Sync to upstream/release/597 (#1054)

# New Type Solver

- Implement bidirectional type inference for higher order functions so
that we can provide a more precise type improving the autocomplete's
human factors.
- We seal all tables, so we changed the stringification to make it a
little lighter on users.
- Fixed a case of array-out-of-bound access.
- Type families no longer depends on `TxnLog` and `Unifier`.
- Type refinements now waits until the free types are sufficiently
solved.

# Native Code Generation

- Remove cached slot lookup for `executeSETTABLEKS` function because it
is a fallback in the event of a cache miss, making the cached slot
lookup redundant.
- Optimized repeated array lookups, e.g. `a[3]` in `a[3] = a[3] / 2` is
done once.

# Misc

- On some platforms, it is necessary to use `gmtime_s` with the
arguments reversed to get the current time. You can now define
`DOCTEST_CONFIG_USE_GMTIME_S` to build and run unit tests on those
platforms.

---------

Co-authored-by: Arseny Kapoulkine <arseny.kapoulkine@gmail.com>
Co-authored-by: Vyacheslav Egorov <vegorov@roblox.com>
Co-authored-by: Andy Friesen <afriesen@roblox.com>
Co-authored-by: Lily Brown <lbrown@roblox.com>
Co-authored-by: Aaron Weiss <aaronweiss@roblox.com>

Analysis/include/Luau/ConstraintGraphBuilder.h

@@ -106,6 +106,14 @@ struct ConstraintGraphBuilder
         std::function<void(const ModuleName&, const ScopePtr&)> prepareModuleScope, DcrLogger* logger, NotNull<DataFlowGraph> dfg,
         std::vector<RequireCycle> requireCycles);
 
+    /**
+     * The entry point to the ConstraintGraphBuilder. This will construct a set
+     * of scopes, constraints, and free types that can be solved later.
+     * @param block the root block to generate constraints for.
+     */
+    void visitModuleRoot(AstStatBlock* block);
+
+private:
     /**
      * Fabricates a new free type belonging to a given scope.
      * @param scope the scope the free type belongs to.
@@ -143,13 +151,6 @@ struct ConstraintGraphBuilder
 
     void applyRefinements(const ScopePtr& scope, Location location, RefinementId refinement);
 
-    /**
-     * The entry point to the ConstraintGraphBuilder. This will construct a set
-     * of scopes, constraints, and free types that can be solved later.
-     * @param block the root block to generate constraints for.
-     */
-    void visit(AstStatBlock* block);
-
     ControlFlow visitBlockWithoutChildScope(const ScopePtr& scope, AstStatBlock* block);
 
     ControlFlow visit(const ScopePtr& scope, AstStat* stat);
@@ -172,7 +173,8 @@ struct ConstraintGraphBuilder
     ControlFlow visit(const ScopePtr& scope, AstStatError* error);
 
     InferencePack checkPack(const ScopePtr& scope, AstArray<AstExpr*> exprs, const std::vector<std::optional<TypeId>>& expectedTypes = {});
-    InferencePack checkPack(const ScopePtr& scope, AstExpr* expr, const std::vector<std::optional<TypeId>>& expectedTypes = {});
+    InferencePack checkPack(
+        const ScopePtr& scope, AstExpr* expr, const std::vector<std::optional<TypeId>>& expectedTypes = {}, bool generalize = true);
 
     InferencePack checkPack(const ScopePtr& scope, AstExprCall* call);
 
@@ -182,10 +184,11 @@ struct ConstraintGraphBuilder
      * @param expr the expression to check.
      * @param expectedType the type of the expression that is expected from its
      *      surrounding context.  Used to implement bidirectional type checking.
+     * @param generalize If true, generalize any lambdas that are encountered.
      * @return the type of the expression.
      */
     Inference check(const ScopePtr& scope, AstExpr* expr, ValueContext context = ValueContext::RValue, std::optional<TypeId> expectedType = {},
-        bool forceSingleton = false);
+        bool forceSingleton = false, bool generalize = true);
 
     Inference check(const ScopePtr& scope, AstExprConstantString* string, std::optional<TypeId> expectedType, bool forceSingleton);
     Inference check(const ScopePtr& scope, AstExprConstantBool* bool_, std::optional<TypeId> expectedType, bool forceSingleton);
@@ -193,7 +196,7 @@ struct ConstraintGraphBuilder
     Inference check(const ScopePtr& scope, AstExprGlobal* global);
     Inference check(const ScopePtr& scope, AstExprIndexName* indexName);
     Inference check(const ScopePtr& scope, AstExprIndexExpr* indexExpr);
-    Inference check(const ScopePtr& scope, AstExprFunction* func, std::optional<TypeId> expectedType);
+    Inference check(const ScopePtr& scope, AstExprFunction* func, std::optional<TypeId> expectedType, bool generalize);
     Inference check(const ScopePtr& scope, AstExprUnary* unary);
     Inference check(const ScopePtr& scope, AstExprBinary* binary, std::optional<TypeId> expectedType);
     Inference check(const ScopePtr& scope, AstExprIfElse* ifElse, std::optional<TypeId> expectedType);

Analysis/include/Luau/NonStrictTypeChecker.h

@@ -0,0 +1,15 @@
+// This file is part of the Luau programming language and is licensed under MIT License; see LICENSE.txt for details
+#pragma once
+
+#include "Luau/Module.h"
+#include "Luau/NotNull.h"
+
+namespace Luau
+{
+
+struct BuiltinTypes;
+
+
+void checkNonStrict(NotNull<BuiltinTypes> builtinTypes, Module* module);
+
+} // namespace Luau

Analysis/src/Clone.cpp

@@ -8,7 +8,6 @@
 #include "Luau/TypePack.h"
 #include "Luau/Unifiable.h"
 
-LUAU_FASTFLAG(DebugLuauCopyBeforeNormalizing)
 LUAU_FASTFLAG(DebugLuauReadWriteProperties)
 
 LUAU_FASTFLAG(DebugLuauDeferredConstraintResolution)
@@ -253,8 +252,10 @@ private:
 
     void cloneChildren(FreeType* t)
     {
-        // TODO: clone lower and upper bounds.
+        if (t->lowerBound)
-        // TODO: In the new solver, we should ice.
+            t->lowerBound = shallowClone(t->lowerBound);
+        if (t->upperBound)
+            t->upperBound = shallowClone(t->upperBound);
     }
 
     void cloneChildren(GenericType* t)
@@ -376,7 +377,11 @@ private:
 
     void cloneChildren(TypeFamilyInstanceType* t)
     {
-        // TODO: In the new solver, we should ice.
+        for (TypeId& ty : t->typeArguments)
+            ty = shallowClone(ty);
+
+        for (TypePackId& tp : t->packArguments)
+            tp = shallowClone(tp);
     }
 
     void cloneChildren(FreeTypePack* t)
@@ -416,7 +421,11 @@ private:
 
     void cloneChildren(TypeFamilyInstanceTypePack* t)
     {
-        // TODO: In the new solver, we should ice.
+        for (TypeId& ty : t->typeArguments)
+            ty = shallowClone(ty);
+
+        for (TypePackId& tp : t->packArguments)
+            tp = shallowClone(tp);
     }
 };
 
@@ -560,8 +569,6 @@ struct TypePackCloner
     void operator()(const Unifiable::Bound<TypePackId>& t)
     {
         TypePackId cloned = clone(t.boundTo, dest, cloneState);
-        if (FFlag::DebugLuauCopyBeforeNormalizing)
-            cloned = dest.addTypePack(TypePackVar{BoundTypePack{cloned}});
         seenTypePacks[typePackId] = cloned;
     }
 
@@ -629,8 +636,6 @@ void TypeCloner::operator()(const GenericType& t)
 void TypeCloner::operator()(const Unifiable::Bound<TypeId>& t)
 {
     TypeId boundTo = clone(t.boundTo, dest, cloneState);
-    if (FFlag::DebugLuauCopyBeforeNormalizing)
-        boundTo = dest.addType(BoundType{boundTo});
     seenTypes[typeId] = boundTo;
 }
 
@@ -701,7 +706,7 @@ void TypeCloner::operator()(const FunctionType& t)
 void TypeCloner::operator()(const TableType& t)
 {
     // If table is now bound to another one, we ignore the content of the original
-    if (!FFlag::DebugLuauCopyBeforeNormalizing && t.boundTo)
+    if (t.boundTo)
     {
         TypeId boundTo = clone(*t.boundTo, dest, cloneState);
         seenTypes[typeId] = boundTo;
@@ -718,9 +723,6 @@ void TypeCloner::operator()(const TableType& t)
 
     ttv->level = TypeLevel{0, 0};
 
-    if (FFlag::DebugLuauCopyBeforeNormalizing && t.boundTo)
-        ttv->boundTo = clone(*t.boundTo, dest, cloneState);
-
     for (const auto& [name, prop] : t.props)
         ttv->props[name] = clone(prop, dest, cloneState);
 

Analysis/src/ConstraintGraphBuilder.cpp

@@ -160,6 +160,25 @@ ConstraintGraphBuilder::ConstraintGraphBuilder(ModulePtr module, NotNull<Normali
     LUAU_ASSERT(module);
 }
 
+void ConstraintGraphBuilder::visitModuleRoot(AstStatBlock* block)
+{
+    LUAU_ASSERT(scopes.empty());
+    LUAU_ASSERT(rootScope == nullptr);
+    ScopePtr scope = std::make_shared<Scope>(globalScope);
+    rootScope = scope.get();
+    scopes.emplace_back(block->location, scope);
+    module->astScopes[block] = NotNull{scope.get()};
+
+    rootScope->returnType = freshTypePack(scope);
+
+    prepopulateGlobalScope(scope, block);
+
+    visitBlockWithoutChildScope(scope, block);
+
+    if (logger)
+        logger->captureGenerationModule(module);
+}
+
 TypeId ConstraintGraphBuilder::freshType(const ScopePtr& scope)
 {
     return Luau::freshType(arena, builtinTypes, scope.get());
@@ -444,25 +463,6 @@ void ConstraintGraphBuilder::applyRefinements(const ScopePtr& scope, Location lo
         addConstraint(scope, location, c);
 }
 
-void ConstraintGraphBuilder::visit(AstStatBlock* block)
-{
-    LUAU_ASSERT(scopes.empty());
-    LUAU_ASSERT(rootScope == nullptr);
-    ScopePtr scope = std::make_shared<Scope>(globalScope);
-    rootScope = scope.get();
-    scopes.emplace_back(block->location, scope);
-    module->astScopes[block] = NotNull{scope.get()};
-
-    rootScope->returnType = freshTypePack(scope);
-
-    prepopulateGlobalScope(scope, block);
-
-    visitBlockWithoutChildScope(scope, block);
-
-    if (logger)
-        logger->captureGenerationModule(module);
-}
-
 ControlFlow ConstraintGraphBuilder::visitBlockWithoutChildScope(const ScopePtr& scope, AstStatBlock* block)
 {
     RecursionCounter counter{&recursionCount};
@@ -616,7 +616,7 @@ ControlFlow ConstraintGraphBuilder::visit(const ScopePtr& scope, AstStatLocal* l
             // See the test TypeInfer/infer_locals_with_nil_value. Better flow
             // awareness should make this obsolete.
 
-            if (!varTypes[i])
+            if (i < varTypes.size() && !varTypes[i])
                 varTypes[i] = freshType(scope);
         }
         // Only function calls and vararg expressions can produce packs.  All
@@ -627,7 +627,7 @@ ControlFlow ConstraintGraphBuilder::visit(const ScopePtr& scope, AstStatLocal* l
             if (hasAnnotation)
                 expectedType = varTypes.at(i);
 
-            TypeId exprType = check(scope, value, ValueContext::RValue, expectedType).ty;
+            TypeId exprType = check(scope, value, ValueContext::RValue, expectedType, /*forceSingleton*/ false, /*generalize*/ true).ty;
             if (i < varTypes.size())
             {
                 if (varTypes[i])
@@ -645,7 +645,7 @@ ControlFlow ConstraintGraphBuilder::visit(const ScopePtr& scope, AstStatLocal* l
             if (hasAnnotation)
                 expectedTypes.insert(begin(expectedTypes), begin(varTypes) + i, end(varTypes));
 
-            TypePackId exprPack = checkPack(scope, value, expectedTypes).tp;
+            TypePackId exprPack = checkPack(scope, value, expectedTypes, /*generalize*/ true).tp;
 
             if (i < local->vars.size)
             {
@@ -1380,7 +1380,8 @@ InferencePack ConstraintGraphBuilder::checkPack(
         return InferencePack{arena->addTypePack(TypePack{std::move(head), tail})};
 }
 
-InferencePack ConstraintGraphBuilder::checkPack(const ScopePtr& scope, AstExpr* expr, const std::vector<std::optional<TypeId>>& expectedTypes)
+InferencePack ConstraintGraphBuilder::checkPack(
+    const ScopePtr& scope, AstExpr* expr, const std::vector<std::optional<TypeId>>& expectedTypes, bool generalize)
 {
     RecursionCounter counter{&recursionCount};
 
@@ -1406,7 +1407,7 @@ InferencePack ConstraintGraphBuilder::checkPack(const ScopePtr& scope, AstExpr*
         std::optional<TypeId> expectedType;
         if (!expectedTypes.empty())
             expectedType = expectedTypes[0];
-        TypeId t = check(scope, expr, ValueContext::RValue, expectedType).ty;
+        TypeId t = check(scope, expr, ValueContext::RValue, expectedType, /*forceSingletons*/ false, generalize).ty;
         result = InferencePack{arena->addTypePack({t})};
     }
 
@@ -1454,51 +1455,25 @@ InferencePack ConstraintGraphBuilder::checkPack(const ScopePtr& scope, AstExprCa
             discriminantTypes.push_back(std::nullopt);
     }
 
-    Checkpoint startCheckpoint = checkpoint(this);
     TypeId fnType = check(scope, call->func).ty;
-    Checkpoint fnEndCheckpoint = checkpoint(this);
 
     std::vector<std::optional<TypeId>> expectedTypesForCall = getExpectedCallTypesForFunctionOverloads(fnType);
 
     module->astOriginalCallTypes[call->func] = fnType;
     module->astOriginalCallTypes[call] = fnType;
 
-    TypePackId expectedArgPack = arena->freshTypePack(scope.get());
-    TypePackId expectedRetPack = arena->freshTypePack(scope.get());
-    TypeId expectedFunctionType = arena->addType(FunctionType{expectedArgPack, expectedRetPack, std::nullopt, call->self});
-
     TypeId instantiatedFnType = arena->addType(BlockedType{});
     addConstraint(scope, call->location, InstantiationConstraint{instantiatedFnType, fnType});
 
-    NotNull<Constraint> extractArgsConstraint = addConstraint(scope, call->location, SubtypeConstraint{instantiatedFnType, expectedFunctionType});
+    Checkpoint argBeginCheckpoint = checkpoint(this);
-
-    // Fully solve fnType, then extract its argument list as expectedArgPack.
-    forEachConstraint(startCheckpoint, fnEndCheckpoint, this, [extractArgsConstraint](const ConstraintPtr& constraint) {
-        extractArgsConstraint->dependencies.emplace_back(constraint.get());
-    });
-
-    const AstExpr* lastArg = exprArgs.size() ? exprArgs[exprArgs.size() - 1] : nullptr;
-    const bool needTail = lastArg && (lastArg->is<AstExprCall>() || lastArg->is<AstExprVarargs>());
-
-    TypePack expectedArgs;
-
-    if (!needTail)
-        expectedArgs = extendTypePack(*arena, builtinTypes, expectedArgPack, exprArgs.size(), expectedTypesForCall);
-    else
-        expectedArgs = extendTypePack(*arena, builtinTypes, expectedArgPack, exprArgs.size() - 1, expectedTypesForCall);
 
     std::vector<TypeId> args;
     std::optional<TypePackId> argTail;
     std::vector<RefinementId> argumentRefinements;
 
-    Checkpoint argCheckpoint = checkpoint(this);
-
     for (size_t i = 0; i < exprArgs.size(); ++i)
     {
         AstExpr* arg = exprArgs[i];
-        std::optional<TypeId> expectedType;
-        if (i < expectedArgs.head.size())
-            expectedType = expectedArgs.head[i];
 
         if (i == 0 && call->self)
         {
@@ -1514,7 +1489,8 @@ InferencePack ConstraintGraphBuilder::checkPack(const ScopePtr& scope, AstExprCa
         }
         else if (i < exprArgs.size() - 1 || !(arg->is<AstExprCall>() || arg->is<AstExprVarargs>()))
         {
-            auto [ty, refinement] = check(scope, arg, ValueContext::RValue, expectedType);
+            auto [ty, refinement] =
+                check(scope, arg, ValueContext::RValue, /*expectedType*/ std::nullopt, /*forceSingleton*/ false, /*generalize*/ false);
             args.push_back(ty);
             argumentRefinements.push_back(refinement);
         }
@@ -1528,12 +1504,6 @@ InferencePack ConstraintGraphBuilder::checkPack(const ScopePtr& scope, AstExprCa
 
     Checkpoint argEndCheckpoint = checkpoint(this);
 
