[clang] support pack expansions for trailing requires clauses (#133190)

clang-tools-extra/clangd/refactor/tweaks/ExtractVariable.cpp

@@ -100,9 +100,9 @@ computeReferencedDecls(const clang::Expr *Expr) {
         TraverseLambdaCapture(LExpr, &Capture, Initializer);
       }
 
-      if (clang::Expr *const RequiresClause =
+      if (const clang::Expr *RequiresClause =
-              LExpr->getTrailingRequiresClause()) {
+              LExpr->getTrailingRequiresClause().ConstraintExpr) {
-        TraverseStmt(RequiresClause);
+        TraverseStmt(const_cast<clang::Expr *>(RequiresClause));
       }
 
       for (auto *const TemplateParam : LExpr->getExplicitTemplateParameters())

clang/docs/ReleaseNotes.rst

@@ -385,6 +385,8 @@ Bug Fixes to C++ Support
 - Improved fix for an issue with pack expansions of type constraints, where this
   now also works if the constraint has non-type or template template parameters.
   (#GH131798)
+- Fix crash when evaluating the trailing requires clause of generic lambdas which are part of
+  a pack expansion.
 - Fixes matching of nested template template parameters. (#GH130362)
 - Correctly diagnoses template template paramters which have a pack parameter
   not in the last position.

clang/include/clang/AST/ASTContext.h

@@ -2907,6 +2907,14 @@ public:
   /// that they may be used in declarations of the same template.
   bool isSameTemplateParameter(const NamedDecl *X, const NamedDecl *Y) const;
 
+  /// Determine whether two 'requires' expressions are similar enough that they
+  /// may be used in re-declarations.
+  ///
+  /// Use of 'requires' isn't mandatory, works with constraints expressed in
+  /// other ways too.
+  bool isSameAssociatedConstraint(const AssociatedConstraint &ACX,
+                                  const AssociatedConstraint &ACY) const;
+
   /// Determine whether two 'requires' expressions are similar enough that they
   /// may be used in re-declarations.
   ///

clang/include/clang/AST/ASTNodeTraverser.h

@@ -538,8 +538,8 @@ public:
       for (const auto *Parameter : D->parameters())
         Visit(Parameter);
 
-    if (const Expr *TRC = D->getTrailingRequiresClause())
+    if (const AssociatedConstraint &TRC = D->getTrailingRequiresClause())
-      Visit(TRC);
+      Visit(TRC.ConstraintExpr);
 
     if (Traversal == TK_IgnoreUnlessSpelledInSource && D->isDefaulted())
       return;

clang/include/clang/AST/Decl.h

@@ -81,13 +81,19 @@ enum class ImplicitParamKind;
 // Holds a constraint expression along with a pack expansion index, if
 // expanded.
 struct AssociatedConstraint {
-  const Expr *ConstraintExpr;
+  const Expr *ConstraintExpr = nullptr;
-  int ArgumentPackSubstitutionIndex;
+  int ArgumentPackSubstitutionIndex = -1;
+
+  constexpr AssociatedConstraint() = default;
 
   explicit AssociatedConstraint(const Expr *ConstraintExpr,
                                 int ArgumentPackSubstitutionIndex = -1)
       : ConstraintExpr(ConstraintExpr),
         ArgumentPackSubstitutionIndex(ArgumentPackSubstitutionIndex) {}
+
+  explicit operator bool() const { return ConstraintExpr != nullptr; }
+
+  bool isNull() const { return !operator bool(); }
 };
 
 /// The top declaration context.
@@ -754,7 +760,7 @@ class DeclaratorDecl : public ValueDecl {
   // and constrained function decls.
   struct ExtInfo : public QualifierInfo {
     TypeSourceInfo *TInfo = nullptr;
-    Expr *TrailingRequiresClause = nullptr;
+    AssociatedConstraint TrailingRequiresClause;
   };
 
   llvm::PointerUnion<TypeSourceInfo *, ExtInfo *> DeclInfo;
@@ -823,17 +829,12 @@ public:
   /// \brief Get the constraint-expression introduced by the trailing
   /// requires-clause in the function/member declaration, or null if no
   /// requires-clause was provided.
-  Expr *getTrailingRequiresClause() {
+  const AssociatedConstraint &getTrailingRequiresClause() const {
-    return hasExtInfo() ? getExtInfo()->TrailingRequiresClause
+    static constexpr AssociatedConstraint Null;
-                        : nullptr;
+    return hasExtInfo() ? getExtInfo()->TrailingRequiresClause : Null;
   }
 
-  const Expr *getTrailingRequiresClause() const {
+  void setTrailingRequiresClause(const AssociatedConstraint &AC);
-    return hasExtInfo() ? getExtInfo()->TrailingRequiresClause
-                        : nullptr;
-  }
-
-  void setTrailingRequiresClause(Expr *TrailingRequiresClause);
 
   unsigned getNumTemplateParameterLists() const {
     return hasExtInfo() ? getExtInfo()->NumTemplParamLists : 0;
@@ -2102,7 +2103,7 @@ protected:
                const DeclarationNameInfo &NameInfo, QualType T,
                TypeSourceInfo *TInfo, StorageClass S, bool UsesFPIntrin,
                bool isInlineSpecified, ConstexprSpecKind ConstexprKind,
-               Expr *TrailingRequiresClause = nullptr);
+               const AssociatedConstraint &TrailingRequiresClause);
 
   using redeclarable_base = Redeclarable<FunctionDecl>;
 
@@ -2138,7 +2139,7 @@ public:
          TypeSourceInfo *TInfo, StorageClass SC, bool UsesFPIntrin = false,
          bool isInlineSpecified = false, bool hasWrittenPrototype = true,
          ConstexprSpecKind ConstexprKind = ConstexprSpecKind::Unspecified,
-         Expr *TrailingRequiresClause = nullptr) {
+         const AssociatedConstraint &TrailingRequiresClause = {}) {
     DeclarationNameInfo NameInfo(N, NLoc);
     return FunctionDecl::Create(C, DC, StartLoc, NameInfo, T, TInfo, SC,
                                 UsesFPIntrin, isInlineSpecified,
@@ -2151,7 +2152,7 @@ public:
          const DeclarationNameInfo &NameInfo, QualType T, TypeSourceInfo *TInfo,
          StorageClass SC, bool UsesFPIntrin, bool isInlineSpecified,
          bool hasWrittenPrototype, ConstexprSpecKind ConstexprKind,
-         Expr *TrailingRequiresClause);
+         const AssociatedConstraint &TrailingRequiresClause);
 
   static FunctionDecl *CreateDeserialized(ASTContext &C, GlobalDeclID ID);
 
@@ -2644,9 +2645,9 @@ public:
   /// Use this instead of getTrailingRequiresClause for concepts APIs that
   /// accept an ArrayRef of constraint expressions.
   void
-  getAssociatedConstraints(SmallVectorImpl<AssociatedConstraint> &AC) const {
+  getAssociatedConstraints(SmallVectorImpl<AssociatedConstraint> &ACs) const {
-    if (auto *TRC = getTrailingRequiresClause())
+    if (const AssociatedConstraint &AC = getTrailingRequiresClause())
-      AC.emplace_back(TRC);
+      ACs.emplace_back(AC);
   }
 
