[clang] remove ClassScopeFunctionSpecializationDecl (#66636)

This removes the `ClassScopeFunctionSpecializationDecl` `Decl` node, and
instead uses `DependentFunctionTemplateSpecializationInfo` to handle
such declarations. `DependentFunctionTemplateSpecializationInfo` is also
changed to store a `const ASTTemplateArgumentListInfo*` to be more in
line with `FunctionTemplateSpecializationInfo`.

This also changes `FunctionDecl::isFunctionTemplateSpecialization` to
return `true` for dependent specializations, and
`FunctionDecl::getTemplateSpecializationKind`/`FunctionDecl::getTemplateSpecializationKindForInstantiation`
to return `TSK_ExplicitSpecialization` for non-friend dependent
specializations (the same behavior as dependent class scope
`ClassTemplateSepcializationDecl` & `VarTemplateSepcializationDecl`).

clang-tools-extra/clang-tidy/modernize/UseEmplaceCheck.cpp

@@ -67,11 +67,9 @@ AST_MATCHER_P(CallExpr, hasLastArgument,
 // function had parameters defined (this is useful to check if there is only one
 // variadic argument).
 AST_MATCHER(CXXMemberCallExpr, hasSameNumArgsAsDeclNumParams) {
-  if (Node.getMethodDecl()->isFunctionTemplateSpecialization())
-    return Node.getNumArgs() == Node.getMethodDecl()
-                                    ->getPrimaryTemplate()
-                                    ->getTemplatedDecl()
-                                    ->getNumParams();
+  if (const FunctionTemplateDecl *Primary =
+          Node.getMethodDecl()->getPrimaryTemplate())
+    return Node.getNumArgs() == Primary->getTemplatedDecl()->getNumParams();
 
   return Node.getNumArgs() == Node.getMethodDecl()->getNumParams();
 }

clang-tools-extra/clangd/SemanticHighlighting.cpp

@@ -715,13 +715,6 @@ class CollectExtraHighlightings
     return true;
   }
 
-  bool VisitClassScopeFunctionSpecializationDecl(
-      ClassScopeFunctionSpecializationDecl *D) {
-    if (auto *Args = D->getTemplateArgsAsWritten())
-      H.addAngleBracketTokens(Args->getLAngleLoc(), Args->getRAngleLoc());
-    return true;
-  }
-
   bool VisitDeclRefExpr(DeclRefExpr *E) {
     H.addAngleBracketTokens(E->getLAngleLoc(), E->getRAngleLoc());
     return true;
@@ -752,8 +745,6 @@ class CollectExtraHighlightings
     }
     if (auto *Args = D->getTemplateSpecializationArgsAsWritten())
       H.addAngleBracketTokens(Args->getLAngleLoc(), Args->getRAngleLoc());
-    if (auto *I = D->getDependentSpecializationInfo())
-      H.addAngleBracketTokens(I->getLAngleLoc(), I->getRAngleLoc());
     return true;
   }
 

clang/docs/ReleaseNotes.rst

@@ -71,6 +71,11 @@ C++ Specific Potentially Breaking Changes
   (`#49884 <https://github.com/llvm/llvm-project/issues/49884>`_), and
   (`#61273 <https://github.com/llvm/llvm-project/issues/61273>`_)
 
+- The `ClassScopeFunctionSpecializationDecl` AST node has been removed. Dependent class scope
+explicit function template specializations now use `DependentFunctionTemplateSpecializationInfo`
+to store candidate primary templates and explicit template arguments. This should not impact users
+of Clang as a compiler, but it may break assumptions in Clang-based tools iterating over the AST.
+
 ABI Changes in This Version
 ---------------------------
 - Following the SystemV ABI for x86-64, ``__int128`` arguments will no longer

clang/include/clang/AST/ASTNodeTraverser.h

@@ -426,8 +426,12 @@ class ASTNodeTraverser
   }
 
   void VisitFunctionDecl(const FunctionDecl *D) {
-    if (const auto *FTSI = D->getTemplateSpecializationInfo())
+    if (FunctionTemplateSpecializationInfo *FTSI =
+            D->getTemplateSpecializationInfo())
       dumpTemplateArgumentList(*FTSI->TemplateArguments);
+    else if (DependentFunctionTemplateSpecializationInfo *DFTSI =
+                 D->getDependentSpecializationInfo())
+      dumpASTTemplateArgumentListInfo(DFTSI->TemplateArgumentsAsWritten);
 
     if (D->param_begin())
       for (const auto *Parameter : D->parameters())
@@ -578,11 +582,6 @@ class ASTNodeTraverser
     dumpTemplateParameters(D->getTemplateParameters());
   }
 
