[clang] AST: remove DependentTemplateSpecializationType (llvm#158109)

A DependentTemplateSpecializationType (DTST) is basically just a
TemplateSpecializationType (TST) with a hardcoded DependentTemplateName
(DTN) as its TemplateName.

This removes the DTST and replaces all uses of it with a TST, removing a
lot of duplication in the implementation.

Technically the hardcoded DTN is an optimization for a most common case,
but the TST implementation is in better shape overall and with other
optimizations, so this patch ends up being an overall performance
positive:
<img width="1465" height="38" alt="image"
src="https://github.com/user-attachments/assets/084b0694-2839-427a-b664-eff400f780b5"
/>

A DTST also didn't allow a template name representing a DTN that was
substituted, such as from an alias template, while the TST does allow it
by the simple fact it can hold an arbitrary TemplateName, so this patch
also increases the amount of sugar retained, while still being faster
overall.

Example (from included test case):
```C++
template<template<class> class TT> using T1 = TT<int>;
template<class T> using T2 = T1<T::template X>;
```

Here we can now represent in the AST that `TT` was substituted for the
dependent template name `T::template X`.

Author: mizvekov

clang-tools-extra/clang-tidy/utils/RenamerClangTidyCheck.cpp

@@ -350,6 +350,8 @@ class RenamerClangTidyVisitor
     const TemplateDecl *Decl =
         Loc.getTypePtr()->getTemplateName().getAsTemplateDecl(
             /*IgnoreDeduced=*/true);
+    if (!Decl)
+      return true;
 
     if (const auto *ClassDecl = dyn_cast<TemplateDecl>(Decl))
       if (const NamedDecl *TemplDecl = ClassDecl->getTemplatedDecl())

clang-tools-extra/clangd/FindTarget.cpp

@@ -406,15 +406,6 @@ struct TargetFinder {
           }
         }
       }
-      void VisitDependentTemplateSpecializationType(
-          const DependentTemplateSpecializationType *DTST) {
-        if (Outer.Resolver) {
-          for (const NamedDecl *ND :
-               Outer.Resolver->resolveTemplateSpecializationType(DTST)) {
-            Outer.add(ND, Flags);
-          }
-        }
-      }
       void VisitTypedefType(const TypedefType *TT) {
         if (shouldSkipTypedef(TT->getDecl()))
           return;
@@ -455,11 +446,13 @@ struct TargetFinder {
         // class template specializations have a (specialized) CXXRecordDecl.
         else if (const CXXRecordDecl *RD = TST->getAsCXXRecordDecl())
           Outer.add(RD, Flags); // add(Decl) will despecialize if needed.
-        else {
+        else if (auto *TD = TST->getTemplateName().getAsTemplateDecl())
           // fallback: the (un-specialized) declaration from primary template.
-          if (auto *TD = TST->getTemplateName().getAsTemplateDecl())
-            Outer.add(TD->getTemplatedDecl(), Flags | Rel::TemplatePattern);
-        }
+          Outer.add(TD->getTemplatedDecl(), Flags | Rel::TemplatePattern);
+        else if (Outer.Resolver)
+          for (const NamedDecl *ND :
+               Outer.Resolver->resolveTemplateSpecializationType(TST))
+            Outer.add(ND, Flags);
       }
       void
       VisitSubstTemplateTypeParmType(const SubstTemplateTypeParmType *STTPT) {
@@ -900,15 +893,6 @@ refInTypeLoc(TypeLoc L, const HeuristicResolver *Resolver) {
                                    DeclRelation::Alias, Resolver)});
     }
 
-    void VisitDependentTemplateSpecializationTypeLoc(
-        DependentTemplateSpecializationTypeLoc L) {
-      Refs.push_back(
-          ReferenceLoc{L.getQualifierLoc(), L.getTemplateNameLoc(),
-                       /*IsDecl=*/false,
-                       explicitReferenceTargets(
-                           DynTypedNode::create(L.getType()), {}, Resolver)});
-    }
-
     void VisitDependentNameTypeLoc(DependentNameTypeLoc L) {
       Refs.push_back(
           ReferenceLoc{L.getQualifierLoc(), L.getNameLoc(),

clang-tools-extra/clangd/SemanticHighlighting.cpp

@@ -728,11 +728,6 @@ class CollectExtraHighlightings
     return true;
   }
 
-  bool VisitTemplateSpecializationTypeLoc(TemplateSpecializationTypeLoc L) {
-    H.addAngleBracketTokens(L.getLAngleLoc(), L.getRAngleLoc());
-    return true;
-  }
-
   bool VisitFunctionDecl(FunctionDecl *D) {
     if (D->isOverloadedOperator()) {
       const auto AddOpDeclToken = [&](SourceLocation Loc) {
@@ -1087,11 +1082,12 @@ class CollectExtraHighlightings
     return true;
   }
 