-    // Do not solve argument constraints until after we have extracted the
-    // expected types from the callable.
-    forEachConstraint(argCheckpoint, argEndCheckpoint, this, [extractArgsConstraint](const ConstraintPtr& constraint) {
-        constraint->dependencies.push_back(extractArgsConstraint);
-    });
-
     if (matchSetmetatable(*call))
     {
         TypePack argTailPack;
@@ -1609,8 +1579,8 @@ InferencePack ConstraintGraphBuilder::checkPack(const ScopePtr& scope, AstExprCa
         // This ensures, for instance, that we start inferring the contents of
         // lambdas under the assumption that their arguments and return types
         // will be compatible with the enclosing function call.
-        forEachConstraint(fnEndCheckpoint, argEndCheckpoint, this, [fcc](const ConstraintPtr& constraint) {
+        forEachConstraint(argBeginCheckpoint, argEndCheckpoint, this, [fcc](const ConstraintPtr& constraint) {
-            fcc->dependencies.emplace_back(constraint.get());
+            constraint->dependencies.emplace_back(fcc);
         });
 
         return InferencePack{rets, {refinementArena.variadic(returnRefinements)}};
@@ -1618,7 +1588,7 @@ InferencePack ConstraintGraphBuilder::checkPack(const ScopePtr& scope, AstExprCa
 }
 
 Inference ConstraintGraphBuilder::check(
-    const ScopePtr& scope, AstExpr* expr, ValueContext context, std::optional<TypeId> expectedType, bool forceSingleton)
+    const ScopePtr& scope, AstExpr* expr, ValueContext context, std::optional<TypeId> expectedType, bool forceSingleton, bool generalize)
 {
     RecursionCounter counter{&recursionCount};
 
@@ -1649,7 +1619,7 @@ Inference ConstraintGraphBuilder::check(
     else if (auto call = expr->as<AstExprCall>())
         result = flattenPack(scope, expr->location, checkPack(scope, call)); // TODO: needs predicates too
     else if (auto a = expr->as<AstExprFunction>())
-        result = check(scope, a, expectedType);
+        result = check(scope, a, expectedType, generalize);
     else if (auto indexName = expr->as<AstExprIndexName>())
         result = check(scope, indexName);
     else if (auto indexExpr = expr->as<AstExprIndexExpr>())
@@ -1817,30 +1787,37 @@ Inference ConstraintGraphBuilder::check(const ScopePtr& scope, AstExprIndexExpr*
         return Inference{result};
 }
 
-Inference ConstraintGraphBuilder::check(const ScopePtr& scope, AstExprFunction* func, std::optional<TypeId> expectedType)
+Inference ConstraintGraphBuilder::check(const ScopePtr& scope, AstExprFunction* func, std::optional<TypeId> expectedType, bool generalize)
 {
     Checkpoint startCheckpoint = checkpoint(this);
     FunctionSignature sig = checkFunctionSignature(scope, func, expectedType);
     checkFunctionBody(sig.bodyScope, func);
     Checkpoint endCheckpoint = checkpoint(this);
 
-    TypeId generalizedTy = arena->addType(BlockedType{});
+    if (generalize)
-    NotNull<Constraint> gc = addConstraint(sig.signatureScope, func->location, GeneralizationConstraint{generalizedTy, sig.signature});
+    {
+        TypeId generalizedTy = arena->addType(BlockedType{});
+        NotNull<Constraint> gc = addConstraint(sig.signatureScope, func->location, GeneralizationConstraint{generalizedTy, sig.signature});
 
-    Constraint* previous = nullptr;
+        Constraint* previous = nullptr;
-    forEachConstraint(startCheckpoint, endCheckpoint, this, [gc, &previous](const ConstraintPtr& constraint) {
+        forEachConstraint(startCheckpoint, endCheckpoint, this, [gc, &previous](const ConstraintPtr& constraint) {
-        gc->dependencies.emplace_back(constraint.get());
+            gc->dependencies.emplace_back(constraint.get());
 
-        if (auto psc = get<PackSubtypeConstraint>(*constraint); psc && psc->returns)
+            if (auto psc = get<PackSubtypeConstraint>(*constraint); psc && psc->returns)
-        {
+            {
-            if (previous)
+                if (previous)
-                constraint->dependencies.push_back(NotNull{previous});
+                    constraint->dependencies.push_back(NotNull{previous});
 
-            previous = constraint.get();
+                previous = constraint.get();
-        }
+            }
-    });
+        });
 
-    return Inference{generalizedTy};
+        return Inference{generalizedTy};
+    }
+    else
+    {
+        return Inference{sig.signature};
+    }
 }
 
 Inference ConstraintGraphBuilder::check(const ScopePtr& scope, AstExprUnary* unary)
@@ -2381,10 +2358,11 @@ ConstraintGraphBuilder::FunctionSignature ConstraintGraphBuilder::checkFunctionS
             argTy = resolveType(signatureScope, local->annotation, /* inTypeArguments */ false, /* replaceErrorWithFresh*/ true);
         else
         {
-            argTy = freshType(signatureScope);
 
             if (i < expectedArgPack.head.size())
-                addConstraint(signatureScope, local->location, SubtypeConstraint{argTy, expectedArgPack.head[i]});
+                argTy = expectedArgPack.head[i];
+            else
+                argTy = freshType(signatureScope);
         }
 
         argTypes.push_back(argTy);

Analysis/src/ConstraintSolver.cpp

@@ -1380,6 +1380,44 @@ bool ConstraintSolver::tryDispatch(const FunctionCallConstraint& c, NotNull<cons
         *asMutable(follow(*ty)) = BoundType{builtinTypes->anyType};
     }
 
+    // We know the type of the function and the arguments it expects to receive.
+    // We also know the TypeIds of the actual arguments that will be passed.
+    //
+    // Bidirectional type checking: Force those TypeIds to be the expected
+    // arguments. If something is incoherent, we'll spot it in type checking.
+    //
+    // Most important detail: If a function argument is a lambda, we also want
+    // to force unannotated argument types of that lambda to be the expected
+    // types.
+
+    // FIXME: Bidirectional type checking of overloaded functions is not yet supported.
+    if (auto ftv = get<FunctionType>(fn))
+    {
+        const std::vector<TypeId> expectedArgs = flatten(ftv->argTypes).first;
+        const std::vector<TypeId> argPackHead = flatten(argsPack).first;
+
+        for (size_t i = 0; i < c.callSite->args.size && i < expectedArgs.size() && i < argPackHead.size(); ++i)
+        {
+            const FunctionType* expectedLambdaTy = get<FunctionType>(follow(expectedArgs[i]));
+            const FunctionType* lambdaTy = get<FunctionType>(follow(argPackHead[i]));
+            const AstExprFunction* lambdaExpr = c.callSite->args.data[i]->as<AstExprFunction>();
+
+            if (expectedLambdaTy && lambdaTy && lambdaExpr)
+            {
+                const std::vector<TypeId> expectedLambdaArgTys = flatten(expectedLambdaTy->argTypes).first;
+                const std::vector<TypeId> lambdaArgTys = flatten(lambdaTy->argTypes).first;
+
+                for (size_t j = 0; j < expectedLambdaArgTys.size() && j < lambdaArgTys.size() && j < lambdaExpr->args.size; ++j)
+                {
+                    if (!lambdaExpr->args.data[j]->annotation && get<FreeType>(follow(lambdaArgTys[j])))
+                    {
+                        asMutable(lambdaArgTys[j])->ty.emplace<BoundType>(expectedLambdaArgTys[j]);
+                    }
+                }
+            }
+        }
+    }
+
     TypeId inferredTy = arena->addType(FunctionType{TypeLevel{}, constraint->scope.get(), argsPack, c.result});
     Unifier2 u2{NotNull{arena}, builtinTypes, constraint->scope, NotNull{&iceReporter}};
 

Analysis/src/Frontend.cpp

@@ -37,6 +37,7 @@ LUAU_FASTFLAGVARIABLE(DebugLuauLogSolverToJson, false)
 LUAU_FASTFLAGVARIABLE(DebugLuauReadWriteProperties, false)
 LUAU_FASTFLAGVARIABLE(LuauTypecheckLimitControls, false)
 LUAU_FASTFLAGVARIABLE(CorrectEarlyReturnInMarkDirty, false)
+LUAU_FASTFLAGVARIABLE(DebugLuauNewNonStrictMode, false)
 
 namespace Luau
 {
@@ -1257,7 +1258,7 @@ ModulePtr check(const SourceModule& sourceModule, const std::vector<RequireCycle
     ConstraintGraphBuilder cgb{result, NotNull{&normalizer}, moduleResolver, builtinTypes, iceHandler, parentScope, std::move(prepareModuleScope),
         logger.get(), NotNull{&dfg}, requireCycles};
 
-    cgb.visit(sourceModule.root);
+    cgb.visitModuleRoot(sourceModule.root);
     result->errors = std::move(cgb.errors);
 
     ConstraintSolver cs{NotNull{&normalizer}, NotNull(cgb.rootScope), borrowConstraints(cgb.constraints), result->humanReadableName, moduleResolver,

Analysis/src/NonStrictTypeChecker.cpp

@@ -0,0 +1,87 @@
+// This file is part of the Luau programming language and is licensed under MIT License; see LICENSE.txt for details
+#include "Luau/NonStrictTypeChecker.h"
+
+#include "Luau/Type.h"
+#include "Luau/Subtyping.h"
+#include "Luau/Normalize.h"
+#include "Luau/Error.h"
+#include "Luau/TypeArena.h"
+#include "Luau/Def.h"
+
+namespace Luau
+{
+
+struct NonStrictContext
+{
+    std::unordered_map<DefId, TypeId> context;
+
+    NonStrictContext() = default;
+
+    NonStrictContext(const NonStrictContext&) = delete;
+    NonStrictContext& operator=(const NonStrictContext&) = delete;
+
+    NonStrictContext(NonStrictContext&&) = default;
+    NonStrictContext& operator=(NonStrictContext&&) = default;
+
+    void unionContexts(const NonStrictContext& other)
+    {
+        // TODO: unimplemented
+    }
+
+    void intersectContexts(const NonStrictContext& other)
+    {
+        // TODO: unimplemented
+    }
+
+    void removeFromContext(const std::vector<DefId>& defs)
+    {
+        // TODO: unimplemented
+    }
+
+    std::optional<TypeId> find(const DefId& def)
+    {
+        // TODO: unimplemented
+        return {};
+    }
+
+    // Satisfies means that for a given DefId n, and an actual type t for `n`, t satisfies the context if t <: context[n]
+    // ice if the DefId is not in the context
+    bool satisfies(const DefId& def, TypeId inferredType)
+    {
+        // TODO: unimplemented
+        return false;
+    }
+
+    bool willRunTimeError(const DefId& def, TypeId inferredType)
+    {
+        return satisfies(def, inferredType);
+    }
+};
+
+struct NonStrictTypeChecker
+{
+
+    NotNull<BuiltinTypes> builtinTypes;
+    const NotNull<InternalErrorReporter> ice;
+    TypeArena arena;
+    Module* module;
+    Normalizer normalizer;
+    Subtyping subtyping;
+
+
+    NonStrictTypeChecker(NotNull<BuiltinTypes> builtinTypes, Subtyping subtyping, const NotNull<InternalErrorReporter> ice,
+        NotNull<UnifierSharedState> unifierState, Module* module)
+        : builtinTypes(builtinTypes)
+        , ice(ice)
+        , module(module)
+        , normalizer{&arena, builtinTypes, unifierState, /* cache inhabitance */ true}
+        , subtyping{builtinTypes, NotNull{&arena}, NotNull(&normalizer), ice, NotNull{module->getModuleScope().get()}}
+    {
+    }
+};
+
+void checkNonStrict(NotNull<BuiltinTypes> builtinTypes, Module* module)
+{
+    // TODO: unimplemented
+}
+} // namespace Luau

Analysis/src/Normalize.cpp

@@ -11,7 +11,6 @@
 #include "Luau/Type.h"
 #include "Luau/Unifier.h"
 
-LUAU_FASTFLAGVARIABLE(DebugLuauCopyBeforeNormalizing, false)
 LUAU_FASTFLAGVARIABLE(DebugLuauCheckNormalizeInvariant, false)
 
 // This could theoretically be 2000 on amd64, but x86 requires this.

Analysis/src/ToString.cpp

@@ -696,16 +696,33 @@ struct TypeStringifier
 
         std::string openbrace = "@@@";
         std::string closedbrace = "@@@?!";
-        switch (state.opts.hideTableKind ? TableState::Unsealed : ttv.state)
+        switch (state.opts.hideTableKind ? (FFlag::DebugLuauDeferredConstraintResolution ? TableState::Sealed : TableState::Unsealed) : ttv.state)
         {
         case TableState::Sealed:
-            state.result.invalid = true;
+            if (FFlag::DebugLuauDeferredConstraintResolution)
-            openbrace = "{|";
+            {
-            closedbrace = "|}";
+                openbrace = "{";
+                closedbrace = "}";
+            }
+            else
+            {
+                state.result.invalid = true;
+                openbrace = "{|";
+                closedbrace = "|}";
+            }
             break;
         case TableState::Unsealed:
-            openbrace = "{";
+            if (FFlag::DebugLuauDeferredConstraintResolution)
-            closedbrace = "}";
+            {
+                state.result.invalid = true;
+                openbrace = "{|";
+                closedbrace = "|}";
+            }
+            else
+            {
+                openbrace = "{";
+                closedbrace = "}";
+            }
             break;
         case TableState::Free:
             state.result.invalid = true;

Analysis/src/Unifier2.cpp

@@ -320,14 +320,26 @@ bool Unifier2::unify(TypePackId subTp, TypePackId superTp)
     for (size_t i = 0; i < maxLength; ++i)
         unify(subTypes[i], superTypes[i]);
 
-    if (!subTail || !superTail)
+    if (subTail && superTail)
-        return true;
+    {
+        TypePackId followedSubTail = follow(*subTail);
+        TypePackId followedSuperTail = follow(*superTail);
 
-    TypePackId followedSubTail = follow(*subTail);
+        if (get<FreeTypePack>(followedSubTail) || get<FreeTypePack>(followedSuperTail))
-    TypePackId followedSuperTail = follow(*superTail);
+            return unify(followedSubTail, followedSuperTail);
-
+    }
-    if (get<FreeTypePack>(followedSubTail) || get<FreeTypePack>(followedSuperTail))
+    else if (subTail)
-        return unify(followedSubTail, followedSuperTail);
+    {
+        TypePackId followedSubTail = follow(*subTail);
+        if (get<FreeTypePack>(followedSubTail))
+            asMutable(followedSubTail)->ty.emplace<BoundTypePack>(builtinTypes->emptyTypePack);
+    }
+    else if (superTail)
+    {
+        TypePackId followedSuperTail = follow(*superTail);
+        if (get<FreeTypePack>(followedSuperTail))
+            asMutable(followedSuperTail)->ty.emplace<BoundTypePack>(builtinTypes->emptyTypePack);
+    }
 
     return true;
 }
@@ -582,13 +594,6 @@ struct MutatingGeneralizer : TypeOnceVisitor
         TableType* tt = getMutable<TableType>(ty);
         LUAU_ASSERT(tt);
 
-        // We only unseal tables if they occur within function argument or
-        // return lists. In principle, we could always seal tables when
-        // generalizing, but that would mean that we'd lose the ability to
-        // report the existence of unsealed tables via things like hovertype.
-        if (tt->state == TableState::Unsealed && !isWithinFunction)
-            return true;
-
         tt->state = TableState::Sealed;
 
         return true;
@@ -611,10 +616,7 @@ std::optional<TypeId> Unifier2::generalize(TypeId ty)
 {
     ty = follow(ty);
 
-    if (ty->owningArena != arena)
+    if (ty->owningArena != arena || ty->persistent)
-        return ty;
-
-    if (ty->persistent)
         return ty;
 
     if (const FunctionType* ft = get<FunctionType>(ty); ft && (!ft->generics.empty() || !ft->genericPacks.empty()))
@@ -627,16 +629,23 @@ std::optional<TypeId> Unifier2::generalize(TypeId ty)
 
     gen.traverse(ty);
 
-    std::optional<TypeId> res = ty;
+    /* MutatingGeneralizer mutates types in place, so it is possible that ty has
+     * been transmuted to a BoundType. We must follow it again and verify that
+     * we are allowed to mutate it before we attach generics to it.
+     */
+    ty = follow(ty);
 
-    FunctionType* ftv = getMutable<FunctionType>(follow(*res));
+    if (ty->owningArena != arena || ty->persistent)
+        return ty;
+
+    FunctionType* ftv = getMutable<FunctionType>(ty);
     if (ftv)
     {
         ftv->generics = std::move(gen.generics);
         ftv->genericPacks = std::move(gen.genericPacks);
     }
 
-    return res;
+    return ty;
 }
 
 TypeId Unifier2::mkUnion(TypeId left, TypeId right)

CodeGen/src/CodeGenUtils.cpp

@@ -477,17 +477,9 @@ const Instruction* executeSETTABLEKS(lua_State* L, const Instruction* pc, StkId
     {
         Table* h = hvalue(rb);
 
-        int slot = LUAU_INSN_C(insn) & h->nodemask8;
+        // we ignore the fast path that checks for the cached slot since IrTranslation already checks for it.
-        LuaNode* n = &h->node[slot];
 
-        // fast-path: value is in expected slot
+        if (fastnotm(h->metatable, TM_NEWINDEX) && !h->readonly)
-        if (LUAU_LIKELY(ttisstring(gkey(n)) && tsvalue(gkey(n)) == tsvalue(kv) && !ttisnil(gval(n)) && !h->readonly))
-        {
-            setobj2t(L, gval(n), ra);
-            luaC_barriert(L, h, ra);
-            return pc;
-        }
-        else if (fastnotm(h->metatable, TM_NEWINDEX) && !h->readonly)
         {
             VM_PROTECT_PC(); // set may fail
 
