.find("w");
+    {
-    REQUIRE_NE(expected->props.end(), propW);
+        CHECK_EQ(toString(result.errors[0]), R"(Type 'X | Y | Z' could not be converted into '{| w: number |}'
-    CHECK_EQ("number", toString(propW->second.type()));
+caused by:
+  Not all union options are compatible.
+Table type 'X' not compatible with type '{| w: number |}' because the former is missing field 'w')");
+    }
 }
 
 TEST_CASE_FIXTURE(Fixture, "error_detailed_union_all")
@@ -545,14 +567,14 @@ TEST_CASE_FIXTURE(BuiltinsFixture, "table_union_write_indirect")
     CheckResult result = check(R"(
         type A = { x: number, y: (number) -> string } | { z: number, y: (number) -> string }
 
-        local a:A = nil
+        function f(a: A)
+            function a.y(x)
+                return tostring(x * 2)
+            end
 
-        function a.y(x)
+            function a.y(x: string): number
-            return tostring(x * 2)
+                return tonumber(x) or 0
-        end
+            end
-
-        function a.y(x: string): number
-            return tonumber(x) or 0
         end
     )");
 
@@ -568,12 +590,13 @@ could not be converted into
 TEST_CASE_FIXTURE(Fixture, "union_true_and_false")
 {
     CheckResult result = check(R"(
-        local x : boolean
+        function f(x : boolean)
-        local y1 : (true | false) = x -- OK
+            local y1 : (true | false) = x -- OK
-        local y2 : (true | false | (string & number)) = x -- OK
+            local y2 : (true | false | (string & number)) = x -- OK
-        local y3 : (true | (string & number) | false) = x -- OK
+            local y3 : (true | (string & number) | false) = x -- OK
-        local y4 : (true | (boolean & true) | false) = x -- OK
+            local y4 : (true | (boolean & true) | false) = x -- OK
-     )");
+        end
+    )");
 
     LUAU_REQUIRE_NO_ERRORS(result);
 }
@@ -581,8 +604,9 @@ TEST_CASE_FIXTURE(Fixture, "union_true_and_false")
 TEST_CASE_FIXTURE(Fixture, "union_of_functions")
 {
     CheckResult result = check(R"(
-        local x : (number) -> number?
+        function f(x : (number) -> number?)
-        local y : ((number?) -> number?) | ((number) -> number) = x -- OK
+            local y : ((number?) -> number?) | ((number) -> number) = x -- OK
+        end
      )");
 
     LUAU_REQUIRE_NO_ERRORS(result);
@@ -591,8 +615,9 @@ TEST_CASE_FIXTURE(Fixture, "union_of_functions")
 TEST_CASE_FIXTURE(Fixture, "union_of_generic_functions")
 {
     CheckResult result = check(R"(
-        local x : <a>(a) -> a?
+        function f(x : <a>(a) -> a?)
-        local y : (<a>(a?) -> a?) | (<b>(b) -> b) = x -- Not OK
+            local y : (<a>(a?) -> a?) | (<b>(b) -> b) = x -- Not OK
+        end
      )");
 
     // TODO: should this example typecheck?
@@ -602,8 +627,9 @@ TEST_CASE_FIXTURE(Fixture, "union_of_generic_functions")
 TEST_CASE_FIXTURE(Fixture, "union_of_generic_typepack_functions")
 {
     CheckResult result = check(R"(
-        local x : <a...>(number, a...) -> (number?, a...)
+        function f(x : <a...>(number, a...) -> (number?, a...))
-        local y : (<a...>(number?, a...) -> (number?, a...)) | (<b...>(number, b...) -> (number, b...)) = x -- Not OK
+            local y : (<a...>(number?, a...) -> (number?, a...)) | (<b...>(number, b...) -> (number, b...)) = x -- Not OK
+        end
      )");
 
     // TODO: should this example typecheck?
@@ -613,12 +639,13 @@ TEST_CASE_FIXTURE(Fixture, "union_of_generic_typepack_functions")
 TEST_CASE_FIXTURE(Fixture, "union_of_functions_mentioning_generics")
 {
     CheckResult result = check(R"(
-      function f<a,b>()
+        function f<a,b>()
-        local x : (a) -> a?
+            function g(x : (a) -> a?)
-        local y : ((a?) -> nil) | ((a) -> a) = x -- OK
+                local y : ((a?) -> nil) | ((a) -> a) = x -- OK
-        local z : ((b?) -> nil) | ((b) -> b) = x -- Not OK
+                local z : ((b?) -> nil) | ((b) -> b) = x -- Not OK
-      end
+            end
-     )");
+        end
+    )");
 