   /// Get the message that indicates why this function was deleted.

clang/include/clang/AST/DeclCXX.h

@@ -1974,7 +1974,7 @@ private:
                         const DeclarationNameInfo &NameInfo, QualType T,
                         TypeSourceInfo *TInfo, SourceLocation EndLocation,
                         CXXConstructorDecl *Ctor, DeductionCandidate Kind,
-                        Expr *TrailingRequiresClause,
+                        const AssociatedConstraint &TrailingRequiresClause,
                         const CXXDeductionGuideDecl *GeneratedFrom,
                         SourceDeductionGuideKind SourceKind)
       : FunctionDecl(CXXDeductionGuide, C, DC, StartLoc, NameInfo, T, TInfo,
@@ -2007,7 +2007,7 @@ public:
          TypeSourceInfo *TInfo, SourceLocation EndLocation,
          CXXConstructorDecl *Ctor = nullptr,
          DeductionCandidate Kind = DeductionCandidate::Normal,
-         Expr *TrailingRequiresClause = nullptr,
+         const AssociatedConstraint &TrailingRequiresClause = {},
          const CXXDeductionGuideDecl *SourceDG = nullptr,
          SourceDeductionGuideKind SK = SourceDeductionGuideKind::None);
 
@@ -2115,7 +2115,7 @@ protected:
                 QualType T, TypeSourceInfo *TInfo, StorageClass SC,
                 bool UsesFPIntrin, bool isInline,
                 ConstexprSpecKind ConstexprKind, SourceLocation EndLocation,
-                Expr *TrailingRequiresClause = nullptr)
+                const AssociatedConstraint &TrailingRequiresClause = {})
       : FunctionDecl(DK, C, RD, StartLoc, NameInfo, T, TInfo, SC, UsesFPIntrin,
                      isInline, ConstexprKind, TrailingRequiresClause) {
     if (EndLocation.isValid())
@@ -2128,7 +2128,7 @@ public:
          const DeclarationNameInfo &NameInfo, QualType T, TypeSourceInfo *TInfo,
          StorageClass SC, bool UsesFPIntrin, bool isInline,
          ConstexprSpecKind ConstexprKind, SourceLocation EndLocation,
-         Expr *TrailingRequiresClause = nullptr);
+         const AssociatedConstraint &TrailingRequiresClause = {});
 
   static CXXMethodDecl *CreateDeserialized(ASTContext &C, GlobalDeclID ID);
 
@@ -2596,7 +2596,7 @@ class CXXConstructorDecl final
                      bool UsesFPIntrin, bool isInline,
                      bool isImplicitlyDeclared, ConstexprSpecKind ConstexprKind,
                      InheritedConstructor Inherited,
-                     Expr *TrailingRequiresClause);
+                     const AssociatedConstraint &TrailingRequiresClause);
 
   void anchor() override;
 
@@ -2639,7 +2639,7 @@ public:
          ExplicitSpecifier ES, bool UsesFPIntrin, bool isInline,
          bool isImplicitlyDeclared, ConstexprSpecKind ConstexprKind,
          InheritedConstructor Inherited = InheritedConstructor(),
-         Expr *TrailingRequiresClause = nullptr);
+         const AssociatedConstraint &TrailingRequiresClause = {});
 
   void setExplicitSpecifier(ExplicitSpecifier ES) {
     assert((!ES.getExpr() ||
@@ -2859,7 +2859,7 @@ class CXXDestructorDecl : public CXXMethodDecl {
                     const DeclarationNameInfo &NameInfo, QualType T,
                     TypeSourceInfo *TInfo, bool UsesFPIntrin, bool isInline,
                     bool isImplicitlyDeclared, ConstexprSpecKind ConstexprKind,
-                    Expr *TrailingRequiresClause = nullptr)
+                    const AssociatedConstraint &TrailingRequiresClause = {})
       : CXXMethodDecl(CXXDestructor, C, RD, StartLoc, NameInfo, T, TInfo,
                       SC_None, UsesFPIntrin, isInline, ConstexprKind,
                       SourceLocation(), TrailingRequiresClause) {
@@ -2874,7 +2874,7 @@ public:
          const DeclarationNameInfo &NameInfo, QualType T, TypeSourceInfo *TInfo,
          bool UsesFPIntrin, bool isInline, bool isImplicitlyDeclared,
          ConstexprSpecKind ConstexprKind,
-         Expr *TrailingRequiresClause = nullptr);
+         const AssociatedConstraint &TrailingRequiresClause = {});
   static CXXDestructorDecl *CreateDeserialized(ASTContext &C, GlobalDeclID ID);
 
   void setOperatorDelete(FunctionDecl *OD, Expr *ThisArg);
@@ -2925,7 +2925,7 @@ class CXXConversionDecl : public CXXMethodDecl {
                     TypeSourceInfo *TInfo, bool UsesFPIntrin, bool isInline,
                     ExplicitSpecifier ES, ConstexprSpecKind ConstexprKind,
                     SourceLocation EndLocation,
-                    Expr *TrailingRequiresClause = nullptr)
+                    const AssociatedConstraint &TrailingRequiresClause = {})
       : CXXMethodDecl(CXXConversion, C, RD, StartLoc, NameInfo, T, TInfo,
                       SC_None, UsesFPIntrin, isInline, ConstexprKind,
                       EndLocation, TrailingRequiresClause),
@@ -2943,7 +2943,7 @@ public:
          const DeclarationNameInfo &NameInfo, QualType T, TypeSourceInfo *TInfo,
          bool UsesFPIntrin, bool isInline, ExplicitSpecifier ES,
          ConstexprSpecKind ConstexprKind, SourceLocation EndLocation,
-         Expr *TrailingRequiresClause = nullptr);
+         const AssociatedConstraint &TrailingRequiresClause = {});
   static CXXConversionDecl *CreateDeserialized(ASTContext &C, GlobalDeclID ID);
 
   ExplicitSpecifier getExplicitSpecifier() {

clang/include/clang/AST/ExprCXX.h

@@ -2129,7 +2129,7 @@ public:
   ArrayRef<NamedDecl *> getExplicitTemplateParameters() const;
 
   /// Get the trailing requires clause, if any.
-  Expr *getTrailingRequiresClause() const;
+  const AssociatedConstraint &getTrailingRequiresClause() const;
 
   /// Whether this is a generic lambda.
   bool isGenericLambda() const { return getTemplateParameterList(); }

clang/include/clang/AST/RecursiveASTVisitor.h

@@ -2253,8 +2253,10 @@ bool RecursiveASTVisitor<Derived>::TraverseFunctionHelper(FunctionDecl *D) {
   }
 