@@ -502,6 +494,7 @@ const Instruction* executeSETTABLEKS(lua_State* L, const Instruction* pc, StkId
         else
         {
             // slow-path, may invoke Lua calls via __newindex metamethod
+            int slot = LUAU_INSN_C(insn) & h->nodemask8;
             L->cachedslot = slot;
             VM_PROTECT(luaV_settable(L, rb, kv, ra));
             // save cachedslot to accelerate future lookups; patches currently executing instruction since pc-2 rolls back two pc++

CodeGen/src/OptimizeConstProp.cpp

@@ -18,6 +18,7 @@ LUAU_FASTFLAGVARIABLE(DebugLuauAbortingChecks, false)
 LUAU_FASTFLAGVARIABLE(LuauReuseHashSlots2, false)
 LUAU_FASTFLAGVARIABLE(LuauKeepVmapLinear, false)
 LUAU_FASTFLAGVARIABLE(LuauMergeTagLoads, false)
+LUAU_FASTFLAGVARIABLE(LuauReuseArrSlots, false)
 
 namespace Luau
 {
@@ -174,14 +175,32 @@ struct ConstPropState
         }
     }
 
+    // Value propagation extends the live range of an SSA register
+    // In some cases we can't propagate earlier values because we can't guarantee that we will be able to find a storage/restore location
+    // As an example, when Luau call is performed, both volatile registers and stack slots might be overwritten
+    void invalidateValuePropagation()
+    {
+        valueMap.clear();
+        tryNumToIndexCache.clear();
+    }
+
+    // If table memory has changed, we can't reuse previously computed and validated table slot lookups
+    // Same goes for table array elements as well
+    void invalidateHeapTableData()
+    {
+        getSlotNodeCache.clear();
+        checkSlotMatchCache.clear();
+
+        getArrAddrCache.clear();
+        checkArraySizeCache.clear();
+    }
+
     void invalidateHeap()
     {
         for (int i = 0; i <= maxReg; ++i)
             invalidateHeap(regs[i]);
 
-        // If table memory has changed, we can't reuse previously computed and validated table slot lookups
+        invalidateHeapTableData();
-        getSlotNodeCache.clear();
-        checkSlotMatchCache.clear();
     }
 
     void invalidateHeap(RegisterInfo& reg)
@@ -203,9 +222,7 @@ struct ConstPropState
         for (int i = 0; i <= maxReg; ++i)
             invalidateTableArraySize(regs[i]);
 
-        // If table memory has changed, we can't reuse previously computed and validated table slot lookups
+        invalidateHeapTableData();
-        getSlotNodeCache.clear();
-        checkSlotMatchCache.clear();
     }
 
     void invalidateTableArraySize(RegisterInfo& reg)
@@ -389,9 +406,9 @@ struct ConstPropState
         checkedGc = false;
 
         instLink.clear();
-        valueMap.clear();
+
-        getSlotNodeCache.clear();
+        invalidateValuePropagation();
-        checkSlotMatchCache.clear();
+        invalidateHeapTableData();
     }
 
     IrFunction& function;
@@ -410,8 +427,15 @@ struct ConstPropState
 
     DenseHashMap<IrInst, uint32_t, IrInstHash, IrInstEq> valueMap;
 
-    std::vector<uint32_t> getSlotNodeCache;
+    // Some instruction re-uses can't be stored in valueMap because of extra requirements
-    std::vector<uint32_t> checkSlotMatchCache;
+    std::vector<uint32_t> tryNumToIndexCache; // Fallback block argument might be different
+
+    // Heap changes might affect table state
+    std::vector<uint32_t> getSlotNodeCache;    // Additionally, pcpos argument might be different
+    std::vector<uint32_t> checkSlotMatchCache; // Additionally, fallback block argument might be different
+
+    std::vector<uint32_t> getArrAddrCache;
+    std::vector<uint32_t> checkArraySizeCache; // Additionally, fallback block argument might be different
 };
 
 static void handleBuiltinEffects(ConstPropState& state, LuauBuiltinFunction bfid, uint32_t firstReturnReg, int nresults)
@@ -873,7 +897,24 @@ static void constPropInInst(ConstPropState& state, IrBuilder& build, IrFunction&
         // These instructions don't have an effect on register/memory state we are tracking
     case IrCmd::NOP:
     case IrCmd::LOAD_ENV:
+        break;
     case IrCmd::GET_ARR_ADDR:
+        if (!FFlag::LuauReuseArrSlots)
+            break;
+
+        for (uint32_t prevIdx : state.getArrAddrCache)
+        {
+            const IrInst& prev = function.instructions[prevIdx];
+
+            if (prev.a == inst.a && prev.b == inst.b)
+            {
+                substitute(function, inst, IrOp{IrOpKind::Inst, prevIdx});
+                return; // Break out from both the loop and the switch
+            }
+        }
+
+        if (int(state.getArrAddrCache.size()) < FInt::LuauCodeGenReuseSlotLimit)
+            state.getArrAddrCache.push_back(index);
         break;
     case IrCmd::GET_SLOT_NODE_ADDR:
         if (!FFlag::LuauReuseHashSlots2)
@@ -929,7 +970,25 @@ static void constPropInInst(ConstPropState& state, IrBuilder& build, IrFunction&
     case IrCmd::STRING_LEN:
     case IrCmd::NEW_TABLE:
     case IrCmd::DUP_TABLE:
+        break;
     case IrCmd::TRY_NUM_TO_INDEX:
+        if (!FFlag::LuauReuseArrSlots)
+            break;
+
+        for (uint32_t prevIdx : state.tryNumToIndexCache)
+        {
+            const IrInst& prev = function.instructions[prevIdx];
+
+            if (prev.a == inst.a)
+            {
+                substitute(function, inst, IrOp{IrOpKind::Inst, prevIdx});
+                return; // Break out from both the loop and the switch
+            }
+        }
+
+        if (int(state.tryNumToIndexCache.size()) < FInt::LuauCodeGenReuseSlotLimit)
+            state.tryNumToIndexCache.push_back(index);
+        break;
     case IrCmd::TRY_CALL_FASTGETTM:
         break;
     case IrCmd::INT_TO_NUM:
@@ -967,8 +1026,42 @@ static void constPropInInst(ConstPropState& state, IrBuilder& build, IrFunction&
                 {
                     replace(function, block, index, {IrCmd::JUMP, inst.c});
                 }
+
+                return; // Break out from both the loop and the switch
             }
         }
+
+        if (!FFlag::LuauReuseArrSlots)
+            break;
+
+        for (uint32_t prevIdx : state.checkArraySizeCache)
+        {
+            const IrInst& prev = function.instructions[prevIdx];
+
+            if (prev.a != inst.a)
+                continue;
+
+            bool sameBoundary = prev.b == inst.b;
+
+            // If arguments are different, in case they are both constant, we can check if a larger bound was already tested
+            if (!sameBoundary && inst.b.kind == IrOpKind::Constant && prev.b.kind == IrOpKind::Constant &&
+                function.intOp(inst.b) < function.intOp(prev.b))
+                sameBoundary = true;
+
+            if (sameBoundary)
+            {
+                if (FFlag::DebugLuauAbortingChecks)
+                    replace(function, inst.c, build.undef());
+                else
+                    kill(function, inst);
+                return; // Break out from both the loop and the switch
+            }
+
+            // TODO: it should be possible to update previous check with a higher bound if current and previous checks are against a constant
+        }
+
+        if (int(state.checkArraySizeCache.size()) < FInt::LuauCodeGenReuseSlotLimit)
+            state.checkArraySizeCache.push_back(index);
         break;
     }
     case IrCmd::CHECK_SLOT_MATCH:
@@ -1053,9 +1146,8 @@ static void constPropInInst(ConstPropState& state, IrBuilder& build, IrFunction&
             replace(function, inst.f, build.constUint(info->knownTableArraySize));
 
         // TODO: this can be relaxed when x64 emitInstSetList becomes aware of register allocator
-        state.valueMap.clear();
+        state.invalidateValuePropagation();
-        state.getSlotNodeCache.clear();
+        state.invalidateHeapTableData();
-        state.checkSlotMatchCache.clear();
         break;
     case IrCmd::CALL:
         state.invalidateRegistersFrom(vmRegOp(inst.a));
@@ -1064,15 +1156,14 @@ static void constPropInInst(ConstPropState& state, IrBuilder& build, IrFunction&
         // We cannot guarantee right now that all live values can be rematerialized from non-stack memory locations
         // To prevent earlier values from being propagated to after the call, we have to clear the map
         // TODO: remove only the values that don't have a guaranteed restore location
-        state.valueMap.clear();
+        state.invalidateValuePropagation();
         break;
     case IrCmd::FORGLOOP:
         state.invalidateRegistersFrom(vmRegOp(inst.a) + 2); // Rn and Rn+1 are not modified
 
         // TODO: this can be relaxed when x64 emitInstForGLoop becomes aware of register allocator
-        state.valueMap.clear();
+        state.invalidateValuePropagation();
-        state.getSlotNodeCache.clear();
+        state.invalidateHeapTableData();
-        state.checkSlotMatchCache.clear();
         break;
     case IrCmd::FORGLOOP_FALLBACK:
         state.invalidateRegistersFrom(vmRegOp(inst.a) + 2); // Rn and Rn+1 are not modified
@@ -1139,11 +1230,10 @@ static void constPropInBlock(IrBuilder& build, IrBlock& block, ConstPropState& s
     if (!FFlag::LuauKeepVmapLinear)
     {
         // Value numbering and load/store propagation is not performed between blocks
-        state.valueMap.clear();
+        state.invalidateValuePropagation();
 
         // Same for table slot data propagation
-        state.getSlotNodeCache.clear();
+        state.invalidateHeapTableData();
-        state.checkSlotMatchCache.clear();
     }
 }
 
@@ -1168,11 +1258,10 @@ static void constPropInBlockChain(IrBuilder& build, std::vector<uint8_t>& visite
         {
             // Value numbering and load/store propagation is not performed between blocks right now
             // This is because cross-block value uses limit creation of linear block (restriction in collectDirectBlockJumpPath)
-            state.valueMap.clear();
+            state.invalidateValuePropagation();
 
             // Same for table slot data propagation
-            state.getSlotNodeCache.clear();
+            state.invalidateHeapTableData();
-            state.checkSlotMatchCache.clear();
         }
 
         // Blocks in a chain are guaranteed to follow each other

Sources.cmake

@@ -178,6 +178,7 @@ target_sources(Luau.Analysis PRIVATE
     Analysis/include/Luau/Metamethods.h
     Analysis/include/Luau/Module.h
     Analysis/include/Luau/ModuleResolver.h
+    Analysis/include/Luau/NonStrictTypeChecker.h
     Analysis/include/Luau/Normalize.h
     Analysis/include/Luau/Predicate.h
     Analysis/include/Luau/Quantify.h
@@ -235,6 +236,7 @@ target_sources(Luau.Analysis PRIVATE
     Analysis/src/Linter.cpp
     Analysis/src/LValue.cpp
     Analysis/src/Module.cpp
+    Analysis/src/NonStrictTypeChecker.cpp        
     Analysis/src/Normalize.cpp
     Analysis/src/Quantify.cpp
     Analysis/src/Refinement.cpp
@@ -398,6 +400,7 @@ if(TARGET Luau.UnitTest)
         tests/LValue.test.cpp
         tests/Module.test.cpp
         tests/NonstrictMode.test.cpp
+        tests/NonStrictTypeChecker.test.cpp	
         tests/Normalize.test.cpp
         tests/NotNull.test.cpp
         tests/Parser.test.cpp

VM/src/loslib.cpp

@@ -22,27 +22,6 @@ static time_t timegm(struct tm* timep)
 {
     return _mkgmtime(timep);
 }
-#elif defined(__FreeBSD__)
-static tm* gmtime_r(const time_t* timep, tm* result)
-{
-    // Note: return is reversed from Windows (0 is success on Windows, but 0/null is failure elsewhere)
-    // Windows https://learn.microsoft.com/en-us/cpp/c-runtime-library/reference/gmtime-s-gmtime32-s-gmtime64-s?view=msvc-170#return-value
-    // Everyone else https://en.cppreference.com/w/c/chrono/gmtime
-    return gmtime_s(timep, result);
-}
-
-static tm* localtime_r(const time_t* timep, tm* result)
-{
-    // Note: return is reversed from Windows (0 is success on Windows, but 0/null is failure elsewhere)
-    // Windows https://learn.microsoft.com/en-us/cpp/c-runtime-library/reference/localtime-s-localtime32-s-localtime64-s?view=msvc-170#return-value
-    // Everyone else https://en.cppreference.com/w/c/chrono/localtime
-    return localtime_s(timep, result);
-}
-
-static time_t timegm(struct tm* timep)
-{
-    return mktime(timep);
-}
 #endif
 
 static int os_clock(lua_State* L)

extern/doctest.h

@@ -3139,7 +3139,11 @@ DOCTEST_MAKE_STD_HEADERS_CLEAN_FROM_WARNINGS_ON_WALL_BEGIN
 #include <unordered_set>
 #include <exception>
 #include <stdexcept>
+
+#if !defined(DOCTEST_CONFIG_NO_POSIX_SIGNALS)
 #include <csignal>
+#endif
+
 #include <cfloat>
 #include <cctype>
 #include <cstdint>
@@ -5667,6 +5671,8 @@ namespace {
                 std::tm timeInfo;
 #ifdef DOCTEST_PLATFORM_WINDOWS
                 gmtime_s(&timeInfo, &rawtime);
+#elif defined(DOCTEST_CONFIG_USE_GMTIME_S)
+                gmtime_s(&rawtime, &timeInfo);
 #else // DOCTEST_PLATFORM_WINDOWS
                 gmtime_r(&rawtime, &timeInfo);
 #endif // DOCTEST_PLATFORM_WINDOWS

tests/Autocomplete.test.cpp

@@ -2162,7 +2162,10 @@ local fp: @1= f
 
     auto ac = autocomplete('1');
 
-    REQUIRE_EQ("({| x: number, y: number |}) -> number", toString(requireType("f")));
+    if (FFlag::DebugLuauDeferredConstraintResolution)
+        REQUIRE_EQ("({ x: number, y: number }) -> number", toString(requireType("f")));
+    else
+        REQUIRE_EQ("({| x: number, y: number |}) -> number", toString(requireType("f")));
     CHECK(ac.entryMap.count("({ x: number, y: number }) -> number"));
 }
 

tests/CodeAllocator.test.cpp

@@ -274,6 +274,9 @@ constexpr X64::RegisterX64 rNonVol4 = X64::r14;
 
 TEST_CASE("GeneratedCodeExecutionX64")
 {
+    if (!Luau::CodeGen::isSupported())
+        return;
+
     using namespace X64;
 
     AssemblyBuilderX64 build(/* logText= */ false);
@@ -315,6 +318,9 @@ static void nonthrowing(int64_t arg)
 
 TEST_CASE("GeneratedCodeExecutionWithThrowX64")
 {
+    if (!Luau::CodeGen::isSupported())
+        return;
+
     using namespace X64;
 
     AssemblyBuilderX64 build(/* logText= */ false);
@@ -513,6 +519,9 @@ TEST_CASE("GeneratedCodeExecutionWithThrowX64Simd")
 
 TEST_CASE("GeneratedCodeExecutionMultipleFunctionsWithThrowX64")
 {
+    if (!Luau::CodeGen::isSupported())
+        return;
+
     using namespace X64;
 
     AssemblyBuilderX64 build(/* logText= */ false);
@@ -650,6 +659,9 @@ TEST_CASE("GeneratedCodeExecutionMultipleFunctionsWithThrowX64")
 
 TEST_CASE("GeneratedCodeExecutionWithThrowOutsideTheGateX64")
 {
+    if (!Luau::CodeGen::isSupported())
+        return;
+
     using namespace X64;
 
     AssemblyBuilderX64 build(/* logText= */ false);

tests/ConstraintGraphBuilderFixture.cpp

@@ -21,7 +21,7 @@ void ConstraintGraphBuilderFixture::generateConstraints(const std::string& code)
     dfg = std::make_unique<DataFlowGraph>(DataFlowGraphBuilder::build(root, NotNull{&ice}));
     cgb = std::make_unique<ConstraintGraphBuilder>(mainModule, NotNull{&normalizer}, NotNull(&moduleResolver), builtinTypes, NotNull(&ice),
         frontend.globals.globalScope, /*prepareModuleScope*/ nullptr, &logger, NotNull{dfg.get()}, std::vector<RequireCycle>());
-    cgb->visit(root);
+    cgb->visitModuleRoot(root);
     rootScope = cgb->rootScope;
     constraints = Luau::borrowConstraints(cgb->constraints);
 }

tests/Differ.test.cpp

@@ -657,8 +657,12 @@ TEST_CASE_FIXTURE(DifferFixture, "function_table_self_referential_cyclic")
     )");
     LUAU_REQUIRE_NO_ERRORS(result);
 