-  bool VisitDependentTemplateSpecializationTypeLoc(
-      DependentTemplateSpecializationTypeLoc L) {
-    H.addToken(L.getTemplateNameLoc(), HighlightingKind::Type)
-        .addModifier(HighlightingModifier::DependentName)
-        .addModifier(HighlightingModifier::ClassScope);
+  bool VisitTemplateSpecializationTypeLoc(TemplateSpecializationTypeLoc L) {
+    if (!L.getTypePtr()->getTemplateName().getAsTemplateDecl(
+            /*IgnoreDeduced=*/true))
+      H.addToken(L.getTemplateNameLoc(), HighlightingKind::Type)
+          .addModifier(HighlightingModifier::DependentName)
+          .addModifier(HighlightingModifier::ClassScope);
     H.addAngleBracketTokens(L.getLAngleLoc(), L.getRAngleLoc());
     return true;
   }

clang-tools-extra/clangd/unittests/FindTargetTests.cpp

@@ -1029,8 +1029,7 @@ TEST_F(TargetDeclTest, DependentTypes) {
         template <typename T>
         void foo(typename A<T>::template [[B]]<int>);
       )cpp";
-  EXPECT_DECLS("DependentTemplateSpecializationTypeLoc",
-               "template <typename> struct B");
+  EXPECT_DECLS("TemplateSpecializationTypeLoc", "template <typename> struct B");
 
   // Dependent name with recursive definition. We don't expect a
   // result, but we shouldn't get into a stack overflow either.

clang-tools-extra/include-cleaner/lib/WalkAST.cpp

@@ -321,6 +321,8 @@ class ASTWalker : public RecursiveASTVisitor<ASTWalker> {
 
   // TypeLoc visitors.
   void reportType(SourceLocation RefLoc, NamedDecl *ND) {
+    if (!ND)
+      return;
     // Reporting explicit references to types nested inside classes can cause
     // issues, e.g. a type accessed through a derived class shouldn't require
     // inclusion of the base.

clang/docs/ReleaseNotes.rst

@@ -458,7 +458,9 @@ AST Matchers
   following the corresponding changes in the clang AST.
 - Ensure ``hasBitWidth`` doesn't crash on bit widths that are dependent on template
   parameters.
-
+- Remove the ``dependentTemplateSpecializationType`` matcher, as the
+  corresponding AST node was removed. This matcher was never very useful, since
+  there was no way to match on its template name.
 - Add a boolean member ``IgnoreSystemHeaders`` to ``MatchFinderOptions``. This
   allows it to ignore nodes in system headers when traversing the AST.
 

clang/include/clang/AST/ASTContext.h

@@ -241,9 +241,6 @@ class ASTContext : public RefCountedBase<ASTContext> {
   mutable llvm::FoldingSet<UsingType> UsingTypes;
   mutable llvm::FoldingSet<FoldingSetPlaceholder<TypedefType>> TypedefTypes;
   mutable llvm::FoldingSet<DependentNameType> DependentNameTypes;
-  mutable llvm::DenseMap<llvm::FoldingSetNodeID,
-                         DependentTemplateSpecializationType *>
-      DependentTemplateSpecializationTypes;
   mutable llvm::FoldingSet<PackExpansionType> PackExpansionTypes;
   mutable llvm::FoldingSet<ObjCObjectTypeImpl> ObjCObjectTypes;
   mutable llvm::FoldingSet<ObjCObjectPointerType> ObjCObjectPointerTypes;
@@ -1904,7 +1901,8 @@ class ASTContext : public RefCountedBase<ASTContext> {
                           TemplateTypeParmDecl *ParmDecl = nullptr) const;
 