   // Visit the trailing requires clause, if any.
-  if (Expr *TrailingRequiresClause = D->getTrailingRequiresClause()) {
+  if (const AssociatedConstraint &TrailingRequiresClause =
-    TRY_TO(TraverseStmt(TrailingRequiresClause));
+          D->getTrailingRequiresClause()) {
+    TRY_TO(TraverseStmt(
+        const_cast<Expr *>(TrailingRequiresClause.ConstraintExpr)));
   }
 
   if (CXXConstructorDecl *Ctor = dyn_cast<CXXConstructorDecl>(D)) {
@@ -2768,7 +2770,8 @@ DEF_TRAVERSE_STMT(LambdaExpr, {
 
     if (S->hasExplicitResultType())
       TRY_TO(TraverseTypeLoc(Proto.getReturnLoc()));
-    TRY_TO_TRAVERSE_OR_ENQUEUE_STMT(S->getTrailingRequiresClause());
+    TRY_TO_TRAVERSE_OR_ENQUEUE_STMT(
+        const_cast<Expr *>(S->getTrailingRequiresClause().ConstraintExpr));
 
     TRY_TO_TRAVERSE_OR_ENQUEUE_STMT(S->getBody());
   }

clang/include/clang/Sema/Sema.h

@@ -8867,12 +8867,14 @@ public:
       CXXMethodDecl *CallOperator, CXXRecordDecl *Class,
       TemplateParameterList *TemplateParams);
 
-  void CompleteLambdaCallOperator(
+  void
-      CXXMethodDecl *Method, SourceLocation LambdaLoc,
+  CompleteLambdaCallOperator(CXXMethodDecl *Method, SourceLocation LambdaLoc,
-      SourceLocation CallOperatorLoc, Expr *TrailingRequiresClause,
+                             SourceLocation CallOperatorLoc,
-      TypeSourceInfo *MethodTyInfo, ConstexprSpecKind ConstexprKind,
+                             const AssociatedConstraint &TrailingRequiresClause,
-      StorageClass SC, ArrayRef<ParmVarDecl *> Params,
+                             TypeSourceInfo *MethodTyInfo,
-      bool HasExplicitResultType);
+                             ConstexprSpecKind ConstexprKind, StorageClass SC,
+                             ArrayRef<ParmVarDecl *> Params,
+                             bool HasExplicitResultType);
 
   /// Returns true if the explicit object parameter was invalid.
   bool DiagnoseInvalidExplicitObjectParameterInLambda(CXXMethodDecl *Method,

clang/lib/AST/ASTContext.cpp

@@ -7070,6 +7070,15 @@ bool ASTContext::hasSameTemplateName(const TemplateName &X,
          getCanonicalTemplateName(Y, IgnoreDeduced);
 }
 
+bool ASTContext::isSameAssociatedConstraint(
+    const AssociatedConstraint &ACX, const AssociatedConstraint &ACY) const {
+  if (ACX.ArgumentPackSubstitutionIndex != ACY.ArgumentPackSubstitutionIndex)
+    return false;
+  if (!isSameConstraintExpr(ACX.ConstraintExpr, ACY.ConstraintExpr))
+    return false;
+  return true;
+}
+
 bool ASTContext::isSameConstraintExpr(const Expr *XCE, const Expr *YCE) const {
   if (!XCE != !YCE)
     return false;
@@ -7386,8 +7395,8 @@ bool ASTContext::isSameEntity(const NamedDecl *X, const NamedDecl *Y) const {
       return false;
     }
 
-    if (!isSameConstraintExpr(FuncX->getTrailingRequiresClause(),
+    if (!isSameAssociatedConstraint(FuncX->getTrailingRequiresClause(),
-                              FuncY->getTrailingRequiresClause()))
+                                    FuncY->getTrailingRequiresClause()))
       return false;
 
     auto GetTypeAsWritten = [](const FunctionDecl *FD) {

clang/lib/AST/ASTImporter.cpp

@@ -3915,8 +3915,9 @@ ExpectedDecl ASTNodeImporter::VisitFunctionDecl(FunctionDecl *D) {
   auto ToEndLoc = importChecked(Err, D->getEndLoc());
   auto ToDefaultLoc = importChecked(Err, D->getDefaultLoc());
   auto ToQualifierLoc = importChecked(Err, D->getQualifierLoc());
-  auto TrailingRequiresClause =
+  AssociatedConstraint TrailingRequiresClause = D->getTrailingRequiresClause();
-      importChecked(Err, D->getTrailingRequiresClause());
+  TrailingRequiresClause.ConstraintExpr =
+      importChecked(Err, TrailingRequiresClause.ConstraintExpr);
   if (Err)
     return std::move(Err);
 

clang/lib/AST/Decl.cpp

@@ -2009,8 +2009,8 @@ void DeclaratorDecl::setQualifierInfo(NestedNameSpecifierLoc QualifierLoc) {
   }
 }
 
-void DeclaratorDecl::setTrailingRequiresClause(Expr *TrailingRequiresClause) {
+void DeclaratorDecl::setTrailingRequiresClause(const AssociatedConstraint &AC) {
-  assert(TrailingRequiresClause);
+  assert(AC);
   // Make sure the extended decl info is allocated.
   if (!hasExtInfo()) {
     // Save (non-extended) type source info pointer.
@@ -2021,7 +2021,7 @@ void DeclaratorDecl::setTrailingRequiresClause(Expr *TrailingRequiresClause) {
     getExtInfo()->TInfo = savedTInfo;
   }
   // Set requires clause info.
-  getExtInfo()->TrailingRequiresClause = TrailingRequiresClause;
+  getExtInfo()->TrailingRequiresClause = AC;
 }
 
 void DeclaratorDecl::setTemplateParameterListsInfo(
@@ -3047,7 +3047,7 @@ FunctionDecl::FunctionDecl(Kind DK, ASTContext &C, DeclContext *DC,
                            TypeSourceInfo *TInfo, StorageClass S,
                            bool UsesFPIntrin, bool isInlineSpecified,
                            ConstexprSpecKind ConstexprKind,
-                           Expr *TrailingRequiresClause)
+                           const AssociatedConstraint &TrailingRequiresClause)
     : DeclaratorDecl(DK, DC, NameInfo.getLoc(), NameInfo.getName(), T, TInfo,
                      StartLoc),
       DeclContext(DK), redeclarable_base(C), Body(), ODRHash(0),
@@ -3571,7 +3571,7 @@ bool FunctionDecl::isMemberLikeConstrainedFriend() const {
     // If these friends don't have constraints, they aren't constrained, and
     // thus don't fall under temp.friend p9. Else the simple presence of a
     // constraint makes them unique.
-    return getTrailingRequiresClause();
+    return !getTrailingRequiresClause().isNull();
   }
 