-    compareTypesNe("foo", "almostFoo",
+    if (FFlag::DebugLuauDeferredConstraintResolution)
-        R"(DiffError: these two types are not equal because the left type at <unlabeled-symbol>.Ret[1].bar.Ret[1] has type t1 where t1 = {| bar: () -> t1 |}, while the right type at <unlabeled-symbol>.Ret[1].bar.Ret[1] has type t1 where t1 = () -> t1)");
+        compareTypesNe("foo", "almostFoo",
+            R"(DiffError: these two types are not equal because the left type at <unlabeled-symbol>.Ret[1].bar.Ret[1] has type t1 where t1 = { bar: () -> t1 }, while the right type at <unlabeled-symbol>.Ret[1].bar.Ret[1] has type t1 where t1 = () -> t1)");
+    else
+        compareTypesNe("foo", "almostFoo",
+            R"(DiffError: these two types are not equal because the left type at <unlabeled-symbol>.Ret[1].bar.Ret[1] has type t1 where t1 = {| bar: () -> t1 |}, while the right type at <unlabeled-symbol>.Ret[1].bar.Ret[1] has type t1 where t1 = () -> t1)");
 }
 
 TEST_CASE_FIXTURE(DifferFixture, "equal_union_cyclic")
@@ -812,8 +816,12 @@ TEST_CASE_FIXTURE(DifferFixture, "union_missing")
     )");
     LUAU_REQUIRE_NO_ERRORS(result);
 
-    compareTypesNe("foo", "almostFoo",
+    if (FFlag::DebugLuauDeferredConstraintResolution)
-        R"(DiffError: these two types are not equal because the left type at <unlabeled-symbol> is a union containing type {| baz: boolean, rot: "singleton" |}, while the right type at <unlabeled-symbol> is a union missing type {| baz: boolean, rot: "singleton" |})");
+        compareTypesNe("foo", "almostFoo",
+            R"(DiffError: these two types are not equal because the left type at <unlabeled-symbol> is a union containing type { baz: boolean, rot: "singleton" }, while the right type at <unlabeled-symbol> is a union missing type { baz: boolean, rot: "singleton" })");
+    else
+        compareTypesNe("foo", "almostFoo",
+            R"(DiffError: these two types are not equal because the left type at <unlabeled-symbol> is a union containing type {| baz: boolean, rot: "singleton" |}, while the right type at <unlabeled-symbol> is a union missing type {| baz: boolean, rot: "singleton" |})");
 }
 
 TEST_CASE_FIXTURE(DifferFixture, "intersection_missing_right")
@@ -848,8 +856,12 @@ TEST_CASE_FIXTURE(DifferFixture, "intersection_tables_missing_right")
     )");
     LUAU_REQUIRE_NO_ERRORS(result);
 
-    compareTypesNe("foo", "almostFoo",
+    if (FFlag::DebugLuauDeferredConstraintResolution)
-        R"(DiffError: these two types are not equal because the left type at <unlabeled-symbol> is an intersection containing type {| x: number |}, while the right type at <unlabeled-symbol> is an intersection missing type {| x: number |})");
+        compareTypesNe("foo", "almostFoo",
+            R"(DiffError: these two types are not equal because the left type at <unlabeled-symbol> is an intersection containing type { x: number }, while the right type at <unlabeled-symbol> is an intersection missing type { x: number })");
+    else
+        compareTypesNe("foo", "almostFoo",
+            R"(DiffError: these two types are not equal because the left type at <unlabeled-symbol> is an intersection containing type {| x: number |}, while the right type at <unlabeled-symbol> is an intersection missing type {| x: number |})");
 }
 
 TEST_CASE_FIXTURE(DifferFixture, "intersection_tables_missing_left")
@@ -860,8 +872,12 @@ TEST_CASE_FIXTURE(DifferFixture, "intersection_tables_missing_left")
     )");
     LUAU_REQUIRE_NO_ERRORS(result);
 
-    compareTypesNe("foo", "almostFoo",
+    if (FFlag::DebugLuauDeferredConstraintResolution)
-        R"(DiffError: these two types are not equal because the left type at <unlabeled-symbol> is an intersection missing type {| z: boolean |}, while the right type at <unlabeled-symbol> is an intersection containing type {| z: boolean |})");
+        compareTypesNe("foo", "almostFoo",
+            R"(DiffError: these two types are not equal because the left type at <unlabeled-symbol> is an intersection missing type { z: boolean }, while the right type at <unlabeled-symbol> is an intersection containing type { z: boolean })");
+    else
+        compareTypesNe("foo", "almostFoo",
+            R"(DiffError: these two types are not equal because the left type at <unlabeled-symbol> is an intersection missing type {| z: boolean |}, while the right type at <unlabeled-symbol> is an intersection containing type {| z: boolean |})");
 }
 
 TEST_CASE_FIXTURE(DifferFixture, "equal_function")

tests/Frontend.test.cpp

@@ -12,6 +12,8 @@
 
 using namespace Luau;
 
+LUAU_FASTFLAG(DebugLuauDeferredConstraintResolution)
+
 namespace
 {
 
@@ -160,7 +162,10 @@ TEST_CASE_FIXTURE(FrontendFixture, "automatically_check_dependent_scripts")
     auto bExports = first(bModule->returnType);
     REQUIRE(!!bExports);
 
-    CHECK_EQ("{| b_value: number |}", toString(*bExports));
+    if (FFlag::DebugLuauDeferredConstraintResolution)
+        CHECK_EQ("{ b_value: number }", toString(*bExports));
+    else
+        CHECK_EQ("{| b_value: number |}", toString(*bExports));
 }
 
 TEST_CASE_FIXTURE(FrontendFixture, "automatically_check_cyclically_dependent_scripts")
@@ -302,7 +307,11 @@ TEST_CASE_FIXTURE(FrontendFixture, "nocheck_cycle_used_by_checked")
 
     std::optional<TypeId> cExports = first(cModule->returnType);
     REQUIRE(bool(cExports));
-    CHECK_EQ("{| a: any, b: any |}", toString(*cExports));
+
+    if (FFlag::DebugLuauDeferredConstraintResolution)
+        CHECK_EQ("{ a: any, b: any }", toString(*cExports));
+    else
+        CHECK_EQ("{| a: any, b: any |}", toString(*cExports));
 }
 
 TEST_CASE_FIXTURE(FrontendFixture, "cycle_detection_disabled_in_nocheck")
@@ -473,20 +482,32 @@ return {mod_b = 2}
     LUAU_REQUIRE_ERRORS(resultB);
 
     TypeId tyB = requireExportedType("game/B", "btype");
-    CHECK_EQ(toString(tyB, opts), "{| x: number |}");
+    if (FFlag::DebugLuauDeferredConstraintResolution)
+        CHECK_EQ(toString(tyB, opts), "{ x: number }");
+    else
+        CHECK_EQ(toString(tyB, opts), "{| x: number |}");
 
     TypeId tyA = requireExportedType("game/A", "atype");
-    CHECK_EQ(toString(tyA, opts), "{| x: any |}");
+    if (FFlag::DebugLuauDeferredConstraintResolution)
+        CHECK_EQ(toString(tyA, opts), "{ x: any }");
+    else
+        CHECK_EQ(toString(tyA, opts), "{| x: any |}");
 
     frontend.markDirty("game/B");
     resultB = frontend.check("game/B");
     LUAU_REQUIRE_ERRORS(resultB);
 
     tyB = requireExportedType("game/B", "btype");
-    CHECK_EQ(toString(tyB, opts), "{| x: number |}");
+    if (FFlag::DebugLuauDeferredConstraintResolution)
+        CHECK_EQ(toString(tyB, opts), "{ x: number }");
+    else
+        CHECK_EQ(toString(tyB, opts), "{| x: number |}");
 
     tyA = requireExportedType("game/A", "atype");
-    CHECK_EQ(toString(tyA, opts), "{| x: any |}");
+    if (FFlag::DebugLuauDeferredConstraintResolution)
+        CHECK_EQ(toString(tyA, opts), "{ x: any }");
+    else
+        CHECK_EQ(toString(tyA, opts), "{| x: any |}");
 }
 
 TEST_CASE_FIXTURE(FrontendFixture, "dont_reparse_clean_file_when_linting")
@@ -559,7 +580,10 @@ TEST_CASE_FIXTURE(FrontendFixture, "recheck_if_dependent_script_is_dirty")
     auto bExports = first(bModule->returnType);
     REQUIRE(!!bExports);
 
-    CHECK_EQ("{| b_value: string |}", toString(*bExports));
+    if (FFlag::DebugLuauDeferredConstraintResolution)
+        CHECK_EQ("{ b_value: string }", toString(*bExports));
+    else
+        CHECK_EQ("{| b_value: string |}", toString(*bExports));
 }
 
 TEST_CASE_FIXTURE(FrontendFixture, "mark_non_immediate_reverse_deps_as_dirty")
@@ -883,8 +907,12 @@ TEST_CASE_FIXTURE(FrontendFixture, "it_should_be_safe_to_stringify_errors_when_f
     // When this test fails, it is because the TypeIds needed by the error have been deallocated.
     // It is thus basically impossible to predict what will happen when this assert is evaluated.
     // It could segfault, or you could see weird type names like the empty string or <VALUELESS BY EXCEPTION>
-    REQUIRE_EQ(
+    if (FFlag::DebugLuauDeferredConstraintResolution)
-        "Table type 'a' not compatible with type '{| Count: number |}' because the former is missing field 'Count'", toString(result.errors[0]));
+        REQUIRE_EQ(
+            "Table type 'a' not compatible with type '{ Count: number }' because the former is missing field 'Count'", toString(result.errors[0]));
+    else
+        REQUIRE_EQ(
+            "Table type 'a' not compatible with type '{| Count: number |}' because the former is missing field 'Count'", toString(result.errors[0]));
 }
 
 TEST_CASE_FIXTURE(FrontendFixture, "trace_requires_in_nonstrict_mode")

tests/IrBuilder.test.cpp

@@ -2060,6 +2060,244 @@ bb_fallback_1:
 )");
 }
 
+TEST_CASE_FIXTURE(IrBuilderFixture, "DuplicateArrayElemChecksSameIndex")
+{
+    ScopedFastFlag luauReuseHashSlots{"LuauReuseArrSlots", true};
+
+    IrOp block = build.block(IrBlockKind::Internal);
+    IrOp fallback = build.block(IrBlockKind::Fallback);
+
+    build.beginBlock(block);
+
+    // This roughly corresponds to 'return t[1] + t[1]'
+    IrOp table1 = build.inst(IrCmd::LOAD_POINTER, build.vmReg(1));
+    build.inst(IrCmd::CHECK_ARRAY_SIZE, table1, build.constInt(0), fallback);
+    IrOp elem1 = build.inst(IrCmd::GET_ARR_ADDR, table1, build.constInt(0));
+    IrOp value1 = build.inst(IrCmd::LOAD_TVALUE, elem1, build.constInt(0));
+    build.inst(IrCmd::STORE_TVALUE, build.vmReg(3), value1);
+
+    build.inst(IrCmd::CHECK_ARRAY_SIZE, table1, build.constInt(0), fallback); // This will be removed
+    IrOp elem2 = build.inst(IrCmd::GET_ARR_ADDR, table1, build.constInt(0));  // And this will be substituted
+    IrOp value1b = build.inst(IrCmd::LOAD_TVALUE, elem2, build.constInt(0));
+    build.inst(IrCmd::STORE_TVALUE, build.vmReg(4), value1b);
+
+    IrOp a = build.inst(IrCmd::LOAD_DOUBLE, build.vmReg(3));
+    IrOp b = build.inst(IrCmd::LOAD_DOUBLE, build.vmReg(4));
+    IrOp sum = build.inst(IrCmd::ADD_NUM, a, b);
+    build.inst(IrCmd::STORE_DOUBLE, build.vmReg(2), sum);
+
+    build.inst(IrCmd::RETURN, build.vmReg(2), build.constUint(1));
+
+    build.beginBlock(fallback);
+    build.inst(IrCmd::RETURN, build.vmReg(0), build.constUint(1));
+
+    updateUseCounts(build.function);
+    constPropInBlockChains(build, true);
+
+    // In the future, we might even see duplicate identical TValue loads go away
+    // In the future, we might even see loads of different VM regs with the same value go away
+    CHECK("\n" + toString(build.function, /* includeUseInfo */ false) == R"(
+bb_0:
+   %0 = LOAD_POINTER R1
+   CHECK_ARRAY_SIZE %0, 0i, bb_fallback_1
+   %2 = GET_ARR_ADDR %0, 0i
+   %3 = LOAD_TVALUE %2, 0i
+   STORE_TVALUE R3, %3
+   %7 = LOAD_TVALUE %2, 0i
+   STORE_TVALUE R4, %7
+   %9 = LOAD_DOUBLE R3
+   %10 = LOAD_DOUBLE R4
+   %11 = ADD_NUM %9, %10
+   STORE_DOUBLE R2, %11
+   RETURN R2, 1u
+
+bb_fallback_1:
+   RETURN R0, 1u
+
+)");
+}
+
+TEST_CASE_FIXTURE(IrBuilderFixture, "DuplicateArrayElemChecksLowerIndex")
+{
+    ScopedFastFlag luauReuseHashSlots{"LuauReuseArrSlots", true};
+
+    IrOp block = build.block(IrBlockKind::Internal);
+    IrOp fallback = build.block(IrBlockKind::Fallback);
+
+    build.beginBlock(block);
+
+    // This roughly corresponds to 'return t[i] + t[i]'
+    IrOp table1 = build.inst(IrCmd::LOAD_POINTER, build.vmReg(1));
+    IrOp index = build.inst(IrCmd::LOAD_DOUBLE, build.vmReg(2));
+    IrOp validIndex = build.inst(IrCmd::TRY_NUM_TO_INDEX, index, fallback);
+    IrOp validOffset = build.inst(IrCmd::SUB_INT, validIndex, build.constInt(1));
+    build.inst(IrCmd::CHECK_ARRAY_SIZE, table1, validOffset, fallback);
+    IrOp elem1 = build.inst(IrCmd::GET_ARR_ADDR, table1, build.constInt(0));
+    IrOp value1 = build.inst(IrCmd::LOAD_TVALUE, elem1, build.constInt(0));
+    build.inst(IrCmd::STORE_TVALUE, build.vmReg(3), value1);
+
+    IrOp validIndex2 = build.inst(IrCmd::TRY_NUM_TO_INDEX, index, fallback);
+    IrOp validOffset2 = build.inst(IrCmd::SUB_INT, validIndex2, build.constInt(1));
+    build.inst(IrCmd::CHECK_ARRAY_SIZE, table1, validOffset2, fallback);     // This will be removed
+    IrOp elem2 = build.inst(IrCmd::GET_ARR_ADDR, table1, build.constInt(0)); // And this will be substituted
+    IrOp value1b = build.inst(IrCmd::LOAD_TVALUE, elem2, build.constInt(0));
+    build.inst(IrCmd::STORE_TVALUE, build.vmReg(4), value1b);
+
+    IrOp a = build.inst(IrCmd::LOAD_DOUBLE, build.vmReg(3));
+    IrOp b = build.inst(IrCmd::LOAD_DOUBLE, build.vmReg(4));
+    IrOp sum = build.inst(IrCmd::ADD_NUM, a, b);
+    build.inst(IrCmd::STORE_DOUBLE, build.vmReg(2), sum);
+
+    build.inst(IrCmd::RETURN, build.vmReg(2), build.constUint(1));
+
+    build.beginBlock(fallback);
+    build.inst(IrCmd::RETURN, build.vmReg(0), build.constUint(1));
+
+    updateUseCounts(build.function);
+    constPropInBlockChains(build, true);
+
+    // In the future, we might even see duplicate identical TValue loads go away
+    // In the future, we might even see loads of different VM regs with the same value go away
+    CHECK("\n" + toString(build.function, /* includeUseInfo */ false) == R"(
+bb_0:
+   %0 = LOAD_POINTER R1
+   %1 = LOAD_DOUBLE R2
+   %2 = TRY_NUM_TO_INDEX %1, bb_fallback_1
+   %3 = SUB_INT %2, 1i
+   CHECK_ARRAY_SIZE %0, %3, bb_fallback_1
+   %5 = GET_ARR_ADDR %0, 0i
+   %6 = LOAD_TVALUE %5, 0i
+   STORE_TVALUE R3, %6
+   %12 = LOAD_TVALUE %5, 0i
+   STORE_TVALUE R4, %12
+   %14 = LOAD_DOUBLE R3
+   %15 = LOAD_DOUBLE R4
+   %16 = ADD_NUM %14, %15
+   STORE_DOUBLE R2, %16
+   RETURN R2, 1u
+
+bb_fallback_1:
+   RETURN R0, 1u
+
+)");
+}
+
+TEST_CASE_FIXTURE(IrBuilderFixture, "DuplicateArrayElemChecksSameValue")
+{
+    ScopedFastFlag luauReuseHashSlots{"LuauReuseArrSlots", true};
+
+    IrOp block = build.block(IrBlockKind::Internal);
+    IrOp fallback = build.block(IrBlockKind::Fallback);
+
+    build.beginBlock(block);
+
+    // This roughly corresponds to 'return t[2] + t[1]'
+    IrOp table1 = build.inst(IrCmd::LOAD_POINTER, build.vmReg(1));
+    build.inst(IrCmd::CHECK_ARRAY_SIZE, table1, build.constInt(1), fallback);
+    IrOp elem1 = build.inst(IrCmd::GET_ARR_ADDR, table1, build.constInt(1));
+    IrOp value1 = build.inst(IrCmd::LOAD_TVALUE, elem1, build.constInt(0));
+    build.inst(IrCmd::STORE_TVALUE, build.vmReg(3), value1);
+
+    build.inst(IrCmd::CHECK_ARRAY_SIZE, table1, build.constInt(0), fallback); // This will be removed
+    IrOp elem2 = build.inst(IrCmd::GET_ARR_ADDR, table1, build.constInt(0));
+    IrOp value1b = build.inst(IrCmd::LOAD_TVALUE, elem2, build.constInt(0));
+    build.inst(IrCmd::STORE_TVALUE, build.vmReg(4), value1b);
+
+    IrOp a = build.inst(IrCmd::LOAD_DOUBLE, build.vmReg(3));
+    IrOp b = build.inst(IrCmd::LOAD_DOUBLE, build.vmReg(4));
+    IrOp sum = build.inst(IrCmd::ADD_NUM, a, b);
+    build.inst(IrCmd::STORE_DOUBLE, build.vmReg(2), sum);
+
+    build.inst(IrCmd::RETURN, build.vmReg(2), build.constUint(1));
+
+    build.beginBlock(fallback);
+    build.inst(IrCmd::RETURN, build.vmReg(0), build.constUint(1));
+
+    updateUseCounts(build.function);
+    constPropInBlockChains(build, true);
+
+    CHECK("\n" + toString(build.function, /* includeUseInfo */ false) == R"(
+bb_0:
+   %0 = LOAD_POINTER R1
+   CHECK_ARRAY_SIZE %0, 1i, bb_fallback_1
+   %2 = GET_ARR_ADDR %0, 1i
+   %3 = LOAD_TVALUE %2, 0i
+   STORE_TVALUE R3, %3
+   %6 = GET_ARR_ADDR %0, 0i
+   %7 = LOAD_TVALUE %6, 0i
+   STORE_TVALUE R4, %7
+   %9 = LOAD_DOUBLE R3
+   %10 = LOAD_DOUBLE R4
+   %11 = ADD_NUM %9, %10
+   STORE_DOUBLE R2, %11
+   RETURN R2, 1u
+
+bb_fallback_1:
+   RETURN R0, 1u
+
+)");
+}
+
+TEST_CASE_FIXTURE(IrBuilderFixture, "DuplicateArrayElemChecksInvalidations")
+{
+    ScopedFastFlag luauReuseHashSlots{"LuauReuseArrSlots", true};
+
+    IrOp block = build.block(IrBlockKind::Internal);
+    IrOp fallback = build.block(IrBlockKind::Fallback);
+
+    build.beginBlock(block);
+
+    // This roughly corresponds to 'return t[1] + t[1]' with a strange table.insert in the middle
+    IrOp table1 = build.inst(IrCmd::LOAD_POINTER, build.vmReg(1));
+    build.inst(IrCmd::CHECK_ARRAY_SIZE, table1, build.constInt(0), fallback);
+    IrOp elem1 = build.inst(IrCmd::GET_ARR_ADDR, table1, build.constInt(0));
+    IrOp value1 = build.inst(IrCmd::LOAD_TVALUE, elem1, build.constInt(0));
+    build.inst(IrCmd::STORE_TVALUE, build.vmReg(3), value1);
+
+    build.inst(IrCmd::TABLE_SETNUM, table1, build.constInt(2));
+
+    build.inst(IrCmd::CHECK_ARRAY_SIZE, table1, build.constInt(0), fallback); // This will be removed
+    IrOp elem2 = build.inst(IrCmd::GET_ARR_ADDR, table1, build.constInt(0));  // And this will be substituted
+    IrOp value1b = build.inst(IrCmd::LOAD_TVALUE, elem2, build.constInt(0));
+    build.inst(IrCmd::STORE_TVALUE, build.vmReg(4), value1b);
+
+    IrOp a = build.inst(IrCmd::LOAD_DOUBLE, build.vmReg(3));
+    IrOp b = build.inst(IrCmd::LOAD_DOUBLE, build.vmReg(4));
+    IrOp sum = build.inst(IrCmd::ADD_NUM, a, b);
+    build.inst(IrCmd::STORE_DOUBLE, build.vmReg(2), sum);
+
+    build.inst(IrCmd::RETURN, build.vmReg(2), build.constUint(1));
+
+    build.beginBlock(fallback);
+    build.inst(IrCmd::RETURN, build.vmReg(0), build.constUint(1));
+
+    updateUseCounts(build.function);
+    constPropInBlockChains(build, true);
+
+    CHECK("\n" + toString(build.function, /* includeUseInfo */ false) == R"(
+bb_0:
+   %0 = LOAD_POINTER R1
+   CHECK_ARRAY_SIZE %0, 0i, bb_fallback_1
+   %2 = GET_ARR_ADDR %0, 0i
+   %3 = LOAD_TVALUE %2, 0i
+   STORE_TVALUE R3, %3
+   %5 = TABLE_SETNUM %0, 2i
+   CHECK_ARRAY_SIZE %0, 0i, bb_fallback_1
+   %7 = GET_ARR_ADDR %0, 0i
+   %8 = LOAD_TVALUE %7, 0i
+   STORE_TVALUE R4, %8
+   %10 = LOAD_DOUBLE R3
+   %11 = LOAD_DOUBLE R4
+   %12 = ADD_NUM %10, %11
+   STORE_DOUBLE R2, %12
+   RETURN R2, 1u
+
+bb_fallback_1:
+   RETURN R0, 1u
+
+)");
+}
+
 TEST_SUITE_END();
 