-  void VisitClassScopeFunctionSpecializationDecl(
-      const ClassScopeFunctionSpecializationDecl *D) {
-    Visit(D->getSpecialization());
-    dumpASTTemplateArgumentListInfo(D->getTemplateArgsAsWritten());
-  }
   void VisitVarTemplateDecl(const VarTemplateDecl *D) { dumpTemplateDecl(D); }
 
   void VisitBuiltinTemplateDecl(const BuiltinTemplateDecl *D) {

clang/include/clang/AST/Decl.h

@@ -2845,9 +2845,7 @@ class FunctionDecl : public DeclaratorDecl,
 
   /// Determine whether this function is a function template
   /// specialization.
-  bool isFunctionTemplateSpecialization() const {
-    return getPrimaryTemplate() != nullptr;
-  }
+  bool isFunctionTemplateSpecialization() const;
 
   /// If this function is actually a function template specialization,
   /// retrieve information about this function template specialization.
@@ -2930,9 +2928,9 @@ class FunctionDecl : public DeclaratorDecl,
 
   /// Specifies that this function declaration is actually a
   /// dependent function template specialization.
-  void setDependentTemplateSpecialization(ASTContext &Context,
-                             const UnresolvedSetImpl &Templates,
-                      const TemplateArgumentListInfo &TemplateArgs);
+  void setDependentTemplateSpecialization(
+      ASTContext &Context, const UnresolvedSetImpl &Templates,
+      const TemplateArgumentListInfo *TemplateArgs);
 
   DependentFunctionTemplateSpecializationInfo *
   getDependentSpecializationInfo() const;

clang/include/clang/AST/DeclTemplate.h

@@ -583,7 +583,7 @@ class FunctionTemplateSpecializationInfo final
   /// \code
   /// template<typename> struct A {
   ///   template<typename> void f();
-  ///   template<> void f<int>(); // ClassScopeFunctionSpecializationDecl
+  ///   template<> void f<int>();
   /// };
   /// \endcode
   ///
@@ -682,82 +682,48 @@ class MemberSpecializationInfo {
 /// Provides information about a dependent function-template
 /// specialization declaration.
 ///
-/// Since explicit function template specialization and instantiation
-/// declarations can only appear in namespace scope, and you can only
-/// specialize a member of a fully-specialized class, the only way to
-/// get one of these is in a friend declaration like the following:
+/// This is used for function templates explicit specializations declared
+/// within class templates:
+///
+/// \code
+/// template<typename> struct A {
+///   template<typename> void f();
+///   template<> void f<int>(); // DependentFunctionTemplateSpecializationInfo
+/// };
+/// \endcode
+///
+/// As well as dependent friend declarations naming function template
+/// specializations declared within class templates:
 ///
 /// \code
 ///   template \<class T> void foo(T);
 ///   template \<class T> class A {
-///     friend void foo<>(T);
+///     friend void foo<>(T); // DependentFunctionTemplateSpecializationInfo
 ///   };
 /// \endcode
 class DependentFunctionTemplateSpecializationInfo final
     : private llvm::TrailingObjects<DependentFunctionTemplateSpecializationInfo,
-                                    TemplateArgumentLoc,
                                     FunctionTemplateDecl *> {
-  /// The number of potential template candidates.
-  unsigned NumTemplates;
-
-  /// The number of template arguments.
-  unsigned NumArgs;
-
-  /// The locations of the left and right angle brackets.
-  SourceRange AngleLocs;
+  friend TrailingObjects;
 
-  size_t numTrailingObjects(OverloadToken<TemplateArgumentLoc>) const {
-    return NumArgs;
-  }
-  size_t numTrailingObjects(OverloadToken<FunctionTemplateDecl *>) const {
-    return NumTemplates;
-  }
+  /// The number of candidates for the primary template.
+  unsigned NumCandidates;
 
   DependentFunctionTemplateSpecializationInfo(
-                                 const UnresolvedSetImpl &Templates,
-                                 const TemplateArgumentListInfo &TemplateArgs);
+      const UnresolvedSetImpl &Candidates,
+      const ASTTemplateArgumentListInfo *TemplateArgsWritten);
 
 public:
-  friend TrailingObjects;
+  /// The template arguments as written in the sources, if provided.
+  const ASTTemplateArgumentListInfo *TemplateArgumentsAsWritten;
 
   static DependentFunctionTemplateSpecializationInfo *
-  Create(ASTContext &Context, const UnresolvedSetImpl &Templates,
-         const TemplateArgumentListInfo &TemplateArgs);
-
-  /// Returns the number of function templates that this might
-  /// be a specialization of.
-  unsigned getNumTemplates() const { return NumTemplates; }
+  Create(ASTContext &Context, const UnresolvedSetImpl &Candidates,
+         const TemplateArgumentListInfo *TemplateArgs);
 