   return FriendConstraintRefersToEnclosingTemplate();
@@ -5453,7 +5453,7 @@ FunctionDecl::Create(ASTContext &C, DeclContext *DC, SourceLocation StartLoc,
                      TypeSourceInfo *TInfo, StorageClass SC, bool UsesFPIntrin,
                      bool isInlineSpecified, bool hasWrittenPrototype,
                      ConstexprSpecKind ConstexprKind,
-                     Expr *TrailingRequiresClause) {
+                     const AssociatedConstraint &TrailingRequiresClause) {
   FunctionDecl *New = new (C, DC) FunctionDecl(
       Function, C, DC, StartLoc, NameInfo, T, TInfo, SC, UsesFPIntrin,
       isInlineSpecified, ConstexprKind, TrailingRequiresClause);
@@ -5464,7 +5464,8 @@ FunctionDecl::Create(ASTContext &C, DeclContext *DC, SourceLocation StartLoc,
 FunctionDecl *FunctionDecl::CreateDeserialized(ASTContext &C, GlobalDeclID ID) {
   return new (C, ID) FunctionDecl(
       Function, C, nullptr, SourceLocation(), DeclarationNameInfo(), QualType(),
-      nullptr, SC_None, false, false, ConstexprSpecKind::Unspecified, nullptr);
+      nullptr, SC_None, false, false, ConstexprSpecKind::Unspecified,
+      /*TrailingRequiresClause=*/{});
 }
 
 BlockDecl *BlockDecl::Create(ASTContext &C, DeclContext *DC, SourceLocation L) {

clang/lib/AST/DeclCXX.cpp

@@ -2304,7 +2304,7 @@ CXXDeductionGuideDecl *CXXDeductionGuideDecl::Create(
     ASTContext &C, DeclContext *DC, SourceLocation StartLoc,
     ExplicitSpecifier ES, const DeclarationNameInfo &NameInfo, QualType T,
     TypeSourceInfo *TInfo, SourceLocation EndLocation, CXXConstructorDecl *Ctor,
-    DeductionCandidate Kind, Expr *TrailingRequiresClause,
+    DeductionCandidate Kind, const AssociatedConstraint &TrailingRequiresClause,
     const CXXDeductionGuideDecl *GeneratedFrom,
     SourceDeductionGuideKind SourceKind) {
   return new (C, DC) CXXDeductionGuideDecl(
@@ -2318,7 +2318,7 @@ CXXDeductionGuideDecl::CreateDeserialized(ASTContext &C, GlobalDeclID ID) {
       C, /*DC=*/nullptr, SourceLocation(), ExplicitSpecifier(),
       DeclarationNameInfo(), QualType(), /*TInfo=*/nullptr, SourceLocation(),
       /*Ctor=*/nullptr, DeductionCandidate::Normal,
-      /*TrailingRequiresClause=*/nullptr,
+      /*TrailingRequiresClause=*/{},
       /*GeneratedFrom=*/nullptr, SourceDeductionGuideKind::None);
 }
 
@@ -2427,7 +2427,7 @@ CXXMethodDecl::Create(ASTContext &C, CXXRecordDecl *RD, SourceLocation StartLoc,
                       TypeSourceInfo *TInfo, StorageClass SC, bool UsesFPIntrin,
                       bool isInline, ConstexprSpecKind ConstexprKind,
                       SourceLocation EndLocation,
-                      Expr *TrailingRequiresClause) {
+                      const AssociatedConstraint &TrailingRequiresClause) {
   return new (C, RD) CXXMethodDecl(
       CXXMethod, C, RD, StartLoc, NameInfo, T, TInfo, SC, UsesFPIntrin,
       isInline, ConstexprKind, EndLocation, TrailingRequiresClause);
@@ -2435,10 +2435,11 @@ CXXMethodDecl::Create(ASTContext &C, CXXRecordDecl *RD, SourceLocation StartLoc,
 
 CXXMethodDecl *CXXMethodDecl::CreateDeserialized(ASTContext &C,
                                                  GlobalDeclID ID) {
-  return new (C, ID) CXXMethodDecl(
+  return new (C, ID)
-      CXXMethod, C, nullptr, SourceLocation(), DeclarationNameInfo(),
+      CXXMethodDecl(CXXMethod, C, nullptr, SourceLocation(),
-      QualType(), nullptr, SC_None, false, false,
+                    DeclarationNameInfo(), QualType(), nullptr, SC_None, false,
-      ConstexprSpecKind::Unspecified, SourceLocation(), nullptr);
+                    false, ConstexprSpecKind::Unspecified, SourceLocation(),
+                    /*TrailingRequiresClause=*/{});
 }
 
 CXXMethodDecl *CXXMethodDecl::getDevirtualizedMethod(const Expr *Base,
@@ -2834,7 +2835,8 @@ CXXConstructorDecl::CXXConstructorDecl(
     const DeclarationNameInfo &NameInfo, QualType T, TypeSourceInfo *TInfo,
     ExplicitSpecifier ES, bool UsesFPIntrin, bool isInline,
     bool isImplicitlyDeclared, ConstexprSpecKind ConstexprKind,
-    InheritedConstructor Inherited, Expr *TrailingRequiresClause)
+    InheritedConstructor Inherited,
+    const AssociatedConstraint &TrailingRequiresClause)
     : CXXMethodDecl(CXXConstructor, C, RD, StartLoc, NameInfo, T, TInfo,
                     SC_None, UsesFPIntrin, isInline, ConstexprKind,
                     SourceLocation(), TrailingRequiresClause) {
@@ -2861,7 +2863,7 @@ CXXConstructorDecl *CXXConstructorDecl::CreateDeserialized(ASTContext &C,
   auto *Result = new (C, ID, Extra) CXXConstructorDecl(
       C, nullptr, SourceLocation(), DeclarationNameInfo(), QualType(), nullptr,
       ExplicitSpecifier(), false, false, false, ConstexprSpecKind::Unspecified,
-      InheritedConstructor(), nullptr);
+      InheritedConstructor(), /*TrailingRequiresClause=*/{});
   Result->setInheritingConstructor(isInheritingConstructor);
   Result->CXXConstructorDeclBits.HasTrailingExplicitSpecifier =
       hasTrailingExplicit;
@@ -2874,7 +2876,8 @@ CXXConstructorDecl *CXXConstructorDecl::Create(
     const DeclarationNameInfo &NameInfo, QualType T, TypeSourceInfo *TInfo,
     ExplicitSpecifier ES, bool UsesFPIntrin, bool isInline,
     bool isImplicitlyDeclared, ConstexprSpecKind ConstexprKind,
-    InheritedConstructor Inherited, Expr *TrailingRequiresClause) {
+    InheritedConstructor Inherited,
+    const AssociatedConstraint &TrailingRequiresClause) {
   assert(NameInfo.getName().getNameKind()
          == DeclarationName::CXXConstructorName &&
          "Name must refer to a constructor");
@@ -3000,14 +3003,16 @@ CXXDestructorDecl *CXXDestructorDecl::CreateDeserialized(ASTContext &C,
                                                          GlobalDeclID ID) {
   return new (C, ID) CXXDestructorDecl(
       C, nullptr, SourceLocation(), DeclarationNameInfo(), QualType(), nullptr,
-      false, false, false, ConstexprSpecKind::Unspecified, nullptr);
+      false, false, false, ConstexprSpecKind::Unspecified,
+      /*TrailingRequiresClause=*/{});
 }
 