 TEST_SUITE_BEGIN("Analysis");

tests/Module.test.cpp

@@ -489,15 +489,20 @@ TEST_CASE_FIXTURE(BuiltinsFixture, "do_not_clone_types_of_reexported_values")
     LUAU_REQUIRE_NO_ERRORS(result);
 
     ModulePtr modA = frontend.moduleResolver.getModule("Module/A");
-    ModulePtr modB = frontend.moduleResolver.getModule("Module/B");
     REQUIRE(modA);
+    ModulePtr modB = frontend.moduleResolver.getModule("Module/B");
     REQUIRE(modB);
+
     std::optional<TypeId> typeA = first(modA->returnType);
-    std::optional<TypeId> typeB = first(modB->returnType);
     REQUIRE(typeA);
+    std::optional<TypeId> typeB = first(modB->returnType);
     REQUIRE(typeB);
+
     TableType* tableA = getMutable<TableType>(*typeA);
+    REQUIRE_MESSAGE(tableA, "Expected a table, but got " << toString(*typeA));
     TableType* tableB = getMutable<TableType>(*typeB);
+    REQUIRE_MESSAGE(tableB, "Expected a table, but got " << toString(*typeB));
+
     CHECK(tableA->props["a"].type() == tableB->props["b"].type());
 }
 

tests/NonStrictTypeChecker.test.cpp

@@ -0,0 +1,17 @@
+// This file is part of the Luau programming language and is licensed under MIT License; see LICENSE.txt for details
+#include "Luau/NonStrictTypeChecker.h"
+
+#include "Fixture.h"
+
+#include "doctest.h"
+
+using namespace Luau;
+
+TEST_SUITE_BEGIN("NonStrictTypeCheckerTest");
+
+TEST_CASE_FIXTURE(Fixture, "basic")
+{
+    Luau::checkNonStrict(builtinTypes, nullptr);
+}
+
+TEST_SUITE_END();

tests/Normalize.test.cpp

@@ -797,7 +797,10 @@ TEST_CASE_FIXTURE(NormalizeFixture, "narrow_union_of_classes_with_intersection")
 
 TEST_CASE_FIXTURE(NormalizeFixture, "intersection_of_metatables_where_the_metatable_is_top_or_bottom")
 {
-    CHECK("{ @metatable *error-type*, {|  |} }" == toString(normal("Mt<{}, any> & Mt<{}, err>")));
+    if (FFlag::DebugLuauDeferredConstraintResolution)
+        CHECK("{ @metatable *error-type*, {  } }" == toString(normal("Mt<{}, any> & Mt<{}, err>")));
+    else
+        CHECK("{ @metatable *error-type*, {|  |} }" == toString(normal("Mt<{}, any> & Mt<{}, err>")));
 }
 
 TEST_CASE_FIXTURE(NormalizeFixture, "recurring_intersection")
@@ -863,7 +866,10 @@ TEST_CASE_FIXTURE(NormalizeFixture, "classes_and_never")
 TEST_CASE_FIXTURE(NormalizeFixture, "top_table_type")
 {
     CHECK("table" == toString(normal("{} | tbl")));
-    CHECK("{|  |}" == toString(normal("{} & tbl")));
+    if (FFlag::DebugLuauDeferredConstraintResolution)
+        CHECK("{  }" == toString(normal("{} & tbl")));
+    else
+        CHECK("{|  |}" == toString(normal("{} & tbl")));
     CHECK("never" == toString(normal("number & tbl")));
 }
 

tests/Simplify.test.cpp

@@ -394,8 +394,8 @@ TEST_CASE_FIXTURE(SimplifyFixture, "table_with_a_tag")
     TypeId t1 = mkTable({{"tag", stringTy}, {"prop", numberTy}});
     TypeId t2 = mkTable({{"tag", helloTy}});
 
-    CHECK("{| prop: number, tag: string |} & {| tag: \"hello\" |}" == intersectStr(t1, t2));
+    CHECK("{ prop: number, tag: string } & { tag: \"hello\" }" == intersectStr(t1, t2));
-    CHECK("{| prop: number, tag: string |} & {| tag: \"hello\" |}" == intersectStr(t2, t1));
+    CHECK("{ prop: number, tag: string } & { tag: \"hello\" }" == intersectStr(t2, t1));
 }
 
 TEST_CASE_FIXTURE(SimplifyFixture, "nested_table_tag_test")

tests/ToString.test.cpp

@@ -50,7 +50,10 @@ TEST_CASE_FIXTURE(Fixture, "cyclic_table")
     TableType* tableOne = getMutable<TableType>(&cyclicTable);
     tableOne->props["self"] = {&cyclicTable};
 
-    CHECK_EQ("t1 where t1 = { self: t1 }", toString(&cyclicTable));
+    if (FFlag::DebugLuauDeferredConstraintResolution)
+        CHECK_EQ("t1 where t1 = {| self: t1 |}", toString(&cyclicTable));
+    else
+        CHECK_EQ("t1 where t1 = { self: t1 }", toString(&cyclicTable));
 }
 
 TEST_CASE_FIXTURE(Fixture, "named_table")
@@ -68,12 +71,18 @@ TEST_CASE_FIXTURE(Fixture, "empty_table")
         local a: {}
     )");
 
-    CHECK_EQ("{|  |}", toString(requireType("a")));
+    if (FFlag::DebugLuauDeferredConstraintResolution)
+        CHECK_EQ("{  }", toString(requireType("a")));
+    else
+        CHECK_EQ("{|  |}", toString(requireType("a")));
 
     // Should stay the same with useLineBreaks enabled
     ToStringOptions opts;
     opts.useLineBreaks = true;
-    CHECK_EQ("{|  |}", toString(requireType("a"), opts));
+    if (FFlag::DebugLuauDeferredConstraintResolution)
+        CHECK_EQ("{  }", toString(requireType("a"), opts));
+    else
+        CHECK_EQ("{|  |}", toString(requireType("a"), opts));
 }
 
 TEST_CASE_FIXTURE(Fixture, "table_respects_use_line_break")
@@ -86,12 +95,20 @@ TEST_CASE_FIXTURE(Fixture, "table_respects_use_line_break")
     opts.useLineBreaks = true;
 
     //clang-format off
-    CHECK_EQ("{|\n"
+    if (FFlag::DebugLuauDeferredConstraintResolution)
-             "    anotherProp: number,\n"
+        CHECK_EQ("{\n"
-             "    prop: string,\n"
+                 "    anotherProp: number,\n"
-             "    thirdProp: boolean\n"
+                 "    prop: string,\n"
-             "|}",
+                 "    thirdProp: boolean\n"
-        toString(requireType("a"), opts));
+                 "}",
+            toString(requireType("a"), opts));
+    else
+        CHECK_EQ("{|\n"
+                 "    anotherProp: number,\n"
+                 "    prop: string,\n"
+                 "    thirdProp: boolean\n"
+                 "|}",
+            toString(requireType("a"), opts));
     //clang-format on
 }
 
@@ -122,7 +139,10 @@ TEST_CASE_FIXTURE(Fixture, "metatable")
     Type table{TypeVariant(TableType())};
     Type metatable{TypeVariant(TableType())};
     Type mtv{TypeVariant(MetatableType{&table, &metatable})};
-    CHECK_EQ("{ @metatable {  }, {  } }", toString(&mtv));
+    if (FFlag::DebugLuauDeferredConstraintResolution)
+        CHECK_EQ("{ @metatable {|  |}, {|  |} }", toString(&mtv));
+    else
+        CHECK_EQ("{ @metatable {  }, {  } }", toString(&mtv));
 }
 
 TEST_CASE_FIXTURE(Fixture, "named_metatable")
@@ -258,7 +278,10 @@ TEST_CASE_FIXTURE(Fixture, "quit_stringifying_table_type_when_length_is_exceeded
     ToStringOptions o;
     o.exhaustive = false;
     o.maxTableLength = 40;
-    CHECK_EQ(toString(&tv, o), "{ a: number, b: number, c: number, d: number, e: number, ... 10 more ... }");
+    if (FFlag::DebugLuauDeferredConstraintResolution)
+        CHECK_EQ(toString(&tv, o), "{| a: number, b: number, c: number, d: number, e: number, ... 10 more ... |}");
+    else
+        CHECK_EQ(toString(&tv, o), "{ a: number, b: number, c: number, d: number, e: number, ... 10 more ... }");
 }
 
 TEST_CASE_FIXTURE(Fixture, "stringifying_table_type_is_still_capped_when_exhaustive")
@@ -272,7 +295,10 @@ TEST_CASE_FIXTURE(Fixture, "stringifying_table_type_is_still_capped_when_exhaust
     ToStringOptions o;
     o.exhaustive = true;
     o.maxTableLength = 40;
-    CHECK_EQ(toString(&tv, o), "{ a: number, b: number, c: number, d: number, e: number, ... 2 more ... }");
+    if (FFlag::DebugLuauDeferredConstraintResolution)
+        CHECK_EQ(toString(&tv, o), "{| a: number, b: number, c: number, d: number, e: number, ... 2 more ... |}");
+    else
+        CHECK_EQ(toString(&tv, o), "{ a: number, b: number, c: number, d: number, e: number, ... 2 more ... }");
 }
 
 TEST_CASE_FIXTURE(Fixture, "quit_stringifying_type_when_length_is_exceeded")
@@ -346,7 +372,10 @@ TEST_CASE_FIXTURE(Fixture, "stringifying_table_type_correctly_use_matching_table
 
     ToStringOptions o;
     o.maxTableLength = 40;
-    CHECK_EQ(toString(&tv, o), "{| a: number, b: number, c: number, d: number, e: number, ... 5 more ... |}");
+    if (FFlag::DebugLuauDeferredConstraintResolution)
+        CHECK_EQ(toString(&tv, o), "{ a: number, b: number, c: number, d: number, e: number, ... 5 more ... }");
+    else
+        CHECK_EQ(toString(&tv, o), "{| a: number, b: number, c: number, d: number, e: number, ... 5 more ... |}");
 }
 
 TEST_CASE_FIXTURE(Fixture, "stringifying_cyclic_union_type_bails_early")
@@ -377,7 +406,10 @@ TEST_CASE_FIXTURE(Fixture, "stringifying_array_uses_array_syntax")
     CHECK_EQ("{string}", toString(Type{ttv}));
 
     ttv.props["A"] = {builtinTypes->numberType};
-    CHECK_EQ("{| [number]: string, A: number |}", toString(Type{ttv}));
+    if (FFlag::DebugLuauDeferredConstraintResolution)
+        CHECK_EQ("{ [number]: string, A: number }", toString(Type{ttv}));
+    else
+        CHECK_EQ("{| [number]: string, A: number |}", toString(Type{ttv}));
 
     ttv.props.clear();
     ttv.state = TableState::Unsealed;
@@ -576,8 +608,17 @@ TEST_CASE_FIXTURE(Fixture, "toString_the_boundTo_table_type_contained_within_a_T
 
     TypePackVar tpv2{TypePack{{&tv2}}};
 
-    CHECK_EQ("{| hello: number, world: number |}", toString(&tpv1));
+
-    CHECK_EQ("{| hello: number, world: number |}", toString(&tpv2));
+    if (FFlag::DebugLuauDeferredConstraintResolution)
+    {
+        CHECK_EQ("{ hello: number, world: number }", toString(&tpv1));
+        CHECK_EQ("{ hello: number, world: number }", toString(&tpv2));
+    }
+    else
+    {
+        CHECK_EQ("{| hello: number, world: number |}", toString(&tpv1));
+        CHECK_EQ("{| hello: number, world: number |}", toString(&tpv2));
+    }
 }
 
 TEST_CASE_FIXTURE(Fixture, "no_parentheses_around_return_type_if_pack_has_an_empty_head_link")
@@ -846,7 +887,24 @@ TEST_CASE_FIXTURE(Fixture, "tostring_error_mismatch")
    end
 
 )");
-    std::string expected = R"(Type
+    //clang-format off
+    std::string expected =
+        (FFlag::DebugLuauDeferredConstraintResolution) ?
+R"(Type
+    '{| a: number, b: string, c: {| d: string |} |}'
+could not be converted into
+    '{ a: number, b: string, c: { d: number } }'
+caused by:
+  Property 'c' is not compatible. 
+Type
+    '{| d: string |}'
+could not be converted into
+    '{ d: number }'
+caused by:
+  Property 'd' is not compatible. 
+Type 'string' could not be converted into 'number' in an invariant context)"
+        :
+R"(Type
     '{ a: number, b: string, c: { d: string } }'
 could not be converted into
     '{| a: number, b: string, c: {| d: number |} |}'
@@ -859,9 +917,12 @@ could not be converted into
 caused by:
   Property 'd' is not compatible. 
 Type 'string' could not be converted into 'number' in an invariant context)";
+    //clang-format on
+    //
     std::string actual = toString(result.errors[0]);
 