     LUAU_REQUIRE_ERROR_COUNT(1, result);
     CHECK_EQ(toString(result.errors[0]),
@@ -628,12 +655,13 @@ TEST_CASE_FIXTURE(Fixture, "union_of_functions_mentioning_generics")
 TEST_CASE_FIXTURE(Fixture, "union_of_functions_mentioning_generic_typepacks")
 {
     CheckResult result = check(R"(
-      function f<a...>()
+        function f<a...>()
-        local x : (number, a...) -> (number?, a...)
+            function g(x : (number, a...) -> (number?, a...))
-        local y : ((number | string, a...) -> (number, a...)) | ((number?, a...) -> (nil, a...)) = x -- OK
+                local y : ((number | string, a...) -> (number, a...)) | ((number?, a...) -> (nil, a...)) = x -- OK
-        local z : ((number) -> number) | ((number?, a...) -> (number?, a...)) = x -- Not OK
+                local z : ((number) -> number) | ((number?, a...) -> (number?, a...)) = x -- Not OK
-      end
+            end
-     )");
+        end
+    )");
 
     LUAU_REQUIRE_ERROR_COUNT(1, result);
     const std::string expected = R"(Type
@@ -646,9 +674,10 @@ could not be converted into
 TEST_CASE_FIXTURE(Fixture, "union_of_functions_with_mismatching_arg_arities")
 {
     CheckResult result = check(R"(
-        local x : (number) -> number?
+        function f(x : (number) -> number?)
-        local y : ((number?) -> number) | ((number | string) -> nil) = x -- OK
+            local y : ((number?) -> number) | ((number | string) -> nil) = x -- OK
-        local z : ((number, string?) -> number) | ((number) -> nil) = x -- Not OK
+            local z : ((number, string?) -> number) | ((number) -> nil) = x -- Not OK
+        end
      )");
 
     LUAU_REQUIRE_ERROR_COUNT(1, result);
@@ -661,11 +690,11 @@ could not be converted into
 
 TEST_CASE_FIXTURE(Fixture, "union_of_functions_with_mismatching_result_arities")
 {
-
     CheckResult result = check(R"(
-        local x : () -> (number | string)
+        function f(x : () -> (number | string))
-        local y : (() -> number) | (() -> string) = x -- OK
+            local y : (() -> number) | (() -> string) = x -- OK
-        local z : (() -> number) | (() -> (string, string)) = x -- Not OK
+            local z : (() -> number) | (() -> (string, string)) = x -- Not OK
+        end
      )");
 
     LUAU_REQUIRE_ERROR_COUNT(1, result);
@@ -678,11 +707,11 @@ could not be converted into
 
 TEST_CASE_FIXTURE(Fixture, "union_of_functions_with_variadics")
 {
-
     CheckResult result = check(R"(
-        local x : (...nil) -> (...number?)
+        function f(x : (...nil) -> (...number?))
-        local y : ((...string?) -> (...number)) | ((...number?) -> nil) = x -- OK
+            local y : ((...string?) -> (...number)) | ((...number?) -> nil) = x -- OK
-        local z : ((...string?) -> (...number)) | ((...string?) -> nil) = x -- OK
+            local z : ((...string?) -> (...number)) | ((...string?) -> nil) = x -- OK
+        end
      )");
 
     LUAU_REQUIRE_ERROR_COUNT(1, result);
@@ -695,11 +724,11 @@ could not be converted into
 
 TEST_CASE_FIXTURE(Fixture, "union_of_functions_with_mismatching_arg_variadics")
 {
-
     CheckResult result = check(R"(
-        local x : (number) -> ()
+        function f(x : (number) -> ())
-        local y : ((number?) -> ()) | ((...number) -> ()) = x -- OK
+            local y : ((number?) -> ()) | ((...number) -> ()) = x -- OK
-        local z : ((number?) -> ()) | ((...number?) -> ()) = x -- Not OK
+            local z : ((number?) -> ()) | ((...number?) -> ()) = x -- Not OK
+        end
      )");
 
     LUAU_REQUIRE_ERROR_COUNT(1, result);
@@ -712,11 +741,11 @@ could not be converted into
 