 CXXDestructorDecl *CXXDestructorDecl::Create(
     ASTContext &C, CXXRecordDecl *RD, SourceLocation StartLoc,
     const DeclarationNameInfo &NameInfo, QualType T, TypeSourceInfo *TInfo,
     bool UsesFPIntrin, bool isInline, bool isImplicitlyDeclared,
-    ConstexprSpecKind ConstexprKind, Expr *TrailingRequiresClause) {
+    ConstexprSpecKind ConstexprKind,
+    const AssociatedConstraint &TrailingRequiresClause) {
   assert(NameInfo.getName().getNameKind()
          == DeclarationName::CXXDestructorName &&
          "Name must refer to a destructor");
@@ -3062,7 +3067,7 @@ CXXConversionDecl *CXXConversionDecl::CreateDeserialized(ASTContext &C,
   return new (C, ID) CXXConversionDecl(
       C, nullptr, SourceLocation(), DeclarationNameInfo(), QualType(), nullptr,
       false, false, ExplicitSpecifier(), ConstexprSpecKind::Unspecified,
-      SourceLocation(), nullptr);
+      SourceLocation(), /*TrailingRequiresClause=*/{});
 }
 
 CXXConversionDecl *CXXConversionDecl::Create(
@@ -3070,7 +3075,7 @@ CXXConversionDecl *CXXConversionDecl::Create(
     const DeclarationNameInfo &NameInfo, QualType T, TypeSourceInfo *TInfo,
     bool UsesFPIntrin, bool isInline, ExplicitSpecifier ES,
     ConstexprSpecKind ConstexprKind, SourceLocation EndLocation,
-    Expr *TrailingRequiresClause) {
+    const AssociatedConstraint &TrailingRequiresClause) {
   assert(NameInfo.getName().getNameKind()
          == DeclarationName::CXXConversionFunctionName &&
          "Name must refer to a conversion function");

clang/lib/AST/DeclPrinter.cpp

@@ -842,10 +842,14 @@ void DeclPrinter::VisitFunctionDecl(FunctionDecl *D) {
     }
     Out << Proto;
 
-    if (Expr *TrailingRequiresClause = D->getTrailingRequiresClause()) {
+    if (const AssociatedConstraint &TrailingRequiresClause =
+            D->getTrailingRequiresClause()) {
       Out << " requires ";
-      TrailingRequiresClause->printPretty(Out, nullptr, SubPolicy, Indentation,
+      // FIXME: The printer could support printing expressions and types as if
-                                          "\n", &Context);
+      // expanded by an index. Pass in the ArgumentPackSubstitutionIndex when
+      // that's supported.
+      TrailingRequiresClause.ConstraintExpr->printPretty(
+          Out, nullptr, SubPolicy, Indentation, "\n", &Context);
     }
   } else {
     Ty.print(Out, Policy, Proto);

clang/lib/AST/DeclTemplate.cpp

@@ -291,7 +291,7 @@ void TemplateDecl::getAssociatedConstraints(
     llvm::SmallVectorImpl<AssociatedConstraint> &ACs) const {
   TemplateParams->getAssociatedConstraints(ACs);
   if (auto *FD = dyn_cast_or_null<FunctionDecl>(getTemplatedDecl()))
-    if (const Expr *TRC = FD->getTrailingRequiresClause())
+    if (const AssociatedConstraint &TRC = FD->getTrailingRequiresClause())
       ACs.emplace_back(TRC);
 }
 
@@ -299,7 +299,7 @@ bool TemplateDecl::hasAssociatedConstraints() const {
   if (TemplateParams->hasAssociatedConstraints())
     return true;
   if (auto *FD = dyn_cast_or_null<FunctionDecl>(getTemplatedDecl()))
-    return FD->getTrailingRequiresClause();
+    return static_cast<bool>(FD->getTrailingRequiresClause());
   return false;
 }
 

clang/lib/AST/ExprCXX.cpp

@@ -1402,7 +1402,7 @@ ArrayRef<NamedDecl *> LambdaExpr::getExplicitTemplateParameters() const {
   return Record->getLambdaExplicitTemplateParameters();
 }
 
-Expr *LambdaExpr::getTrailingRequiresClause() const {
+const AssociatedConstraint &LambdaExpr::getTrailingRequiresClause() const {
   return getCallOperator()->getTrailingRequiresClause();
 }
 

clang/lib/AST/ItaniumMangle.cpp

@@ -3781,7 +3781,7 @@ void CXXNameMangler::mangleBareFunctionType(const FunctionProtoType *Proto,
 
   if (FD) {
     FunctionTypeDepth.enterResultType();
-    mangleRequiresClause(FD->getTrailingRequiresClause());
+    mangleRequiresClause(FD->getTrailingRequiresClause().ConstraintExpr);
   }
 
   FunctionTypeDepth.pop(saved);

clang/lib/ASTMatchers/ASTMatchFinder.cpp

@@ -584,7 +584,8 @@ public:
 
         if (LE->hasExplicitResultType())
           TraverseTypeLoc(Proto.getReturnLoc());
-        TraverseStmt(LE->getTrailingRequiresClause());
+        TraverseStmt(
+            const_cast<Expr *>(LE->getTrailingRequiresClause().ConstraintExpr));
       }
 
       TraverseStmt(LE->getBody());

clang/lib/Index/IndexDecl.cpp

@@ -132,8 +132,8 @@ public:
         }
       }
     }
-    if (auto *C = D->getTrailingRequiresClause())
+    if (const AssociatedConstraint &C = D->getTrailingRequiresClause())
-      IndexCtx.indexBody(C, Parent);
+      IndexCtx.indexBody(C.ConstraintExpr, Parent);
   }
 
   bool handleObjCMethod(const ObjCMethodDecl *D,

clang/lib/Sema/SemaConcept.cpp

@@ -848,10 +848,8 @@ bool Sema::CheckFunctionConstraints(const FunctionDecl *FD,
       ForOverloadResolution);
 
   return CheckConstraintSatisfaction(
-      FD,
+      FD, FD->getTrailingRequiresClause(), *MLTAL,
-      AssociatedConstraint(FD->getTrailingRequiresClause(),
+      SourceRange(UsageLoc.isValid() ? UsageLoc : FD->getLocation()),
-                           ArgumentPackSubstitutionIndex),
-      *MLTAL, SourceRange(UsageLoc.isValid() ? UsageLoc : FD->getLocation()),
       Satisfaction);
 }
 

clang/lib/Sema/SemaDecl.cpp

@@ -9353,7 +9353,7 @@ static FunctionDecl *CreateNewFunctionDecl(Sema &SemaRef, Declarator &D,
         SemaRef.Context, DC, D.getBeginLoc(), NameInfo, R, TInfo, SC,
         SemaRef.getCurFPFeatures().isFPConstrained(), isInline, HasPrototype,
         ConstexprSpecKind::Unspecified,
-        /*TrailingRequiresClause=*/nullptr);
+        /*TrailingRequiresClause=*/{});
     if (D.isInvalidType())
       NewFD->setInvalidDecl();
 