     LUAU_REQUIRE_ERROR_COUNT(1, result);
+
     CHECK(expected == actual);
 }
 TEST_SUITE_END();

tests/TypeInfer.aliases.test.cpp

@@ -223,7 +223,7 @@ TEST_CASE_FIXTURE(Fixture, "dependent_generic_aliases")
     const std::string expected = R"(Type 'bad' could not be converted into 'U<number>'
 caused by:
   Property 't' is not compatible. 
-Type '{ v: string }' could not be converted into 'T<number>'
+Type '{| v: string |}' could not be converted into 'T<number>'
 caused by:
   Property 'v' is not compatible. 
 Type 'string' could not be converted into 'number' in an invariant context)";
@@ -332,7 +332,10 @@ TEST_CASE_FIXTURE(Fixture, "stringify_type_alias_of_recursive_template_table_typ
 
     TypeMismatch* tm = get<TypeMismatch>(result.errors[0]);
     REQUIRE(tm);
-    CHECK_EQ("t1 where t1 = ({| a: t1 |}) -> string", toString(tm->wantedType));
+    if (FFlag::DebugLuauDeferredConstraintResolution)
+        CHECK_EQ("t1 where t1 = ({ a: t1 }) -> string", toString(tm->wantedType));
+    else
+        CHECK_EQ("t1 where t1 = ({| a: t1 |}) -> string", toString(tm->wantedType));
     CHECK_EQ(builtinTypes->numberType, tm->givenType);
 }
 
@@ -815,7 +818,10 @@ TEST_CASE_FIXTURE(Fixture, "forward_declared_alias_is_not_clobbered_by_prior_uni
         local d: FutureType = { smth = true } -- missing error, 'd' is resolved to 'any'
     )");
 
-    CHECK_EQ("{| foo: number |}", toString(requireType("d"), {true}));
+    if (FFlag::DebugLuauDeferredConstraintResolution)
+        CHECK_EQ("{ foo: number }", toString(requireType("d"), {true}));
+    else
+        CHECK_EQ("{| foo: number |}", toString(requireType("d"), {true}));
 
     LUAU_REQUIRE_ERROR_COUNT(1, result);
 }

tests/TypeInfer.builtins.test.cpp

@@ -398,7 +398,10 @@ TEST_CASE_FIXTURE(BuiltinsFixture, "table_pack")
     )");
 
     LUAU_REQUIRE_NO_ERRORS(result);
-    CHECK_EQ("{| [number]: boolean | number | string, n: number |}", toString(requireType("t")));
+    if (FFlag::DebugLuauDeferredConstraintResolution)
+        CHECK_EQ("{ [number]: boolean | number | string, n: number }", toString(requireType("t")));
+    else
+        CHECK_EQ("{| [number]: boolean | number | string, n: number |}", toString(requireType("t")));
 }
 
 TEST_CASE_FIXTURE(BuiltinsFixture, "table_pack_variadic")
@@ -413,7 +416,10 @@ local t = table.pack(f())
     )");
 
     LUAU_REQUIRE_NO_ERRORS(result);
-    CHECK_EQ("{| [number]: number | string, n: number |}", toString(requireType("t")));
+    if (FFlag::DebugLuauDeferredConstraintResolution)
+        CHECK_EQ("{ [number]: number | string, n: number }", toString(requireType("t")));
+    else
+        CHECK_EQ("{| [number]: number | string, n: number |}", toString(requireType("t")));
 }
 
 TEST_CASE_FIXTURE(BuiltinsFixture, "table_pack_reduce")
@@ -423,14 +429,20 @@ TEST_CASE_FIXTURE(BuiltinsFixture, "table_pack_reduce")
     )");
 
     LUAU_REQUIRE_NO_ERRORS(result);
-    CHECK_EQ("{| [number]: boolean | number, n: number |}", toString(requireType("t")));
+    if (FFlag::DebugLuauDeferredConstraintResolution)
+        CHECK_EQ("{ [number]: boolean | number, n: number }", toString(requireType("t")));
+    else
+        CHECK_EQ("{| [number]: boolean | number, n: number |}", toString(requireType("t")));
 
     result = check(R"(
         local t = table.pack("a", "b", "c")
     )");
 
     LUAU_REQUIRE_NO_ERRORS(result);
-    CHECK_EQ("{| [number]: string, n: number |}", toString(requireType("t")));
+    if (FFlag::DebugLuauDeferredConstraintResolution)
+        CHECK_EQ("{ [number]: string, n: number }", toString(requireType("t")));
+    else
+        CHECK_EQ("{| [number]: string, n: number |}", toString(requireType("t")));
 }
 
 TEST_CASE_FIXTURE(BuiltinsFixture, "gcinfo")

tests/TypeInfer.functions.test.cpp

@@ -2077,7 +2077,7 @@ TEST_CASE_FIXTURE(Fixture, "attempt_to_call_an_intersection_of_tables")
     LUAU_REQUIRE_ERROR_COUNT(1, result);
 
     if (FFlag::DebugLuauDeferredConstraintResolution)
-        CHECK_EQ(toString(result.errors[0]), "Cannot call non-function {| x: number |} & {| y: string |}");
+        CHECK_EQ(toString(result.errors[0]), "Cannot call non-function { x: number } & { y: string }");
     else
         CHECK_EQ(toString(result.errors[0]), "Cannot call non-function {| x: number |}");
 }

tests/TypeInfer.intersectionTypes.test.cpp

@@ -164,7 +164,7 @@ TEST_CASE_FIXTURE(Fixture, "index_on_an_intersection_type_with_property_guarante
 
     LUAU_REQUIRE_NO_ERRORS(result);
     if (FFlag::DebugLuauDeferredConstraintResolution)
-        CHECK("{| y: number |}" == toString(requireType("r")));
+        CHECK("{ y: number }" == toString(requireType("r")));
     else
         CHECK("{| y: number |} & {| y: number |}" == toString(requireType("r")));
 }
@@ -513,7 +513,13 @@ TEST_CASE_FIXTURE(Fixture, "intersection_of_tables")
     )");
 
     LUAU_REQUIRE_ERROR_COUNT(1, result);
-    const std::string expected = R"(Type
+    const std::string expected =
+        (FFlag::DebugLuauDeferredConstraintResolution) ?
+        "Type "
+        "'{ p: number?, q: number?, r: number? } & { p: number?, q: string? }'"
+        " could not be converted into "
+        "'{ p: nil }'; none of the intersection parts are compatible" :
+        R"(Type
     '{| p: number?, q: number?, r: number? |} & {| p: number?, q: string? |}'
 could not be converted into
     '{| p: nil |}'; none of the intersection parts are compatible)";
@@ -581,7 +587,13 @@ TEST_CASE_FIXTURE(Fixture, "overloaded_functions_returning_intersections")
     )");
 
     LUAU_REQUIRE_ERROR_COUNT(1, result);
-    const std::string expected = R"(Type
+    const std::string expected =
+        (FFlag::DebugLuauDeferredConstraintResolution) ?
+        R"(Type
+    '((number?) -> { p: number } & { q: number }) & ((string?) -> { p: number } & { r: number })'
+could not be converted into
+    '(number?) -> { p: number, q: number, r: number }'; none of the intersection parts are compatible)" :
+        R"(Type
     '((number?) -> {| p: number |} & {| q: number |}) & ((string?) -> {| p: number |} & {| r: number |})'
 could not be converted into
     '(number?) -> {| p: number, q: number, r: number |}'; none of the intersection parts are compatible)";
@@ -933,7 +945,7 @@ TEST_CASE_FIXTURE(Fixture, "less_greedy_unification_with_intersection_types_2")
 
     LUAU_REQUIRE_NO_ERRORS(result);
 
-    CHECK_EQ("({| x: number |} & {| x: string |}) -> never", toString(requireType("f")));
+    CHECK_EQ("({ x: number } & { x: string }) -> never", toString(requireType("f")));
 }
 
 TEST_CASE_FIXTURE(BuiltinsFixture, "index_property_table_intersection_1")

tests/TypeInfer.modules.test.cpp

@@ -225,7 +225,10 @@ local tbl: string = require(game.A)
 
     CheckResult result = frontend.check("game/B");
     LUAU_REQUIRE_ERROR_COUNT(1, result);
-    CHECK_EQ("Type '{| def: number |}' could not be converted into 'string'", toString(result.errors[0]));
+    if (FFlag::DebugLuauDeferredConstraintResolution)
+        CHECK_EQ("Type '{ def: number }' could not be converted into 'string'", toString(result.errors[0]));
+    else
+        CHECK_EQ("Type '{| def: number |}' could not be converted into 'string'", toString(result.errors[0]));
 }
 
 TEST_CASE_FIXTURE(Fixture, "bound_free_table_export_is_ok")

tests/TypeInfer.oop.test.cpp

@@ -410,7 +410,7 @@ TEST_CASE_FIXTURE(BuiltinsFixture, "cycle_between_object_constructor_and_alias")
     CHECK_MESSAGE(get<MetatableType>(follow(aliasType)), "Expected metatable type but got: " << toString(aliasType));
 }
 
-TEST_CASE_FIXTURE(BuiltinsFixture, "promise_type_error_too_complex" * doctest::timeout(0.5))
+TEST_CASE_FIXTURE(BuiltinsFixture, "promise_type_error_too_complex" * doctest::timeout(2))
 {
     // TODO: LTI changes to function call resolution have rendered this test impossibly slow
     // shared self should fix it, but there may be other mitigations possible as well

tests/TypeInfer.operators.test.cpp

@@ -571,6 +571,8 @@ TEST_CASE_FIXTURE(BuiltinsFixture, "typecheck_unary_len_error")
     CHECK_EQ("number", toString(requireType("a")));
 
     TypeMismatch* tm = get<TypeMismatch>(result.errors[0]);
+    REQUIRE_MESSAGE(tm, "Expected a TypeMismatch but got " << result.errors[0]);
+
     REQUIRE_EQ(*tm->wantedType, *builtinTypes->numberType);
     REQUIRE_EQ(*tm->givenType, *builtinTypes->stringType);
 }

tests/TypeInfer.provisional.test.cpp

@@ -251,10 +251,22 @@ TEST_CASE_FIXTURE(Fixture, "discriminate_from_x_not_equal_to_nil")
 
     LUAU_REQUIRE_NO_ERRORS(result);
 
-    CHECK_EQ("{| x: string, y: number |}", toString(requireTypeAtPosition({5, 28})));
+    if (FFlag::DebugLuauDeferredConstraintResolution)
+    {
+
+        CHECK_EQ("{ x: string, y: number }", toString(requireTypeAtPosition({5, 28})));
+
+        // Should be { x: nil, y: nil }
+        CHECK_EQ("{ x: nil, y: nil } | { x: string, y: number }", toString(requireTypeAtPosition({7, 28})));
+    }
+    else
+    {
+        CHECK_EQ("{| x: string, y: number |}", toString(requireTypeAtPosition({5, 28})));
+
+        // Should be {| x: nil, y: nil |}
+        CHECK_EQ("{| x: nil, y: nil |} | {| x: string, y: number |}", toString(requireTypeAtPosition({7, 28})));
+    }
 
-    // Should be {| x: nil, y: nil |}
-    CHECK_EQ("{| x: nil, y: nil |} | {| x: string, y: number |}", toString(requireTypeAtPosition({7, 28})));
 }
 
 TEST_CASE_FIXTURE(BuiltinsFixture, "bail_early_if_unification_is_too_complicated" * doctest::timeout(0.5))

tests/TypeInfer.refinements.test.cpp

@@ -535,7 +535,10 @@ TEST_CASE_FIXTURE(Fixture, "unknown_lvalue_is_not_synonymous_with_other_on_not_e
     LUAU_REQUIRE_NO_ERRORS(result);
 
     CHECK_EQ(toString(requireTypeAtPosition({3, 33})), "any");              // a ~= b
-    CHECK_EQ(toString(requireTypeAtPosition({3, 36})), "{| x: number |}?"); // a ~= b
+    if (FFlag::DebugLuauDeferredConstraintResolution)
+        CHECK_EQ(toString(requireTypeAtPosition({3, 36})), "{ x: number }?"); // a ~= b
+    else
+        CHECK_EQ(toString(requireTypeAtPosition({3, 36})), "{| x: number |}?"); // a ~= b
 }
 
 TEST_CASE_FIXTURE(Fixture, "string_not_equal_to_string_or_nil")
@@ -658,7 +661,10 @@ TEST_CASE_FIXTURE(BuiltinsFixture, "typeguard_narrows_for_table")
 
     LUAU_REQUIRE_NO_ERRORS(result);
 
-    CHECK_EQ("{| x: number |} | {| y: boolean |}", toString(requireTypeAtPosition({3, 28}))); // type(x) == "table"
+    if (FFlag::DebugLuauDeferredConstraintResolution)
+        CHECK_EQ("{ x: number } | { y: boolean }", toString(requireTypeAtPosition({3, 28}))); // type(x) == "table"
+    else
+        CHECK_EQ("{| x: number |} | {| y: boolean |}", toString(requireTypeAtPosition({3, 28}))); // type(x) == "table"
     CHECK_EQ("string", toString(requireTypeAtPosition({5, 28})));                             // type(x) ~= "table"
 }
 
@@ -697,7 +703,10 @@ TEST_CASE_FIXTURE(BuiltinsFixture, "type_guard_can_filter_for_intersection_of_ta
 
     ToStringOptions opts;
     opts.exhaustive = true;
-    CHECK_EQ("{| x: number |} & {| y: number |}", toString(requireTypeAtPosition({4, 28}), opts));
+    if (FFlag::DebugLuauDeferredConstraintResolution)
+        CHECK_EQ("{ x: number } & { y: number }", toString(requireTypeAtPosition({4, 28}), opts));
+    else
+        CHECK_EQ("{| x: number |} & {| y: number |}", toString(requireTypeAtPosition({4, 28}), opts));
     CHECK_EQ("nil", toString(requireTypeAtPosition({6, 28})));
 }
 
@@ -1216,8 +1225,17 @@ TEST_CASE_FIXTURE(RefinementClassFixture, "discriminate_from_isa_of_x")
 
     LUAU_REQUIRE_NO_ERRORS(result);
 
-    CHECK_EQ(R"({| tag: "Part", x: Part |})", toString(requireTypeAtPosition({5, 28})));
+
-    CHECK_EQ(R"({| tag: "Folder", x: Folder |})", toString(requireTypeAtPosition({7, 28})));
+    if (FFlag::DebugLuauDeferredConstraintResolution)
+    {
+        CHECK_EQ(R"({ tag: "Part", x: Part })", toString(requireTypeAtPosition({5, 28})));
+        CHECK_EQ(R"({ tag: "Folder", x: Folder })", toString(requireTypeAtPosition({7, 28})));
+    }
+    else
+    {
+        CHECK_EQ(R"({| tag: "Part", x: Part |})", toString(requireTypeAtPosition({5, 28})));
+        CHECK_EQ(R"({| tag: "Folder", x: Folder |})", toString(requireTypeAtPosition({7, 28})));
+    }
 }
 
 TEST_CASE_FIXTURE(RefinementClassFixture, "typeguard_cast_free_table_to_vector")

tests/TypeInfer.tables.test.cpp

@@ -91,7 +91,10 @@ TEST_CASE_FIXTURE(Fixture, "cannot_augment_sealed_table")
 
     // TODO: better, more robust comparison of type vars
     auto s = toString(error->tableType, ToStringOptions{/*exhaustive*/ true});
-    CHECK_EQ(s, "{| prop: number |}");
+    if (FFlag::DebugLuauDeferredConstraintResolution)
+        CHECK_EQ(s, "{ prop: number }");
+    else
+        CHECK_EQ(s, "{| prop: number |}");
     CHECK_EQ(error->prop, "foo");
     CHECK_EQ(error->context, CannotExtendTable::Property);
 }
@@ -733,7 +736,10 @@ TEST_CASE_FIXTURE(Fixture, "infer_indexer_from_value_property_in_literal")
     CHECK(bool(retType->indexer));
 
     const TableIndexer& indexer = *retType->indexer;
-    CHECK_EQ("{| __name: string |}", toString(indexer.indexType));
+    if (FFlag::DebugLuauDeferredConstraintResolution)
+        CHECK_EQ("{ __name: string }", toString(indexer.indexType));
+    else
+        CHECK_EQ("{| __name: string |}", toString(indexer.indexType));
 }
 
 TEST_CASE_FIXTURE(Fixture, "infer_indexer_from_its_variable_type_and_unifiable")
@@ -775,7 +781,10 @@ TEST_CASE_FIXTURE(Fixture, "indexer_mismatch")
     TypeMismatch* tm = get<TypeMismatch>(result.errors[0]);
     REQUIRE(tm != nullptr);
     CHECK(toString(tm->wantedType) == "{number}");
-    CHECK(toString(tm->givenType) == "{| [string]: string |}");
+    if (FFlag::DebugLuauDeferredConstraintResolution)
+        CHECK(toString(tm->givenType) == "{ [string]: string }");
+    else
+        CHECK(toString(tm->givenType) == "{| [string]: string |}");
 