 TEST_CASE_FIXTURE(Fixture, "union_of_functions_with_mismatching_result_variadics")
 {
-
     CheckResult result = check(R"(
-        local x : () -> (number?, ...number)
+        function f(x : () -> (number?, ...number))
-        local y : (() -> (...number)) | (() -> nil) = x -- OK
+            local y : (() -> (...number)) | (() -> nil) = x -- OK
-        local z : (() -> (...number)) | (() -> number) = x -- OK
+            local z : (() -> (...number)) | (() -> number) = x -- OK
+        end
      )");
 
     LUAU_REQUIRE_ERROR_COUNT(1, result);
@@ -782,9 +811,9 @@ TEST_CASE_FIXTURE(Fixture, "union_table_any_property")
 TEST_CASE_FIXTURE(Fixture, "union_function_any_args")
 {
     CheckResult result = check(R"(
-        local sup : ((...any) -> (...any))?
+        function f(sup : ((...any) -> (...any))?, sub : ((number) -> (...any)))
-        local sub : ((number) -> (...any))
+            sup = sub
-        sup = sub
+        end
     )");
 
     LUAU_REQUIRE_NO_ERRORS(result);
@@ -793,9 +822,9 @@ TEST_CASE_FIXTURE(Fixture, "union_function_any_args")
 TEST_CASE_FIXTURE(Fixture, "optional_any")
 {
     CheckResult result = check(R"(
-        local sup : any?
+        function f(sup : any?, sub : number)
-        local sub : number
+            sup = sub
-        sup = sub
+        end
     )");
 
     LUAU_REQUIRE_NO_ERRORS(result);
@@ -844,9 +873,9 @@ TEST_CASE_FIXTURE(Fixture, "suppress_errors_for_prop_lookup_of_a_union_that_incl
     registerHiddenTypes(&frontend);
 
     CheckResult result = check(R"(
-        local a = 5 :: err | Not<nil>
+        function f(a: err | Not<nil>)
-
+            local b = a.foo
-        local b = a.foo
+        end
     )");
 
     LUAU_REQUIRE_NO_ERRORS(result);

tests/TypeInfer.unknownnever.test.cpp

@@ -215,8 +215,9 @@ TEST_CASE_FIXTURE(Fixture, "assign_to_global_which_is_never")
 TEST_CASE_FIXTURE(Fixture, "assign_to_prop_which_is_never")
 {
     CheckResult result = check(R"(
-        local t: never
+        local function f(t: never)
-        t.x = 5
+            t.x = 5
+        end
     )");
 
     LUAU_REQUIRE_NO_ERRORS(result);
@@ -225,8 +226,9 @@ TEST_CASE_FIXTURE(Fixture, "assign_to_prop_which_is_never")
 TEST_CASE_FIXTURE(Fixture, "assign_to_subscript_which_is_never")
 {
     CheckResult result = check(R"(
-        local t: never
+        local function f(t: never)
-        t[5] = 7
+            t[5] = 7
+        end
     )");
 
     LUAU_REQUIRE_NO_ERRORS(result);
@@ -257,8 +259,12 @@ TEST_CASE_FIXTURE(Fixture, "index_on_union_of_tables_for_properties_that_is_neve
 {
     CheckResult result = check(R"(
         type Disjoint = {foo: never, bar: unknown, tag: "ok"} | {foo: never, baz: unknown, tag: "err"}
-        local disjoint: Disjoint = {foo = 5 :: never, bar = true, tag = "ok"}
+
-        local foo = disjoint.foo
+        function f(disjoint: Disjoint)
+            return disjoint.foo
+        end
+
+        local foo = f({foo = 5 :: never, bar = true, tag = "ok"})
     )");
 
     LUAU_REQUIRE_NO_ERRORS(result);
@@ -270,8 +276,12 @@ TEST_CASE_FIXTURE(Fixture, "index_on_union_of_tables_for_properties_that_is_sort
 {
     CheckResult result = check(R"(
         type Disjoint = {foo: string, bar: unknown, tag: "ok"} | {foo: never, baz: unknown, tag: "err"}
-        local disjoint: Disjoint = {foo = 5 :: never, bar = true, tag = "ok"}
+
-        local foo = disjoint.foo
+        function f(disjoint: Disjoint)
+            return disjoint.foo
+        end
+
+        local foo = f({foo = 5 :: never, bar = true, tag = "ok"})
     )");
 