@@ -9361,7 +9361,7 @@ static FunctionDecl *CreateNewFunctionDecl(Sema &SemaRef, Declarator &D,
   }
 
   ExplicitSpecifier ExplicitSpecifier = D.getDeclSpec().getExplicitSpecifier();
-  Expr *TrailingRequiresClause = D.getTrailingRequiresClause();
+  AssociatedConstraint TrailingRequiresClause(D.getTrailingRequiresClause());
 
   SemaRef.CheckExplicitObjectMemberFunction(DC, D, Name, R);
 
@@ -10531,7 +10531,7 @@ Sema::ActOnFunctionDeclarator(Scope *S, Declarator &D, DeclContext *DC,
            diag::ext_operator_new_delete_declared_inline)
         << NewFD->getDeclName();
 
-    if (Expr *TRC = NewFD->getTrailingRequiresClause()) {
+    if (const Expr *TRC = NewFD->getTrailingRequiresClause().ConstraintExpr) {
       // C++20 [dcl.decl.general]p4:
       //   The optional requires-clause in an init-declarator or
       //   member-declarator shall be present only if the declarator declares a
@@ -12261,7 +12261,7 @@ bool Sema::CheckFunctionDeclaration(Scope *S, FunctionDecl *NewFD,
       if (Method->isVirtual() && NewFD->getTrailingRequiresClause())
         // C++2a [class.virtual]p6
         // A virtual method shall not have a requires-clause.
-        Diag(NewFD->getTrailingRequiresClause()->getBeginLoc(),
+        Diag(NewFD->getTrailingRequiresClause().ConstraintExpr->getBeginLoc(),
              diag::err_constrained_virtual_method);
 
       if (Method->isStatic())
@@ -19085,8 +19085,7 @@ static void SetEligibleMethods(Sema &S, CXXRecordDecl *Record,
   SmallVector<bool, 4> SatisfactionStatus;
 
   for (CXXMethodDecl *Method : Methods) {
-    const Expr *Constraints = Method->getTrailingRequiresClause();
+    if (!Method->getTrailingRequiresClause())
-    if (!Constraints)
       SatisfactionStatus.push_back(true);
     else {
       ConstraintSatisfaction Satisfaction;
@@ -19105,7 +19104,7 @@ static void SetEligibleMethods(Sema &S, CXXRecordDecl *Record,
     if (FunctionDecl *MF = OrigMethod->getInstantiatedFromMemberFunction())
       OrigMethod = cast<CXXMethodDecl>(MF);
 
-    const Expr *Constraints = OrigMethod->getTrailingRequiresClause();
+    AssociatedConstraint Orig = OrigMethod->getTrailingRequiresClause();
     bool AnotherMethodIsMoreConstrained = false;
     for (size_t j = 0; j < Methods.size(); j++) {
       if (i == j || !SatisfactionStatus[j])
@@ -19118,15 +19117,13 @@ static void SetEligibleMethods(Sema &S, CXXRecordDecl *Record,
                                              CSM))
         continue;
 
-      const Expr *OtherConstraints = OtherMethod->getTrailingRequiresClause();
+      AssociatedConstraint Other = OtherMethod->getTrailingRequiresClause();
-      if (!OtherConstraints)
+      if (!Other)
         continue;
-      if (!Constraints) {
+      if (!Orig) {
         AnotherMethodIsMoreConstrained = true;
         break;
       }
-      AssociatedConstraint Other(OtherConstraints);
-      AssociatedConstraint Orig(Constraints);
       if (S.IsAtLeastAsConstrained(OtherMethod, {Other}, OrigMethod, {Orig},
                                    AnotherMethodIsMoreConstrained)) {
         // There was an error with the constraints comparison. Exit the loop

clang/lib/Sema/SemaDeclCXX.cpp

@@ -18995,8 +18995,8 @@ bool Sema::checkThisInStaticMemberFunctionType(CXXMethodDecl *Method) {
     return true;
 
   // Check the trailing requires clause
-  if (Expr *E = Method->getTrailingRequiresClause())
+  if (const AssociatedConstraint &TRC = Method->getTrailingRequiresClause())
-    if (!Finder.TraverseStmt(E))
+    if (!Finder.TraverseStmt(const_cast<Expr *>(TRC.ConstraintExpr)))
       return true;
 
   return checkThisInStaticMemberFunctionAttributes(Method);

clang/lib/Sema/SemaFunctionEffects.cpp

@@ -990,7 +990,7 @@ private:
         followDestructor(dyn_cast<CXXRecordDecl>(Dtor->getParent()), Dtor);
 
       if (auto *FD = dyn_cast<FunctionDecl>(CurrentCaller.CDecl)) {
-        TrailingRequiresClause = FD->getTrailingRequiresClause();
+        TrailingRequiresClause = FD->getTrailingRequiresClause().ConstraintExpr;
 
         // Note that FD->getType->getAs<FunctionProtoType>() can yield a
         // noexcept Expr which has been boiled down to a constant expression.

clang/lib/Sema/SemaLambda.cpp

@@ -1015,7 +1015,7 @@ CXXMethodDecl *Sema::CreateLambdaCallOperator(SourceRange IntroducerRange,
       QualType(), /*Tinfo=*/nullptr, SC_None,
       getCurFPFeatures().isFPConstrained(),
       /*isInline=*/true, ConstexprSpecKind::Unspecified, SourceLocation(),
-      /*TrailingRequiresClause=*/nullptr);
+      /*TrailingRequiresClause=*/{});
   Method->setAccess(AS_public);
   return Method;
 }
@@ -1033,7 +1033,8 @@ void Sema::AddTemplateParametersToLambdaCallOperator(
 
 void Sema::CompleteLambdaCallOperator(
     CXXMethodDecl *Method, SourceLocation LambdaLoc,
-    SourceLocation CallOperatorLoc, Expr *TrailingRequiresClause,
+    SourceLocation CallOperatorLoc,
+    const AssociatedConstraint &TrailingRequiresClause,
     TypeSourceInfo *MethodTyInfo, ConstexprSpecKind ConstexprKind,
     StorageClass SC, ArrayRef<ParmVarDecl *> Params,
     bool HasExplicitResultType) {
@@ -1461,8 +1462,9 @@ void Sema::ActOnStartOfLambdaDefinition(LambdaIntroducer &Intro,
 
   CompleteLambdaCallOperator(
       Method, Intro.Range.getBegin(), CallOperatorLoc,
-      ParamInfo.getTrailingRequiresClause(), MethodTyInfo,
+      AssociatedConstraint(ParamInfo.getTrailingRequiresClause(),
-      ParamInfo.getDeclSpec().getConstexprSpecifier(),
+                           /*ArgumentPackSubstitutionIndex=*/-1),
+      MethodTyInfo, ParamInfo.getDeclSpec().getConstexprSpecifier(),
       IsLambdaStatic ? SC_Static : SC_None, Params, ExplicitResultType);
 