-  /// Returns the i'th template candidate.
-  FunctionTemplateDecl *getTemplate(unsigned I) const {
-    assert(I < getNumTemplates() && "template index out of range");
-    return getTrailingObjects<FunctionTemplateDecl *>()[I];
-  }
-
-  /// Returns the explicit template arguments that were given.
-  const TemplateArgumentLoc *getTemplateArgs() const {
-    return getTrailingObjects<TemplateArgumentLoc>();
-  }
-
-  /// Returns the number of explicit template arguments that were given.
-  unsigned getNumTemplateArgs() const { return NumArgs; }
-
-  llvm::ArrayRef<TemplateArgumentLoc> arguments() const {
-    return llvm::ArrayRef(getTemplateArgs(), getNumTemplateArgs());
-  }
-
-  /// Returns the nth template argument.
-  const TemplateArgumentLoc &getTemplateArg(unsigned I) const {
-    assert(I < getNumTemplateArgs() && "template arg index out of range");
-    return getTemplateArgs()[I];
-  }
-
-  SourceLocation getLAngleLoc() const {
-    return AngleLocs.getBegin();
-  }
-
-  SourceLocation getRAngleLoc() const {
-    return AngleLocs.getEnd();
+  /// Returns the candidates for the primary function template.
+  ArrayRef<FunctionTemplateDecl *> getCandidates() const {
+    return {getTrailingObjects<FunctionTemplateDecl *>(), NumCandidates};
   }
 };
 
@@ -2613,70 +2579,6 @@ class TypeAliasTemplateDecl : public RedeclarableTemplateDecl {
   static bool classofKind(Kind K) { return K == TypeAliasTemplate; }
 };
 
-/// Declaration of a function specialization at template class scope.
-///
-/// For example:
-/// \code
-/// template <class T>
-/// class A {
-///    template <class U> void foo(U a) { }
-///    template<> void foo(int a) { }
-/// }
-/// \endcode
-///
-/// "template<> foo(int a)" will be saved in Specialization as a normal
-/// CXXMethodDecl. Then during an instantiation of class A, it will be
-/// transformed into an actual function specialization.
-///
-/// FIXME: This is redundant; we could store the same information directly on
-/// the CXXMethodDecl as a DependentFunctionTemplateSpecializationInfo.
-class ClassScopeFunctionSpecializationDecl : public Decl {
-  CXXMethodDecl *Specialization;
-  const ASTTemplateArgumentListInfo *TemplateArgs;
-
-  ClassScopeFunctionSpecializationDecl(
-      DeclContext *DC, SourceLocation Loc, CXXMethodDecl *FD,
-      const ASTTemplateArgumentListInfo *TemplArgs)
-      : Decl(Decl::ClassScopeFunctionSpecialization, DC, Loc),
-        Specialization(FD), TemplateArgs(TemplArgs) {}
-
-  ClassScopeFunctionSpecializationDecl(EmptyShell Empty)
-      : Decl(Decl::ClassScopeFunctionSpecialization, Empty) {}
-
-  virtual void anchor();
-
-public:
-  friend class ASTDeclReader;
-  friend class ASTDeclWriter;
-
-  CXXMethodDecl *getSpecialization() const { return Specialization; }
-  bool hasExplicitTemplateArgs() const { return TemplateArgs; }
-  const ASTTemplateArgumentListInfo *getTemplateArgsAsWritten() const {
-    return TemplateArgs;
-  }
-
-  static ClassScopeFunctionSpecializationDecl *
-  Create(ASTContext &C, DeclContext *DC, SourceLocation Loc, CXXMethodDecl *FD,
-         bool HasExplicitTemplateArgs,
-         const TemplateArgumentListInfo &TemplateArgs) {
-    return new (C, DC) ClassScopeFunctionSpecializationDecl(
-        DC, Loc, FD,
-        HasExplicitTemplateArgs
-            ? ASTTemplateArgumentListInfo::Create(C, TemplateArgs)
-            : nullptr);
-  }
-
-  static ClassScopeFunctionSpecializationDecl *
-  CreateDeserialized(ASTContext &Context, unsigned ID);
-
-  // Implement isa/cast/dyncast/etc.
-  static bool classof(const Decl *D) { return classofKind(D->getKind()); }
-
-  static bool classofKind(Kind K) {
-    return K == Decl::ClassScopeFunctionSpecialization;
-  }
-};
-
 /// Represents a variable template specialization, which refers to
 /// a variable template with a given set of template arguments.
 ///

clang/include/clang/AST/RecursiveASTVisitor.h

@@ -1563,16 +1563,6 @@ DEF_TRAVERSE_DECL(FriendTemplateDecl, {
   }
 })
 
-DEF_TRAVERSE_DECL(ClassScopeFunctionSpecializationDecl, {
-  TRY_TO(TraverseDecl(D->getSpecialization()));
-
-  if (D->hasExplicitTemplateArgs()) {
-    TRY_TO(TraverseTemplateArgumentLocsHelper(
-        D->getTemplateArgsAsWritten()->getTemplateArgs(),
-        D->getTemplateArgsAsWritten()->NumTemplateArgs));
-  }
-})
-
 DEF_TRAVERSE_DECL(LinkageSpecDecl, {})
 