     LUAU_REQUIRE_NO_ERRORS(result);

tests/require/with_config/src/global_library_requirer.luau

@@ -1,2 +1,2 @@
 -- should be required using the paths array in the parent directory's .luaurc
-return require("@global_library")
+return require("global_library")

tests/require/with_config/src/requirer.luau

@@ -1,2 +1,2 @@
 -- should be required using the paths array in .luaurc
-return require("@library")
+return require("library")

tests/require/without_config/module.luau

@@ -1,3 +1,3 @@
-local result = require("./dependency")
+local result = require("dependency")
 result[#result+1] = "required into module"
 return result

tools/faillist.txt

@@ -1,17 +1,13 @@
 AnnotationTests.typeof_expr
 AstQuery.last_argument_function_call_type
-AstQuery::getDocumentationSymbolAtPosition.class_method
-AstQuery::getDocumentationSymbolAtPosition.overloaded_class_method
 AstQuery::getDocumentationSymbolAtPosition.table_overloaded_function_prop
 AutocompleteTest.anonymous_autofilled_generic_on_argument_type_pack_vararg
 AutocompleteTest.anonymous_autofilled_generic_type_pack_vararg
 AutocompleteTest.autocomplete_interpolated_string_as_singleton
-AutocompleteTest.autocomplete_response_perf1
 AutocompleteTest.autocomplete_string_singleton_equality
 AutocompleteTest.autocomplete_string_singleton_escape
 AutocompleteTest.autocomplete_string_singletons
 AutocompleteTest.do_wrong_compatible_nonself_calls
-AutocompleteTest.no_incompatible_self_calls_on_class
 AutocompleteTest.string_singleton_in_if_statement
 AutocompleteTest.suggest_external_module_type
 AutocompleteTest.type_correct_expected_argument_type_pack_suggestion
@@ -33,7 +29,6 @@ BuiltinTests.assert_returns_false_and_string_iff_it_knows_the_first_argument_can
 BuiltinTests.bad_select_should_not_crash
 BuiltinTests.coroutine_resume_anything_goes
 BuiltinTests.debug_info_is_crazy
-BuiltinTests.dont_add_definitions_to_persistent_types
 BuiltinTests.global_singleton_types_are_sealed
 BuiltinTests.gmatch_capture_types
 BuiltinTests.gmatch_capture_types2
@@ -62,7 +57,6 @@ BuiltinTests.string_format_as_method
 BuiltinTests.string_format_correctly_ordered_types
 BuiltinTests.string_format_report_all_type_errors_at_correct_positions
 BuiltinTests.string_format_use_correct_argument2
-BuiltinTests.string_match
 BuiltinTests.table_concat_returns_string
 BuiltinTests.table_dot_remove_optionally_returns_generic
 BuiltinTests.table_freeze_is_generic
@@ -100,16 +94,15 @@ ControlFlowAnalysis.if_not_x_then_assert_false
 ControlFlowAnalysis.if_not_x_then_error
 ControlFlowAnalysis.prototyping_and_visiting_alias_has_the_same_scope_breaking
 ControlFlowAnalysis.prototyping_and_visiting_alias_has_the_same_scope_continuing
+ControlFlowAnalysis.tagged_unions
 ControlFlowAnalysis.tagged_unions_breaking
 ControlFlowAnalysis.tagged_unions_continuing
 ControlFlowAnalysis.type_alias_does_not_leak_out_breaking
 ControlFlowAnalysis.type_alias_does_not_leak_out_continuing
-DefinitionTests.class_definition_function_prop
 DefinitionTests.class_definition_indexer
 DefinitionTests.class_definition_overload_metamethods
 DefinitionTests.class_definition_string_props
 DefinitionTests.declaring_generic_functions
-DefinitionTests.definition_file_class_function_args
 DefinitionTests.definition_file_classes
 Differ.equal_generictp_cyclic
 Differ.equal_table_A_B_C
@@ -142,7 +135,6 @@ GenericsTests.apply_type_function_nested_generics1
 GenericsTests.apply_type_function_nested_generics2
 GenericsTests.better_mismatch_error_messages
 GenericsTests.bound_tables_do_not_clone_original_fields
-GenericsTests.calling_self_generic_methods
 GenericsTests.check_generic_function
 GenericsTests.check_generic_local_function