     CHECK_NE(*t1, *t2);
 }
@@ -1564,8 +1573,16 @@ TEST_CASE_FIXTURE(Fixture, "casting_sealed_tables_with_props_into_table_with_ind
     ToStringOptions o{/* exhaustive= */ true};
     TypeMismatch* tm = get<TypeMismatch>(result.errors[0]);
     REQUIRE(tm);
-    CHECK_EQ("{| [string]: string |}", toString(tm->wantedType, o));
+    if (FFlag::DebugLuauDeferredConstraintResolution)
-    CHECK_EQ("{| foo: number |}", toString(tm->givenType, o));
+    {
+        CHECK_EQ("{ [string]: string }", toString(tm->wantedType, o));
+        CHECK_EQ("{ foo: number }", toString(tm->givenType, o));
+    }
+    else
+    {
+        CHECK_EQ("{| [string]: string |}", toString(tm->wantedType, o));
+        CHECK_EQ("{| foo: number |}", toString(tm->givenType, o));
+    }
 }
 
 TEST_CASE_FIXTURE(Fixture, "casting_tables_with_props_into_table_with_indexer2")
@@ -1803,8 +1820,16 @@ TEST_CASE_FIXTURE(Fixture, "hide_table_error_properties")
 
     LUAU_REQUIRE_ERROR_COUNT(2, result);
 
-    CHECK_EQ("Cannot add property 'a' to table '{| x: number |}'", toString(result.errors[0]));
+    if (FFlag::DebugLuauDeferredConstraintResolution)
-    CHECK_EQ("Cannot add property 'b' to table '{| x: number |}'", toString(result.errors[1]));
+    {
+        CHECK_EQ("Cannot add property 'a' to table '{ x: number }'", toString(result.errors[0]));
+        CHECK_EQ("Cannot add property 'b' to table '{ x: number }'", toString(result.errors[1]));
+    }
+    else
+    {
+        CHECK_EQ("Cannot add property 'a' to table '{| x: number |}'", toString(result.errors[0]));
+        CHECK_EQ("Cannot add property 'b' to table '{| x: number |}'", toString(result.errors[1]));
+    }
 }
 
 TEST_CASE_FIXTURE(BuiltinsFixture, "builtin_table_names")
@@ -2969,7 +2994,10 @@ TEST_CASE_FIXTURE(BuiltinsFixture, "access_index_metamethod_that_returns_variadi
 
     ToStringOptions o;
     o.exhaustive = true;
-    CHECK_EQ("{| x: string |}", toString(requireType("foo"), o));
+    if (FFlag::DebugLuauDeferredConstraintResolution)
+        CHECK_EQ("{ x: string }", toString(requireType("foo"), o));
+    else
+        CHECK_EQ("{| x: string |}", toString(requireType("foo"), o));
 }
 
 TEST_CASE_FIXTURE(Fixture, "dont_invalidate_the_properties_iterator_of_free_table_when_rolled_back")
@@ -3029,7 +3057,10 @@ TEST_CASE_FIXTURE(Fixture, "accidentally_checked_prop_in_opposite_branch")
     )");
 
     LUAU_REQUIRE_ERROR_COUNT(1, result);
-    CHECK_EQ("Value of type '{| x: number? |}?' could be nil", toString(result.errors[0]));
+    if (FFlag::DebugLuauDeferredConstraintResolution)
+        CHECK_EQ("Value of type '{ x: number? }?' could be nil", toString(result.errors[0]));
+    else
+        CHECK_EQ("Value of type '{| x: number? |}?' could be nil", toString(result.errors[0]));
     CHECK_EQ("boolean", toString(requireType("u")));
 }
 
@@ -3260,7 +3291,10 @@ TEST_CASE_FIXTURE(Fixture, "prop_access_on_unions_of_indexers_where_key_whose_ty
     )");
 
     LUAU_REQUIRE_ERROR_COUNT(1, result);
-    CHECK_EQ("Type '{number} | {| [boolean]: number |}' does not have key 'x'", toString(result.errors[0]));
+    if (FFlag::DebugLuauDeferredConstraintResolution)
+        CHECK_EQ("Type '{ [boolean]: number } | {number}' does not have key 'x'", toString(result.errors[0]));
+    else
+        CHECK_EQ("Type '{number} | {| [boolean]: number |}' does not have key 'x'", toString(result.errors[0]));
 }
 
 TEST_CASE_FIXTURE(BuiltinsFixture, "quantify_metatables_of_metatables_of_table")
@@ -3824,4 +3858,24 @@ TEST_CASE_FIXTURE(Fixture, "cli_84607_missing_prop_in_array_or_dict")
     CHECK_EQ("prop", error2->properties[0]);
 }
 
+TEST_CASE_FIXTURE(Fixture, "simple_method_definition")
+{
+    CheckResult result = check(R"(
+        local T = {}
+
+        function T:m()
+            return 5
+        end
+
+        return T
+    )");
+
+    LUAU_REQUIRE_NO_ERRORS(result);
+
+    if (FFlag::DebugLuauDeferredConstraintResolution)
+        CHECK_EQ("{ m: (unknown) -> number }", toString(getMainModule()->returnType, ToStringOptions{true}));
+    else
+        CHECK_EQ("{| m: <a>(a) -> number |}", toString(getMainModule()->returnType, ToStringOptions{true}));
+}
+
 TEST_SUITE_END();

tests/TypeInfer.test.cpp

@@ -69,6 +69,18 @@ TEST_CASE_FIXTURE(Fixture, "infer_locals_with_nil_value")
     CHECK_EQ(getPrimitiveType(ty), PrimitiveType::String);
 }
 
+TEST_CASE_FIXTURE(Fixture, "infer_locals_with_nil_value_2")
+{
+    CheckResult result = check(R"(
+        local a = 2
+        local b = a,nil
+    )");
+
+    LUAU_REQUIRE_NO_ERRORS(result);
+    CHECK_EQ("number", toString(requireType("a")));
+    CHECK_EQ("number", toString(requireType("b")));
+}
+
 TEST_CASE_FIXTURE(Fixture, "infer_locals_via_assignment_from_its_call_site")
 {
     CheckResult result = check(R"(
@@ -1168,6 +1180,28 @@ TEST_CASE_FIXTURE(Fixture, "bidirectional_checking_of_higher_order_function")
     CHECK(location.end.line == 4);
 }
 
+TEST_CASE_FIXTURE(Fixture, "bidirectional_checking_of_callback_property")
+{
+    CheckResult result = check(R"(
+        local print: (number) -> ()
+
+        type Point = {x: number, y: number}
+        local T : {callback: ((Point) -> ())?} = {}
+
+        T.callback = function(p) -- No error here
+            print(p.z)           -- error here.  Point has no property z
+        end
+    )");
+
+    LUAU_REQUIRE_ERROR_COUNT(1, result);
+
+    CHECK_MESSAGE(get<UnknownProperty>(result.errors[0]), "Expected UnknownProperty but got " << result.errors[0]);
+
+    Location location = result.errors[0].location;
+    CHECK(location.begin.line == 7);
+    CHECK(location.end.line == 7);
+}
+
 TEST_CASE_FIXTURE(BuiltinsFixture, "it_is_ok_to_have_inconsistent_number_of_return_values_in_nonstrict")
 {
     CheckResult result = check(R"(

tests/TypeInfer.tryUnify.test.cpp

@@ -373,12 +373,22 @@ TEST_CASE_FIXTURE(TryUnifyFixture, "metatables_unify_against_shape_of_free_table
     state.log.commit();
 
     REQUIRE_EQ(state.errors.size(), 1);
-    const std::string expected = R"(Type
+    // clang-format off
+    const std::string expected =
+        (FFlag::DebugLuauDeferredConstraintResolution) ?
+R"(Type
+    '{ @metatable { __index: { foo: string } }, {|  |} }'
+could not be converted into
+    '{- foo: number -}'
+caused by:
+  Type 'number' could not be converted into 'string')" :
+R"(Type
     '{ @metatable {| __index: {| foo: string |} |}, {  } }'
 could not be converted into
     '{- foo: number -}'
 caused by:
   Type 'number' could not be converted into 'string')";
+    // clang-format on
     CHECK_EQ(expected, toString(state.errors[0]));
 }
 

tests/TypeInfer.typePacks.cpp

@@ -308,12 +308,18 @@ local c: Packed<string, number, boolean>
     tf = lookupType("Packed");
     REQUIRE(tf);
     CHECK_EQ(toString(*tf), "Packed<T, U...>");
-    CHECK_EQ(toString(*tf, {true}), "{| f: (T, U...) -> (T, U...) |}");
+    if (FFlag::DebugLuauDeferredConstraintResolution)
+        CHECK_EQ(toString(*tf, {true}), "{ f: (T, U...) -> (T, U...) }");
+    else
+        CHECK_EQ(toString(*tf, {true}), "{| f: (T, U...) -> (T, U...) |}");
 
     auto ttvA = get<TableType>(requireType("a"));
     REQUIRE(ttvA);
     CHECK_EQ(toString(requireType("a")), "Packed<number>");
-    CHECK_EQ(toString(requireType("a"), {true}), "{| f: (number) -> number |}");
+    if (FFlag::DebugLuauDeferredConstraintResolution)
+        CHECK_EQ(toString(requireType("a"), {true}), "{ f: (number) -> number }");
+    else
+        CHECK_EQ(toString(requireType("a"), {true}), "{| f: (number) -> number |}");
     REQUIRE(ttvA->instantiatedTypeParams.size() == 1);
     REQUIRE(ttvA->instantiatedTypePackParams.size() == 1);
     CHECK_EQ(toString(ttvA->instantiatedTypeParams[0], {true}), "number");
@@ -322,7 +328,10 @@ local c: Packed<string, number, boolean>
     auto ttvB = get<TableType>(requireType("b"));
     REQUIRE(ttvB);
     CHECK_EQ(toString(requireType("b")), "Packed<string, number>");
-    CHECK_EQ(toString(requireType("b"), {true}), "{| f: (string, number) -> (string, number) |}");
+    if (FFlag::DebugLuauDeferredConstraintResolution)
+        CHECK_EQ(toString(requireType("b"), {true}), "{ f: (string, number) -> (string, number) }");
+    else
+        CHECK_EQ(toString(requireType("b"), {true}), "{| f: (string, number) -> (string, number) |}");
     REQUIRE(ttvB->instantiatedTypeParams.size() == 1);
     REQUIRE(ttvB->instantiatedTypePackParams.size() == 1);
     CHECK_EQ(toString(ttvB->instantiatedTypeParams[0], {true}), "string");
@@ -331,7 +340,10 @@ local c: Packed<string, number, boolean>
     auto ttvC = get<TableType>(requireType("c"));
     REQUIRE(ttvC);
     CHECK_EQ(toString(requireType("c")), "Packed<string, number, boolean>");
-    CHECK_EQ(toString(requireType("c"), {true}), "{| f: (string, number, boolean) -> (string, number, boolean) |}");
+    if (FFlag::DebugLuauDeferredConstraintResolution)
+        CHECK_EQ(toString(requireType("c"), {true}), "{ f: (string, number, boolean) -> (string, number, boolean) }");
+    else
+        CHECK_EQ(toString(requireType("c"), {true}), "{| f: (string, number, boolean) -> (string, number, boolean) |}");
     REQUIRE(ttvC->instantiatedTypeParams.size() == 1);
     REQUIRE(ttvC->instantiatedTypePackParams.size() == 1);
     CHECK_EQ(toString(ttvC->instantiatedTypeParams[0], {true}), "string");
@@ -360,12 +372,25 @@ local d: { a: typeof(c) }
     auto tf = lookupImportedType("Import", "Packed");
     REQUIRE(tf);
     CHECK_EQ(toString(*tf), "Packed<T, U...>");
-    CHECK_EQ(toString(*tf, {true}), "{| a: T, b: (U...) -> () |}");
 
-    CHECK_EQ(toString(requireType("a"), {true}), "{| a: number, b: () -> () |}");
+    if (FFlag::DebugLuauDeferredConstraintResolution)
-    CHECK_EQ(toString(requireType("b"), {true}), "{| a: string, b: (number) -> () |}");
+    {
-    CHECK_EQ(toString(requireType("c"), {true}), "{| a: string, b: (number, boolean) -> () |}");
+        CHECK_EQ(toString(*tf, {true}), "{ a: T, b: (U...) -> () }");
-    CHECK_EQ(toString(requireType("d")), "{| a: Packed<string, number, boolean> |}");
+
+        CHECK_EQ(toString(requireType("a"), {true}), "{ a: number, b: () -> () }");
+        CHECK_EQ(toString(requireType("b"), {true}), "{ a: string, b: (number) -> () }");
+        CHECK_EQ(toString(requireType("c"), {true}), "{ a: string, b: (number, boolean) -> () }");
+        CHECK_EQ(toString(requireType("d")), "{ a: Packed<string, number, boolean> }");
+    }
+    else
+    {
+        CHECK_EQ(toString(*tf, {true}), "{| a: T, b: (U...) -> () |}");
+
+        CHECK_EQ(toString(requireType("a"), {true}), "{| a: number, b: () -> () |}");
+        CHECK_EQ(toString(requireType("b"), {true}), "{| a: string, b: (number) -> () |}");
+        CHECK_EQ(toString(requireType("c"), {true}), "{| a: string, b: (number, boolean) -> () |}");
+        CHECK_EQ(toString(requireType("d")), "{| a: Packed<string, number, boolean> |}");
+    }
 }
 
 TEST_CASE_FIXTURE(BuiltinsFixture, "type_pack_type_parameters")
@@ -388,19 +413,31 @@ type C<X...> = Import.Packed<string, (number, X...)>
     auto tf = lookupType("Alias");
     REQUIRE(tf);
     CHECK_EQ(toString(*tf), "Alias<S, T, R...>");
-    CHECK_EQ(toString(*tf, {true}), "{| a: S, b: (T, R...) -> () |}");
+    if (FFlag::DebugLuauDeferredConstraintResolution)
+        CHECK_EQ(toString(*tf, {true}), "{ a: S, b: (T, R...) -> () }");
+    else
+        CHECK_EQ(toString(*tf, {true}), "{| a: S, b: (T, R...) -> () |}");
 
-    CHECK_EQ(toString(requireType("a"), {true}), "{| a: string, b: (number, boolean) -> () |}");
+    if (FFlag::DebugLuauDeferredConstraintResolution)
+        CHECK_EQ(toString(requireType("a"), {true}), "{ a: string, b: (number, boolean) -> () }");
+    else
+        CHECK_EQ(toString(requireType("a"), {true}), "{| a: string, b: (number, boolean) -> () |}");
 
     tf = lookupType("B");
     REQUIRE(tf);
     CHECK_EQ(toString(*tf), "B<X...>");
-    CHECK_EQ(toString(*tf, {true}), "{| a: string, b: (X...) -> () |}");
+    if (FFlag::DebugLuauDeferredConstraintResolution)
+        CHECK_EQ(toString(*tf, {true}), "{ a: string, b: (X...) -> () }");
+    else
+        CHECK_EQ(toString(*tf, {true}), "{| a: string, b: (X...) -> () |}");
 
     tf = lookupType("C");
     REQUIRE(tf);
     CHECK_EQ(toString(*tf), "C<X...>");
-    CHECK_EQ(toString(*tf, {true}), "{| a: string, b: (number, X...) -> () |}");
+    if (FFlag::DebugLuauDeferredConstraintResolution)
+        CHECK_EQ(toString(*tf, {true}), "{ a: string, b: (number, X...) -> () }");
+    else
+        CHECK_EQ(toString(*tf, {true}), "{| a: string, b: (number, X...) -> () |}");
 }
 
 TEST_CASE_FIXTURE(Fixture, "type_alias_type_packs_nested")
@@ -867,7 +904,10 @@ type R = { m: F<R> }
 
     LUAU_REQUIRE_NO_ERRORS(result);
 
-    CHECK_EQ(toString(*lookupType("R"), {true}), "t1 where t1 = {| m: (t1) -> (t1) -> () |}");
+    if (FFlag::DebugLuauDeferredConstraintResolution)
+        CHECK_EQ(toString(*lookupType("R"), {true}), "t1 where t1 = { m: (t1) -> (t1) -> () }");
+    else
+        CHECK_EQ(toString(*lookupType("R"), {true}), "t1 where t1 = {| m: (t1) -> (t1) -> () |}");
 }
 
 TEST_CASE_FIXTURE(Fixture, "pack_tail_unification_check")

tests/TypeInfer.unionTypes.test.cpp

@@ -356,7 +356,10 @@ a.x = 2
 
     LUAU_REQUIRE_ERROR_COUNT(1, result);
     auto s = toString(result.errors[0]);
-    CHECK_EQ("Value of type '({| x: number |} & {| y: number |})?' could be nil", s);
+    if (FFlag::DebugLuauDeferredConstraintResolution)
+        CHECK_EQ("Value of type '({ x: number } & { y: number })?' could be nil", s);
+    else
+        CHECK_EQ("Value of type '({| x: number |} & {| y: number |})?' could be nil", s);
 }
 
 TEST_CASE_FIXTURE(Fixture, "optional_length_error")
@@ -471,11 +474,20 @@ local b: { w: number } = a
     )");
 