   CheckCXXDefaultArguments(Method);
@@ -1545,7 +1547,7 @@ void Sema::ActOnStartOfLambdaDefinition(LambdaIntroducer &Intro,
   // The optional requires-clause ([temp.pre]) in an init-declarator or
   // member-declarator shall be present only if the declarator declares a
   // templated function ([dcl.fct]).
-  if (Expr *TRC = Method->getTrailingRequiresClause()) {
+  if (const AssociatedConstraint &TRC = Method->getTrailingRequiresClause()) {
     // [temp.pre]/8:
     // An entity is templated if it is
     // - a template,
@@ -1568,7 +1570,8 @@ void Sema::ActOnStartOfLambdaDefinition(LambdaIntroducer &Intro,
     // applies to the call operator, which we already know is a member function,
     // AND defined.
     if (!Method->getDescribedFunctionTemplate() && !Method->isTemplated()) {
-      Diag(TRC->getBeginLoc(), diag::err_constrained_non_templated_function);
+      Diag(TRC.ConstraintExpr->getBeginLoc(),
+           diag::err_constrained_non_templated_function);
     }
   }
 
@@ -1791,7 +1794,8 @@ static void addFunctionPointerConversion(Sema &S, SourceRange IntroducerRange,
 
   // A non-generic lambda may still be a templated entity. We need to preserve
   // constraints when converting the lambda to a function pointer. See GH63181.
-  if (Expr *Requires = CallOperator->getTrailingRequiresClause())
+  if (const AssociatedConstraint &Requires =
+          CallOperator->getTrailingRequiresClause())
     Conversion->setTrailingRequiresClause(Requires);
 
   if (Class->isGenericLambda()) {

clang/lib/Sema/SemaOverload.cpp

@@ -1551,12 +1551,16 @@ static bool IsOverloadOrOverrideImpl(Sema &SemaRef, FunctionDecl *New,
 
   if (!UseOverrideRules &&
       New->getTemplateSpecializationKind() != TSK_ExplicitSpecialization) {
-    Expr *NewRC = New->getTrailingRequiresClause(),
+    AssociatedConstraint NewRC = New->getTrailingRequiresClause(),
-         *OldRC = Old->getTrailingRequiresClause();
+                         OldRC = Old->getTrailingRequiresClause();
-    if ((NewRC != nullptr) != (OldRC != nullptr))
+    if (!NewRC != !OldRC)
+      return true;
+    if (NewRC.ArgumentPackSubstitutionIndex !=
+        OldRC.ArgumentPackSubstitutionIndex)
       return true;
     if (NewRC &&
-        !SemaRef.AreConstraintExpressionsEqual(OldDecl, OldRC, NewDecl, NewRC))
+        !SemaRef.AreConstraintExpressionsEqual(OldDecl, OldRC.ConstraintExpr,
+                                               NewDecl, NewRC.ConstraintExpr))
       return true;
   }
 

clang/lib/Sema/SemaTemplateDeductionGuide.cpp

@@ -200,7 +200,7 @@ buildDeductionGuide(Sema &SemaRef, TemplateDecl *OriginalTemplate,
                     TypeSourceInfo *TInfo, SourceLocation LocStart,
                     SourceLocation Loc, SourceLocation LocEnd, bool IsImplicit,
                     llvm::ArrayRef<TypedefNameDecl *> MaterializedTypedefs = {},
-                    Expr *FunctionTrailingRC = nullptr) {
+                    const AssociatedConstraint &FunctionTrailingRC = {}) {
   DeclContext *DC = OriginalTemplate->getDeclContext();
   auto DeductionGuideName =
       SemaRef.Context.DeclarationNames.getCXXDeductionGuideName(
@@ -356,7 +356,8 @@ struct ConvertConstructorToDeductionGuideTransform {
     TemplateParameterList *TemplateParams =
         SemaRef.GetTemplateParameterList(Template);
     SmallVector<TemplateArgument, 16> Depth1Args;
-    Expr *OuterRC = TemplateParams->getRequiresClause();
+    AssociatedConstraint OuterRC(TemplateParams->getRequiresClause(),
+                                 /*ArgumentPackSubstitutionIndex=*/-1);
     if (FTD) {
       TemplateParameterList *InnerParams = FTD->getTemplateParameters();
       SmallVector<NamedDecl *, 16> AllParams;
@@ -456,18 +457,20 @@ struct ConvertConstructorToDeductionGuideTransform {
     // At this point, the function parameters are already 'instantiated' in the
     // current scope. Substitute into the constructor's trailing
     // requires-clause, if any.
-    Expr *FunctionTrailingRC = nullptr;
+    AssociatedConstraint FunctionTrailingRC;
-    if (Expr *RC = CD->getTrailingRequiresClause()) {
+    if (const AssociatedConstraint &RC = CD->getTrailingRequiresClause()) {
       MultiLevelTemplateArgumentList Args;
       Args.setKind(TemplateSubstitutionKind::Rewrite);
       Args.addOuterTemplateArguments(Depth1Args);
       Args.addOuterRetainedLevel();
       if (NestedPattern)
         Args.addOuterRetainedLevels(NestedPattern->getTemplateDepth());
-      ExprResult E = SemaRef.SubstConstraintExprWithoutSatisfaction(RC, Args);
+      ExprResult E = SemaRef.SubstConstraintExprWithoutSatisfaction(
+          const_cast<Expr *>(RC.ConstraintExpr), Args);
       if (!E.isUsable())
         return nullptr;
-      FunctionTrailingRC = E.get();
+      FunctionTrailingRC =
+          AssociatedConstraint(E.get(), RC.ArgumentPackSubstitutionIndex);
     }
 
     // C++ [over.match.class.deduct]p1:
@@ -480,13 +483,19 @@ struct ConvertConstructorToDeductionGuideTransform {
     if (OuterRC) {
       // The outer template parameters are not transformed, so their
       // associated constraints don't need substitution.
+      // FIXME: Should simply add another field for the OuterRC, instead of
+      // combining them like this.
       if (!FunctionTrailingRC)
         FunctionTrailingRC = OuterRC;
       else
-        FunctionTrailingRC = BinaryOperator::Create(
+        FunctionTrailingRC = AssociatedConstraint(
-            SemaRef.Context, /*lhs=*/OuterRC, /*rhs=*/FunctionTrailingRC,
+            BinaryOperator::Create(
-            BO_LAnd, SemaRef.Context.BoolTy, VK_PRValue, OK_Ordinary,
+                SemaRef.Context,
-            TemplateParams->getTemplateLoc(), FPOptionsOverride());
+                /*lhs=*/const_cast<Expr *>(OuterRC.ConstraintExpr),
+                /*rhs=*/const_cast<Expr *>(FunctionTrailingRC.ConstraintExpr),
+                BO_LAnd, SemaRef.Context.BoolTy, VK_PRValue, OK_Ordinary,
+                TemplateParams->getTemplateLoc(), FPOptionsOverride()),
+            FunctionTrailingRC.ArgumentPackSubstitutionIndex);
     }
 