 GenericsTests.check_generic_typepack_function
@@ -230,6 +222,7 @@ Linter.DeprecatedApiFenv
 Linter.FormatStringTyped
 Linter.TableOperationsIndexer
 ModuleTests.clone_self_property
+Negations.cofinite_strings_can_be_compared_for_equality
 Negations.negated_string_is_a_subtype_of_string
 NonstrictModeTests.inconsistent_module_return_types_are_ok
 NonstrictModeTests.infer_nullary_function
@@ -244,6 +237,7 @@ Normalize.negations_of_tables
 Normalize.specific_functions_cannot_be_negated
 ProvisionalTests.assign_table_with_refined_property_with_a_similar_type_is_illegal
 ProvisionalTests.choose_the_right_overload_for_pcall
+ProvisionalTests.discriminate_from_x_not_equal_to_nil
 ProvisionalTests.do_not_ice_when_trying_to_pick_first_of_generic_type_pack
 ProvisionalTests.error_on_eq_metamethod_returning_a_type_other_than_boolean
 ProvisionalTests.expected_type_should_be_a_helpful_deduction_guide_for_function_calls
@@ -272,19 +266,27 @@ RefinementTest.assert_non_binary_expressions_actually_resolve_constraints
 RefinementTest.correctly_lookup_a_shadowed_local_that_which_was_previously_refined
 RefinementTest.correctly_lookup_property_whose_base_was_previously_refined
 RefinementTest.dataflow_analysis_can_tell_refinements_when_its_appropriate_to_refine_into_nil_or_never
+RefinementTest.discriminate_from_isa_of_x
 RefinementTest.discriminate_from_truthiness_of_x
+RefinementTest.discriminate_tag
+RefinementTest.discriminate_tag_with_implicit_else
 RefinementTest.either_number_or_string
+RefinementTest.else_with_no_explicit_expression_should_also_refine_the_tagged_union
 RefinementTest.fail_to_refine_a_property_of_subscript_expression
 RefinementTest.falsiness_of_TruthyPredicate_narrows_into_nil
 RefinementTest.function_call_with_colon_after_refining_not_to_be_nil
 RefinementTest.impossible_type_narrow_is_not_an_error
 RefinementTest.index_on_a_refined_property
 RefinementTest.isa_type_refinement_must_be_known_ahead_of_time
+RefinementTest.luau_polyfill_isindexkey_refine_conjunction
+RefinementTest.luau_polyfill_isindexkey_refine_conjunction_variant
 RefinementTest.merge_should_be_fully_agnostic_of_hashmap_ordering
 RefinementTest.narrow_property_of_a_bounded_variable
 RefinementTest.nonoptional_type_can_narrow_to_nil_if_sense_is_true
 RefinementTest.not_t_or_some_prop_of_t
+RefinementTest.refine_a_param_that_got_resolved_during_constraint_solving_stage
 RefinementTest.refine_a_property_of_some_global
+RefinementTest.refine_param_of_type_folder_or_part_without_using_typeof
 RefinementTest.refine_unknown_to_table_then_clone_it
 RefinementTest.refinements_should_preserve_error_suppression
 RefinementTest.string_not_equal_to_string_or_nil
@@ -297,6 +299,7 @@ RefinementTest.typeguard_cast_free_table_to_vector
 RefinementTest.typeguard_in_assert_position
 RefinementTest.typeguard_in_if_condition_position
 RefinementTest.x_as_any_if_x_is_instance_elseif_x_is_table
+RefinementTest.x_is_not_instance_or_else_not_part
 TableTests.a_free_shape_can_turn_into_a_scalar_directly
 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
@@ -459,7 +462,6 @@ TypeInfer.cli_50041_committing_txnlog_in_apollo_client_error
 TypeInfer.dont_ice_when_failing_the_occurs_check
 TypeInfer.dont_report_type_errors_within_an_AstExprError
 TypeInfer.dont_report_type_errors_within_an_AstStatError
-TypeInfer.follow_on_new_types_in_substitution
 TypeInfer.globals
 TypeInfer.globals2
 TypeInfer.globals_are_banned_in_strict_mode
@@ -555,6 +557,7 @@ TypeInferFunctions.other_things_are_not_related_to_function
 TypeInferFunctions.param_1_and_2_both_takes_the_same_generic_but_their_arguments_are_incompatible