     LUAU_REQUIRE_ERROR_COUNT(1, result);
-    const std::string expected = R"(Type 'X | Y | Z' could not be converted into '{| w: number |}'
+    TypeMismatch* tm = get<TypeMismatch>(result.errors[0]);
-caused by:
+    REQUIRE(tm);
-  Not all union options are compatible. 
+
-Table type 'X' not compatible with type '{| w: number |}' because the former is missing field 'w')";
+    CHECK_EQ(tm->reason, "Not all union options are compatible.");
-    CHECK_EQ(expected, toString(result.errors[0]));
+
+    CHECK_EQ("X | Y | Z", toString(tm->givenType));
+
+    const TableType* expected = get<TableType>(tm->wantedType);
+    REQUIRE(expected);
+    CHECK_EQ(TableState::Sealed, expected->state);
+    CHECK_EQ(1, expected->props.size());
+    auto propW = expected->props.find("w");
+    REQUIRE_NE(expected->props.end(), propW);
+    CHECK_EQ("number", toString(propW->second.type()));
 }
 
 TEST_CASE_FIXTURE(Fixture, "error_detailed_union_all")
@@ -744,7 +756,10 @@ TEST_CASE_FIXTURE(Fixture, "less_greedy_unification_with_union_types_2")
 
     LUAU_REQUIRE_NO_ERRORS(result);
 
-    CHECK_EQ("({| x: number |} | {| x: string |}) -> number | string", toString(requireType("f")));
+    if (FFlag::DebugLuauDeferredConstraintResolution)
+        CHECK_EQ("({ x: number } | { x: string }) -> number | string", toString(requireType("f")));
+    else
+        CHECK_EQ("({| x: number |} | {| x: string |}) -> number | string", toString(requireType("f")));
 }
 
 TEST_CASE_FIXTURE(Fixture, "union_table_any_property")

tests/TypeVar.test.cpp

@@ -298,7 +298,10 @@ TEST_CASE_FIXTURE(Fixture, "substitution_skip_failure")
 
     REQUIRE(!anyification.normalizationTooComplex);
     REQUIRE(any.has_value());
-    CHECK_EQ("{| f: t1 |} where t1 = () -> {| f: () -> {| f: ({| f: t1 |}) -> (), signal: {| f: (any) -> () |} |} |}", toString(*any));
+    if (FFlag::DebugLuauDeferredConstraintResolution)
+        CHECK_EQ("{ f: t1 } where t1 = () -> { f: () -> { f: ({ f: t1 }) -> (), signal: { f: (any) -> () } } }", toString(*any));
+    else
+        CHECK_EQ("{| f: t1 |} where t1 = () -> {| f: () -> {| f: ({| f: t1 |}) -> (), signal: {| f: (any) -> () |} |} |}", toString(*any));
 }
 
 TEST_CASE("tagging_tables")

tests/Unifier2.test.cpp

@@ -38,6 +38,11 @@ struct Unifier2Fixture
     {
         return ::Luau::toString(ty, opts);
     }
+
+    std::string toString(TypePackId ty)
+    {
+        return ::Luau::toString(ty, opts);
+    }
 };
 
 TEST_SUITE_BEGIN("Unifier2");
@@ -99,6 +104,32 @@ TEST_CASE_FIXTURE(Unifier2Fixture, "(string) -> () <: (X) -> Y...")
     CHECK(!yPack->tail);
 }
 
+TEST_CASE_FIXTURE(Unifier2Fixture, "unify_binds_free_subtype_tail_pack")
+{
+    TypePackId numberPack = arena.addTypePack({builtinTypes.numberType});
+
+    TypePackId freeTail = arena.freshTypePack(&scope);
+    TypeId freeHead = arena.addType(FreeType{&scope, builtinTypes.neverType, builtinTypes.unknownType});
+    TypePackId freeAndFree = arena.addTypePack({freeHead}, freeTail);
+
+    u2.unify(freeAndFree, numberPack);
+
+    CHECK("('a <: number)" == toString(freeAndFree));
+}
+
+TEST_CASE_FIXTURE(Unifier2Fixture, "unify_binds_free_supertype_tail_pack")
+{
+    TypePackId numberPack = arena.addTypePack({builtinTypes.numberType});
+
+    TypePackId freeTail = arena.freshTypePack(&scope);
+    TypeId freeHead = arena.addType(FreeType{&scope, builtinTypes.neverType, builtinTypes.unknownType});
+    TypePackId freeAndFree = arena.addTypePack({freeHead}, freeTail);
+
+    u2.unify(numberPack, freeAndFree);
+
+    CHECK("(number <: 'a)" == toString(freeAndFree));
+}
+
 TEST_CASE_FIXTURE(Unifier2Fixture, "generalize_a_type_that_is_bounded_by_another_generalizable_type")
 {
     auto [t1, ft1] = freshType();

tests/conformance/datetime.lua

@@ -18,6 +18,11 @@ assert(os.date(string.rep("%d", 1000), t) ==
 assert(os.date(string.rep("%", 200)) == string.rep("%", 100))
 assert(os.date("", -1) == nil)
 
+assert(os.time({ year = 1969, month = 12, day = 31, hour = 23, min = 59, sec = 59}) == nil) -- just before start
+assert(os.time({ year = 1970, month = 1, day = 1, hour = 0, min = 0, sec = 0}) == 0) -- start
+assert(os.time({ year = 3000, month = 12, day = 31, hour = 23, min = 59, sec = 59}) == 32535215999) -- just before Windows max range
+assert(os.time({ year = 1970, month = 1, day = 1, hour = 0, min = 0, sec = -1}) == nil) -- going before using time fields
+
 local function checkDateTable (t)
   local D = os.date("!*t", t)
   assert(os.time(D) == t)

tests/conformance/native.lua

@@ -171,4 +171,38 @@ end
 
 nilInvalidatesSlot()
 
+local function arraySizeOpt1(a)
+  a[1] += 2
+  a[1] *= 3
+
+  table.insert(a, 3)
+  table.insert(a, 4)
+  table.insert(a, 5)
+  table.insert(a, 6)
+
+  a[1] += 4
+  a[1] *= 5
+
+  return a[1] + a[5]
+end
+
+assert(arraySizeOpt1({1}) == 71)
+
+local function arraySizeOpt2(a, i)
+  a[i] += 2
+  a[i] *= 3
+
+  table.insert(a, 3)
+  table.insert(a, 4)
+  table.insert(a, 5)
+  table.insert(a, 6)
+
+  a[i] += 4
+  a[i] *= 5
+
+  return a[i] + a[5]
+end
+
+assert(arraySizeOpt1({1}, 1) == 71)
+
 return('OK')

tests/main.cpp

@@ -94,6 +94,8 @@ static int testAssertionHandler(const char* expr, const char* file, int line, co
     return 1;
 }
 
+
+
 struct BoostLikeReporter : doctest::IReporter
 {
     const doctest::TestCaseData* currentTest = nullptr;
@@ -255,6 +257,8 @@ int main(int argc, char** argv)
 {
     Luau::assertHandler() = testAssertionHandler;
 
+
+
     doctest::registerReporter<BoostLikeReporter>("boost", 0, true);
 
     doctest::Context context;

tools/faillist.txt

@@ -14,9 +14,12 @@ AutocompleteTest.type_correct_expected_argument_type_pack_suggestion
 AutocompleteTest.type_correct_expected_argument_type_suggestion
 AutocompleteTest.type_correct_expected_argument_type_suggestion_optional
 AutocompleteTest.type_correct_expected_argument_type_suggestion_self
+AutocompleteTest.type_correct_expected_return_type_pack_suggestion
+AutocompleteTest.type_correct_expected_return_type_suggestion
 AutocompleteTest.type_correct_function_no_parenthesis
 AutocompleteTest.type_correct_function_return_types
 AutocompleteTest.type_correct_keywords
+AutocompleteTest.type_correct_suggestion_for_overloads
 AutocompleteTest.type_correct_suggestion_in_argument
 AutocompleteTest.unsealed_table_2
 BuiltinTests.aliased_string_format
@@ -55,8 +58,35 @@ BuiltinTests.table_dot_remove_optionally_returns_generic
 BuiltinTests.table_freeze_is_generic
 BuiltinTests.table_insert_correctly_infers_type_of_array_2_args_overload
 BuiltinTests.table_insert_correctly_infers_type_of_array_3_args_overload
+BuiltinTests.table_pack_variadic
 BuiltinTests.trivial_select
 BuiltinTests.xpcall
+ControlFlowAnalysis.for_record_do_if_not_x_break
+ControlFlowAnalysis.for_record_do_if_not_x_continue
+ControlFlowAnalysis.if_not_x_break
+ControlFlowAnalysis.if_not_x_break_elif_not_y_break
+ControlFlowAnalysis.if_not_x_break_elif_not_y_continue
+ControlFlowAnalysis.if_not_x_break_elif_not_y_fallthrough_elif_not_z_break
+ControlFlowAnalysis.if_not_x_break_elif_rand_break_elif_not_y_break
+ControlFlowAnalysis.if_not_x_break_elif_rand_break_elif_not_y_fallthrough
+ControlFlowAnalysis.if_not_x_break_if_not_y_break
+ControlFlowAnalysis.if_not_x_break_if_not_y_continue
+ControlFlowAnalysis.if_not_x_continue
+ControlFlowAnalysis.if_not_x_continue_elif_not_y_continue
+ControlFlowAnalysis.if_not_x_continue_elif_not_y_fallthrough_elif_not_z_continue
+ControlFlowAnalysis.if_not_x_continue_elif_not_y_throw_elif_not_z_fallthrough
+ControlFlowAnalysis.if_not_x_continue_elif_rand_continue_elif_not_y_continue
+ControlFlowAnalysis.if_not_x_continue_elif_rand_continue_elif_not_y_fallthrough
+ControlFlowAnalysis.if_not_x_continue_if_not_y_continue
+ControlFlowAnalysis.if_not_x_continue_if_not_y_throw
+ControlFlowAnalysis.if_not_x_return_elif_not_y_break
+ControlFlowAnalysis.if_not_x_return_elif_not_y_fallthrough_elif_not_z_break
+ControlFlowAnalysis.prototyping_and_visiting_alias_has_the_same_scope_breaking
+ControlFlowAnalysis.prototyping_and_visiting_alias_has_the_same_scope_continuing
+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_indexer
 DefinitionTests.class_definition_overload_metamethods
 DefinitionTests.class_definition_string_props
@@ -81,7 +111,6 @@ GenericsTests.generic_argument_count_too_many
 GenericsTests.generic_functions_dont_cache_type_parameters
 GenericsTests.generic_functions_should_be_memory_safe
 GenericsTests.generic_type_pack_parentheses
-GenericsTests.generic_type_pack_unification2
 GenericsTests.higher_rank_polymorphism_should_not_accept_instantiated_arguments
 GenericsTests.hof_subtype_instantiation_regression
 GenericsTests.infer_generic_function_function_argument
@@ -90,9 +119,6 @@ GenericsTests.infer_generic_function_function_argument_3
 GenericsTests.infer_generic_function_function_argument_overloaded
 GenericsTests.infer_generic_lib_function_function_argument
 GenericsTests.infer_generic_property
-GenericsTests.instantiate_cyclic_generic_function
-GenericsTests.instantiate_generic_function_in_assignments
-GenericsTests.instantiate_generic_function_in_assignments2
 GenericsTests.instantiated_function_argument_names
 GenericsTests.mutable_state_polymorphism
 GenericsTests.no_stack_overflow_from_quantifying
@@ -135,7 +161,6 @@ RefinementTest.isa_type_refinement_must_be_known_ahead_of_time
 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_2
 RefinementTest.refine_a_property_of_some_global
 RefinementTest.truthy_constraint_on_properties
 RefinementTest.type_narrow_to_vector
@@ -193,9 +218,9 @@ TableTests.shared_selfs
 TableTests.shared_selfs_from_free_param
 TableTests.shared_selfs_through_metatables
 TableTests.table_call_metamethod_basic
+TableTests.table_call_metamethod_generic
 TableTests.table_param_width_subtyping_1
 TableTests.table_param_width_subtyping_2
-TableTests.table_param_width_subtyping_3
 TableTests.table_simple_call
 TableTests.table_subtyping_with_extra_props_dont_report_multiple_errors
 TableTests.table_subtyping_with_missing_props_dont_report_multiple_errors
@@ -213,9 +238,7 @@ ToString.named_metatable_toStringNamedFunction
 ToString.pick_distinct_names_for_mixed_explicit_and_implicit_generics
 ToString.toStringDetailed2
 ToString.toStringErrorPack
-ToString.toStringGenericPack
 ToString.toStringNamedFunction_generic_pack
-ToString.toStringNamedFunction_map
 TryUnifyTests.members_of_failed_typepack_unification_are_unified_with_errorType
 TryUnifyTests.result_of_failed_typepack_unification_is_constrained
 TryUnifyTests.typepack_unification_should_trim_free_tails
@@ -239,8 +262,7 @@ TypeFamilyTests.function_internal_families
 TypeFamilyTests.internal_families_raise_errors
 TypeFamilyTests.table_internal_families
 TypeFamilyTests.unsolvable_family
-TypeInfer.be_sure_to_use_active_txnlog_when_evaluating_a_variadic_overload
+TypeInfer.bidirectional_checking_of_callback_property
-TypeInfer.bidirectional_checking_of_higher_order_function
 TypeInfer.check_expr_recursion_limit
 TypeInfer.check_type_infer_recursion_count
 TypeInfer.cli_39932_use_unifier_in_ensure_methods
@@ -249,14 +271,12 @@ TypeInfer.dont_report_type_errors_within_an_AstExprError
 TypeInfer.dont_report_type_errors_within_an_AstStatError
 TypeInfer.fuzz_free_table_type_change_during_index_check
 TypeInfer.infer_assignment_value_types_mutable_lval
-TypeInfer.infer_locals_via_assignment_from_its_call_site
-TypeInfer.interesting_local_type_inference_case
 TypeInfer.no_stack_overflow_from_isoptional
 TypeInfer.promote_tail_type_packs
 TypeInfer.recursive_function_that_invokes_itself_with_a_refinement_of_its_parameter
 TypeInfer.recursive_function_that_invokes_itself_with_a_refinement_of_its_parameter_2
 TypeInfer.tc_after_error_recovery_no_replacement_name_in_error
-TypeInfer.type_infer_cache_limit_normalizer
+TypeInfer.tc_if_else_expressions_expected_type_3
 TypeInfer.type_infer_recursion_limit_no_ice
 TypeInfer.type_infer_recursion_limit_normalizer
 TypeInferAnyError.can_subscript_any
@@ -274,7 +294,6 @@ TypeInferClasses.class_type_mismatch_with_name_conflict
 TypeInferClasses.detailed_class_unification_error
 TypeInferClasses.index_instance_property
 TypeInferClasses.table_class_unification_reports_sane_errors_for_missing_properties
-TypeInferClasses.table_indexers_are_invariant
 TypeInferFunctions.apply_of_lambda_with_inferred_and_explicit_types
 TypeInferFunctions.cannot_hoist_interior_defns_into_signature
 TypeInferFunctions.dont_assert_when_the_tarjan_limit_is_exceeded_during_generalization
@@ -295,7 +314,6 @@ TypeInferFunctions.infer_anonymous_function_arguments
 TypeInferFunctions.infer_generic_function_function_argument
 TypeInferFunctions.infer_generic_function_function_argument_overloaded
 TypeInferFunctions.infer_generic_lib_function_function_argument
-TypeInferFunctions.infer_higher_order_function
 TypeInferFunctions.infer_return_type_from_selected_overload
 TypeInferFunctions.infer_that_function_does_not_return_a_table
 TypeInferFunctions.it_is_ok_to_oversaturate_a_higher_order_function_argument
@@ -336,6 +354,7 @@ TypeInferLoops.properly_infer_iteratee_is_a_free_table
 TypeInferLoops.unreachable_code_after_infinite_loop
 TypeInferLoops.varlist_declared_by_for_in_loop_should_be_free
 TypeInferModules.bound_free_table_export_is_ok
+TypeInferModules.do_not_modify_imported_types_4
 TypeInferModules.do_not_modify_imported_types_5
 TypeInferModules.module_type_conflict
 TypeInferModules.module_type_conflict_instantiated
@@ -358,6 +377,7 @@ TypeInferOperators.concat_op_on_string_lhs_and_free_rhs
 TypeInferOperators.disallow_string_and_types_without_metatables_from_arithmetic_binary_ops
 TypeInferOperators.luau_polyfill_is_array
 TypeInferOperators.operator_eq_completely_incompatible
+TypeInferOperators.reducing_and
 TypeInferOperators.strict_binary_op_where_lhs_unknown
 TypeInferOperators.typecheck_overloaded_multiply_that_is_an_intersection
 TypeInferOperators.typecheck_overloaded_multiply_that_is_an_intersection_on_rhs
@@ -370,7 +390,7 @@ TypeInferPrimitives.CheckMethodsOfNumber
 TypeInferPrimitives.string_index
 TypeInferUnknownNever.length_of_never
 TypeInferUnknownNever.math_operators_and_never
-TypePackTests.higher_order_function
+TypePackTests.fuzz_typepack_iter_follow_2
 TypePackTests.pack_tail_unification_check
 TypePackTests.type_alias_backwards_compatible
 TypePackTests.type_alias_default_type_errors
@@ -378,6 +398,7 @@ TypePackTests.type_packs_with_tails_in_vararg_adjustment
 TypeSingletons.function_args_infer_singletons
 TypeSingletons.function_call_with_singletons
 TypeSingletons.function_call_with_singletons_mismatch
+TypeSingletons.overloaded_function_call_with_singletons
 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
@@ -385,3 +406,4 @@ TypeSingletons.widening_happens_almost_everywhere
 UnionTypes.index_on_a_union_type_with_missing_property
 UnionTypes.less_greedy_unification_with_union_types
 UnionTypes.table_union_write_indirect
+UnionTypes.unify_unsealed_table_union_check