 DEF_TRAVERSE_DECL(ExportDecl, {})
@@ -2154,6 +2144,13 @@ bool RecursiveASTVisitor<Derived>::TraverseFunctionHelper(FunctionDecl *D) {
                                                   TALI->NumTemplateArgs));
       }
     }
+  } else if (const DependentFunctionTemplateSpecializationInfo *DFSI =
+                 D->getDependentSpecializationInfo()) {
+    if (const ASTTemplateArgumentListInfo *TALI =
+            DFSI->TemplateArgumentsAsWritten) {
+      TRY_TO(TraverseTemplateArgumentLocsHelper(TALI->getTemplateArgs(),
+                                                TALI->NumTemplateArgs));
+    }
   }
 
   // Visit the function type itself, which can be either

clang/include/clang/Basic/DeclNodes.td

@@ -102,7 +102,6 @@ def FriendTemplate : DeclNode<Decl>;
 def StaticAssert : DeclNode<Decl>;
 def Block : DeclNode<Decl, "blocks">, DeclContext;
 def Captured : DeclNode<Decl>, DeclContext;
-def ClassScopeFunctionSpecialization : DeclNode<Decl>;
 def Import : DeclNode<Decl>;
 def OMPThreadPrivate : DeclNode<Decl>;
 def OMPAllocate : DeclNode<Decl>;

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -5232,11 +5232,11 @@ def err_explicit_specialization_inconsistent_storage_class : Error<
   "'%select{none|extern|static|__private_extern__|auto|register}0'">;
 def err_dependent_function_template_spec_no_match : Error<
   "no candidate function template was found for dependent"
-  " friend function template specialization">;
+  " %select{member|friend}0 function template specialization">;
 def note_dependent_function_template_spec_discard_reason : Note<
-  "candidate ignored: %select{not a function template"
-  "|not a member of the enclosing namespace;"
-  " did you mean to explicitly qualify the specialization?}0">;
+  "candidate ignored: %select{not a function template|"
+  "not a member of the enclosing %select{class template|"
+  "namespace; did you mean to explicitly qualify the specialization?}1}0">;
 
 // C++ class template specializations and out-of-line definitions
 def err_template_spec_needs_header : Error<

clang/include/clang/Sema/Sema.h

@@ -8451,9 +8451,9 @@ class Sema final {
                                          SourceLocation PrevPtOfInstantiation,
                                          bool &SuppressNew);
 
-  bool CheckDependentFunctionTemplateSpecialization(FunctionDecl *FD,
-                    const TemplateArgumentListInfo &ExplicitTemplateArgs,
-                                                    LookupResult &Previous);
+  bool CheckDependentFunctionTemplateSpecialization(
+      FunctionDecl *FD, const TemplateArgumentListInfo *ExplicitTemplateArgs,
+      LookupResult &Previous);
 
   bool CheckFunctionTemplateSpecialization(
       FunctionDecl *FD, TemplateArgumentListInfo *ExplicitTemplateArgs,

clang/include/clang/Sema/Template.h

@@ -630,8 +630,6 @@ enum class TemplateSubstitutionKind : char {
     // A few supplemental visitor functions.
     Decl *VisitCXXMethodDecl(CXXMethodDecl *D,
                              TemplateParameterList *TemplateParams,
-                             std::optional<const ASTTemplateArgumentListInfo *>
-                                 ClassScopeSpecializationArgs = std::nullopt,
                              RewriteKind RK = RewriteKind::None);
     Decl *VisitFunctionDecl(FunctionDecl *D,
                             TemplateParameterList *TemplateParams,

clang/include/clang/Serialization/ASTBitCodes.h

@@ -1475,10 +1475,6 @@ enum DeclCode {
   /// template template parameter pack.
   DECL_EXPANDED_TEMPLATE_TEMPLATE_PARM_PACK,
 
-  /// A ClassScopeFunctionSpecializationDecl record a class scope
-  /// function specialization. (Microsoft extension).
-  DECL_CLASS_SCOPE_FUNCTION_SPECIALIZATION,
-
   /// An ImportDecl recording a module import.
   DECL_IMPORT,
 

clang/include/clang/Serialization/ASTRecordReader.h

@@ -155,6 +155,8 @@ class ASTRecordReader
   /// Reads a TemplateArgumentLoc, advancing Idx.
   TemplateArgumentLoc readTemplateArgumentLoc();
 
+  void readTemplateArgumentListInfo(TemplateArgumentListInfo &Result);
+
   const ASTTemplateArgumentListInfo*
   readASTTemplateArgumentListInfo();