   QualType getCanonicalTemplateSpecializationType(
-      TemplateName T, ArrayRef<TemplateArgument> CanonicalArgs) const;
+      ElaboratedTypeKeyword Keyword, TemplateName T,
+      ArrayRef<TemplateArgument> CanonicalArgs) const;
 
   QualType
   getTemplateSpecializationType(ElaboratedTypeKeyword Keyword, TemplateName T,
@@ -1935,13 +1933,6 @@ class ASTContext : public RefCountedBase<ASTContext> {
                                 NestedNameSpecifier NNS,
                                 const IdentifierInfo *Name) const;
 
-  QualType getDependentTemplateSpecializationType(
-      ElaboratedTypeKeyword Keyword, const DependentTemplateStorage &Name,
-      ArrayRef<TemplateArgumentLoc> Args) const;
-  QualType getDependentTemplateSpecializationType(
-      ElaboratedTypeKeyword Keyword, const DependentTemplateStorage &Name,
-      ArrayRef<TemplateArgument> Args, bool IsCanonical = false) const;
-
   TemplateArgument getInjectedTemplateArg(NamedDecl *ParamDecl) const;
 
   /// Form a pack expansion type with the given pattern.

clang/include/clang/AST/ASTNodeTraverser.h

@@ -533,11 +533,6 @@ class ASTNodeTraverser
     for (unsigned I=0, N=TL.getNumArgs(); I < N; ++I)
       dumpTemplateArgumentLoc(TL.getArgLoc(I));
   }
-  void VisitDependentTemplateSpecializationTypeLoc(
-      DependentTemplateSpecializationTypeLoc TL) {
-    for (unsigned I=0, N=TL.getNumArgs(); I < N; ++I)
-      dumpTemplateArgumentLoc(TL.getArgLoc(I));
-  }
 
   void VisitTypedefDecl(const TypedefDecl *D) { Visit(D->getUnderlyingType()); }
 

clang/include/clang/AST/RecursiveASTVisitor.h

@@ -1192,13 +1192,6 @@ DEF_TRAVERSE_TYPE(DependentNameType, {
     TRY_TO(TraverseNestedNameSpecifier(T->getQualifier()));
 })
 
-DEF_TRAVERSE_TYPE(DependentTemplateSpecializationType, {
-  const DependentTemplateStorage &S = T->getDependentTemplateName();
-  if (TraverseQualifier)
-    TRY_TO(TraverseNestedNameSpecifier(S.getQualifier()));
-  TRY_TO(TraverseTemplateArguments(T->template_arguments()));
-})
-
 DEF_TRAVERSE_TYPE(TemplateSpecializationType, {
   if (TraverseQualifier) {
     TRY_TO(TraverseTemplateName(T->getTemplateName()));
@@ -1546,15 +1539,6 @@ DEF_TRAVERSE_TYPELOC(DependentNameType, {
     TRY_TO(TraverseNestedNameSpecifierLoc(TL.getQualifierLoc()));
 })
 
-DEF_TRAVERSE_TYPELOC(DependentTemplateSpecializationType, {
-  if (TraverseQualifier)
-    TRY_TO(TraverseNestedNameSpecifierLoc(TL.getQualifierLoc()));
-
-  for (unsigned I = 0, E = TL.getNumArgs(); I != E; ++I) {
-    TRY_TO(TraverseTemplateArgumentLoc(TL.getArgLoc(I)));
-  }
-})
-
 DEF_TRAVERSE_TYPELOC(TemplateSpecializationType, {
   if (TraverseQualifier)
     TRY_TO(TraverseNestedNameSpecifierLoc(TL.getQualifierLoc()));

clang/include/clang/AST/TemplateName.h

@@ -297,10 +297,10 @@ class TemplateName {
   /// set of function templates, returns NULL.
   TemplateDecl *getAsTemplateDecl(bool IgnoreDeduced = false) const;
 
-  /// Retrieves the underlying template declaration that
+  /// Retrieves the underlying template name that
   /// this template name refers to, along with the
   /// deduced default arguments, if any.
-  std::pair<TemplateDecl *, DefaultArguments>
+  std::pair<TemplateName, DefaultArguments>
   getTemplateDeclAndDefaultArgs() const;
 
   /// Retrieve the underlying, overloaded function template