     return buildDeductionGuide(
@@ -1238,14 +1247,20 @@ void DeclareImplicitDeductionGuidesForTypeAlias(
       // FIXME: Here the synthesized deduction guide is not a templated
       // function. Per [dcl.decl]p4, the requires-clause shall be present only
       // if the declarator declares a templated function, a bug in standard?
-      auto *Constraint = buildIsDeducibleConstraint(
+      AssociatedConstraint Constraint(
-          SemaRef, AliasTemplate, Transformed->getReturnType(), {});
+          buildIsDeducibleConstraint(SemaRef, AliasTemplate,
-      if (auto *RC = DG->getTrailingRequiresClause()) {
+                                     Transformed->getReturnType(), {}),
-        auto Conjunction =
+          /*ArgumentPackSubstitutionIndex=*/-1);
-            SemaRef.BuildBinOp(SemaRef.getCurScope(), SourceLocation{},
+      if (const AssociatedConstraint &RC = DG->getTrailingRequiresClause()) {
-                               BinaryOperatorKind::BO_LAnd, RC, Constraint);
+        auto Conjunction = SemaRef.BuildBinOp(
-        if (!Conjunction.isInvalid())
+            SemaRef.getCurScope(), SourceLocation{},
-          Constraint = Conjunction.getAs<Expr>();
+            BinaryOperatorKind::BO_LAnd, const_cast<Expr *>(RC.ConstraintExpr),
+            const_cast<Expr *>(Constraint.ConstraintExpr));
+        if (!Conjunction.isInvalid()) {
+          Constraint.ConstraintExpr = Conjunction.getAs<Expr>();
+          Constraint.ArgumentPackSubstitutionIndex =
+              RC.ArgumentPackSubstitutionIndex;
+        }
       }
       Transformed->setTrailingRequiresClause(Constraint);
       continue;

clang/lib/Sema/SemaTemplateInstantiateDecl.cpp

@@ -2671,7 +2671,7 @@ Decl *TemplateDeclInstantiator::VisitFunctionDecl(
       return nullptr;
   }
 
-  Expr *TrailingRequiresClause = D->getTrailingRequiresClause();
+  AssociatedConstraint TrailingRequiresClause = D->getTrailingRequiresClause();
 
   // If we're instantiating a local function declaration, put the result
   // in the enclosing namespace; otherwise we need to find the instantiated
@@ -3102,7 +3102,7 @@ Decl *TemplateDeclInstantiator::VisitCXXMethodDecl(
   }
 
   CXXRecordDecl *Record = cast<CXXRecordDecl>(DC);
-  Expr *TrailingRequiresClause = D->getTrailingRequiresClause();
+  AssociatedConstraint TrailingRequiresClause = D->getTrailingRequiresClause();
 
   DeclarationNameInfo NameInfo
     = SemaRef.SubstDeclarationNameInfo(D->getNameInfo(), TemplateArgs);

clang/lib/Sema/TreeTransform.h

@@ -15659,10 +15659,13 @@ TreeTransform<Derived>::TransformLambdaExpr(LambdaExpr *E) {
   auto FPTL = NewCallOpTSI->getTypeLoc().getAsAdjusted<FunctionProtoTypeLoc>();
   assert(FPTL && "Not a FunctionProtoType?");
 
+  AssociatedConstraint TRC = E->getCallOperator()->getTrailingRequiresClause();
+  if (TRC.ArgumentPackSubstitutionIndex == -1)
+    TRC.ArgumentPackSubstitutionIndex = SemaRef.ArgumentPackSubstitutionIndex;
+
   getSema().CompleteLambdaCallOperator(
       NewCallOperator, E->getCallOperator()->getLocation(),
-      E->getCallOperator()->getInnerLocStart(),
+      E->getCallOperator()->getInnerLocStart(), TRC, NewCallOpTSI,
-      E->getCallOperator()->getTrailingRequiresClause(), NewCallOpTSI,
       E->getCallOperator()->getConstexprKind(),
       E->getCallOperator()->getStorageClass(), FPTL.getParams(),
       E->hasExplicitResultType());

clang/lib/Serialization/ASTReaderDecl.cpp

@@ -904,7 +904,8 @@ void ASTDeclReader::VisitDeclaratorDecl(DeclaratorDecl *DD) {
   if (Record.readInt()) { // hasExtInfo
     auto *Info = new (Reader.getContext()) DeclaratorDecl::ExtInfo();
     Record.readQualifierInfo(*Info);
-    Info->TrailingRequiresClause = Record.readExpr();
+    Info->TrailingRequiresClause =
+        AssociatedConstraint(Record.readExpr(), int(Record.readInt()));
     DD->DeclInfo = Info;
   }
   QualType TSIType = Record.readType();

clang/lib/Serialization/ASTWriterDecl.cpp

@@ -728,7 +728,10 @@ void ASTDeclWriter::VisitDeclaratorDecl(DeclaratorDecl *D) {
   if (D->hasExtInfo()) {
     DeclaratorDecl::ExtInfo *Info = D->getExtInfo();
     Record.AddQualifierInfo(*Info);
-    Record.AddStmt(Info->TrailingRequiresClause);
+    Record.AddStmt(
+        const_cast<Expr *>(Info->TrailingRequiresClause.ConstraintExpr));
+    Record.push_back(
+        Info->TrailingRequiresClause.ArgumentPackSubstitutionIndex);
   }
   // The location information is deferred until the end of the record.
   Record.AddTypeRef(D->getTypeSourceInfo() ? D->getTypeSourceInfo()->getType()

clang/test/SemaCXX/fold_lambda_with_variadics.cpp

@@ -267,6 +267,15 @@ static_assert(bazz<1, 2>()(1));
 // expected-error@-1 {{is ambiguous}}
 // expected-note@#bazz 2{{candidate function [with value:auto = int]}}
 
+template <class T> concept C2 = sizeof(T) >= sizeof(int);
+template <class... Ts> static constexpr auto trailing() {
+  return Overloaded{[](auto) requires (C2<Ts> && C2<int>) { return 0; }...}; // #trailing
+}
+static_assert(trailing<int, long>()(0));
+// expected-error@-1 {{is ambiguous}}
+// expected-note@#trailing 2{{candidate function [with auto:1 = int]}}
+
+
 } // namespace GH101754
 
 namespace GH131798 {

clang/tools/libclang/CIndex.cpp

@@ -872,7 +872,7 @@ bool CursorVisitor::VisitFunctionDecl(FunctionDecl *ND) {
     // FIXME: Attributes?
   }
 
-  if (auto *E = ND->getTrailingRequiresClause()) {
+  if (auto *E = ND->getTrailingRequiresClause().ConstraintExpr) {
     if (Visit(E))
       return true;
   }