[clang] AST: remove DependentTemplateSpecializationType

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:

A DTST also didn't allow a template name representing a DependentTemplateName
that was substituted from an alias templates, while the TST does 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/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

clang/include/clang/AST/TypeBase.h

@@ -2250,22 +2250,6 @@ class alignas(TypeAlignment) Type : public ExtQualsTypeCommonBase {
     unsigned NumArgs;
   };
 
-  class DependentTemplateSpecializationTypeBitfields {
-    friend class DependentTemplateSpecializationType;
-
-    LLVM_PREFERRED_TYPE(KeywordWrapperBitfields)
-    unsigned : NumTypeWithKeywordBits;
-
-    /// The number of template arguments named in this class template
-    /// specialization, which is expected to be able to hold at least 1024
-    /// according to [implimits]. However, as this limit is somewhat easy to
-    /// hit with template metaprogramming we'd prefer to keep it as large
-    /// as possible. At the moment it has been left as a non-bitfield since
-    /// this type safely fits in 64 bits as an unsigned, so there is no reason
-    /// to introduce the performance impact of a bitfield.
-    unsigned NumArgs;
-  };
-
   class PackExpansionTypeBitfields {
     friend class PackExpansionType;
 
@@ -2346,8 +2330,6 @@ class alignas(TypeAlignment) Type : public ExtQualsTypeCommonBase {
     SubstTemplateTypeParmTypeBitfields SubstTemplateTypeParmTypeBits;
     SubstPackTypeBitfields SubstPackTypeBits;
     TemplateSpecializationTypeBitfields TemplateSpecializationTypeBits;
-    DependentTemplateSpecializationTypeBitfields
-      DependentTemplateSpecializationTypeBits;
     PackExpansionTypeBitfields PackExpansionTypeBits;
     CountAttributedTypeBitfields CountAttributedTypeBits;
     PresefinedSugarTypeBitfields PredefinedSugarTypeBits;
@@ -7366,9 +7348,9 @@ class TemplateSpecializationType : public TypeWithKeyword,
   }
 
   void Profile(llvm::FoldingSetNodeID &ID, const ASTContext &Ctx);
-  static void Profile(llvm::FoldingSetNodeID &ID, TemplateName T,
-                      ArrayRef<TemplateArgument> Args, QualType Underlying,
-                      const ASTContext &Context);
+  static void Profile(llvm::FoldingSetNodeID &ID, ElaboratedTypeKeyword Keyword,
+                      TemplateName T, ArrayRef<TemplateArgument> Args,
+                      QualType Underlying, const ASTContext &Context);
 
   static bool classof(const Type *T) {
     return T->getTypeClass() == TemplateSpecialization;
@@ -7459,46 +7441,6 @@ class DependentNameType : public TypeWithKeyword, public llvm::FoldingSetNode {
   }
 };
 
-/// Represents a template specialization type whose template cannot be
-/// resolved, e.g.
-///   A<T>::template B<T>
-class DependentTemplateSpecializationType : public TypeWithKeyword {
-  friend class ASTContext; // ASTContext creates these
-
-  DependentTemplateStorage Name;
-
-  DependentTemplateSpecializationType(ElaboratedTypeKeyword Keyword,
-                                      const DependentTemplateStorage &Name,
-                                      ArrayRef<TemplateArgument> Args,
-                                      QualType Canon);
-
-public:
-  const DependentTemplateStorage &getDependentTemplateName() const {
-    return Name;
-  }
-
-  ArrayRef<TemplateArgument> template_arguments() const {
-    return {reinterpret_cast<const TemplateArgument *>(this + 1),
-            DependentTemplateSpecializationTypeBits.NumArgs};
-  }
-
-  bool isSugared() const { return false; }
-  QualType desugar() const { return QualType(this, 0); }
-
-  void Profile(llvm::FoldingSetNodeID &ID, const ASTContext &Context) {
-    Profile(ID, Context, getKeyword(), Name, template_arguments());
-  }
-
-  static void Profile(llvm::FoldingSetNodeID &ID, const ASTContext &Context,
-                      ElaboratedTypeKeyword Keyword,
-                      const DependentTemplateStorage &Name,
-                      ArrayRef<TemplateArgument> Args);
-
-  static bool classof(const Type *T) {
-    return T->getTypeClass() == DependentTemplateSpecialization;
-  }
-};
-
 /// Represents a pack expansion of types.
 ///
 /// Pack expansions are part of C++11 variadic templates. A pack