 TypeInferFunctions.param_1_and_2_both_takes_the_same_generic_but_their_arguments_are_incompatible_2
 TypeInferFunctions.record_matching_overload
+TypeInferFunctions.regex_benchmark_string_format_minimization
 TypeInferFunctions.report_exiting_without_return_nonstrict
 TypeInferFunctions.report_exiting_without_return_strict
 TypeInferFunctions.return_type_by_overload
@@ -600,6 +603,7 @@ TypeInferLoops.unreachable_code_after_infinite_loop
 TypeInferLoops.varlist_declared_by_for_in_loop_should_be_free
 TypeInferLoops.while_loop
 TypeInferModules.bound_free_table_export_is_ok
+TypeInferModules.custom_require_global
 TypeInferModules.do_not_modify_imported_types
 TypeInferModules.do_not_modify_imported_types_5
 TypeInferModules.require
@@ -645,7 +649,6 @@ TypeInferPrimitives.CheckMethodsOfNumber
 TypeInferPrimitives.string_function_indirect
 TypeInferPrimitives.string_index
 TypeInferUnknownNever.assign_to_local_which_is_never
-TypeInferUnknownNever.assign_to_prop_which_is_never
 TypeInferUnknownNever.index_on_union_of_tables_for_properties_that_is_never
 TypeInferUnknownNever.index_on_union_of_tables_for_properties_that_is_sorta_never
 TypeInferUnknownNever.length_of_never
@@ -663,42 +666,25 @@ TypeSingletons.error_detailed_tagged_union_mismatch_string
 TypeSingletons.function_args_infer_singletons
 TypeSingletons.function_call_with_singletons
 TypeSingletons.function_call_with_singletons_mismatch
-TypeSingletons.overloaded_function_call_with_singletons_mismatch
 TypeSingletons.return_type_of_f_is_not_widened
 TypeSingletons.table_properties_type_error_escapes
 TypeSingletons.widen_the_supertype_if_it_is_free_and_subtype_has_singleton
-TypeSingletons.widening_happens_almost_everywhere
 TypeStatesTest.invalidate_type_refinements_upon_assignments
-TypeStatesTest.local_t_is_assigned_a_fresh_table_with_x_assigned_a_union_and_then_assert_restricts_actual_outflow_of_types
-UnionTypes.disallow_less_specific_assign
-UnionTypes.disallow_less_specific_assign2
 UnionTypes.error_detailed_optional
 UnionTypes.error_detailed_union_all
-UnionTypes.error_detailed_union_part
 UnionTypes.error_takes_optional_arguments
 UnionTypes.generic_function_with_optional_arg
 UnionTypes.index_on_a_union_type_with_missing_property
-UnionTypes.index_on_a_union_type_with_mixed_types
-UnionTypes.index_on_a_union_type_with_one_optional_property
-UnionTypes.index_on_a_union_type_with_one_property_of_type_any
-UnionTypes.index_on_a_union_type_with_property_guaranteed_to_exist
-UnionTypes.index_on_a_union_type_works_at_arbitrary_depth
 UnionTypes.less_greedy_unification_with_union_types
 UnionTypes.optional_arguments_table
-UnionTypes.optional_assignment_errors
-UnionTypes.optional_call_error
-UnionTypes.optional_field_access_error
-UnionTypes.optional_index_error
 UnionTypes.optional_length_error
-UnionTypes.optional_missing_key_error_details
-UnionTypes.optional_union_follow
 UnionTypes.optional_union_functions
 UnionTypes.optional_union_members
 UnionTypes.optional_union_methods
 UnionTypes.return_types_can_be_disjoint
 UnionTypes.table_union_write_indirect
 UnionTypes.unify_unsealed_table_union_check
-UnionTypes.union_of_functions
+UnionTypes.union_function_any_args
 UnionTypes.union_of_functions_mentioning_generic_typepacks
 UnionTypes.union_of_functions_mentioning_generics
 UnionTypes.union_of_functions_with_mismatching_arg_arities
@@ -706,5 +692,4 @@ UnionTypes.union_of_functions_with_mismatching_arg_variadics
 UnionTypes.union_of_functions_with_mismatching_result_arities
 UnionTypes.union_of_functions_with_mismatching_result_variadics
 UnionTypes.union_of_functions_with_variadics
-UnionTypes.union_true_and_false
 VisitType.throw_when_limit_is_exceeded