[clang] improved preservation of template keyword

This makes it so clang can better represent the abscence of
the template keyword for a template which cannot be resolved
due to a dependent prefix.

The tracking of the template keyword is removed from the
NestedNameSpecifier, and added to the last type node which had
it missing, the DependentTemplateSpecializationType (DTST).

The DTST and the DependentTemplateName are refactored to share
most of their implementation.

That refactoring along with the removal of the TypeSpecWithTemplate kind
from the nested name specifiers amounts to a large amount of code
removed, making this patch a small performance improvement overall.

This will also enable future further simplifications.

As a drive-by, this removes some special cases from the type printer,
allowing template arguments within NestedNameSpecifiers to not have
their nested names suppressed when printing.

Author: mizvekov

clang-tools-extra/clangd/AST.cpp

@@ -119,8 +119,7 @@ getQualification(ASTContext &Context, const DeclContext *DestContext,
       // There can't be any more tag parents after hitting a namespace.
       assert(!ReachedNS);
       (void)ReachedNS;
-      NNS = NestedNameSpecifier::Create(Context, nullptr, false,
-                                        TD->getTypeForDecl());
+      NNS = NestedNameSpecifier::Create(Context, nullptr, TD->getTypeForDecl());
     } else if (auto *NSD = llvm::dyn_cast<NamespaceDecl>(CurContext)) {
       ReachedNS = true;
       NNS = NestedNameSpecifier::Create(Context, nullptr, NSD);

clang-tools-extra/clangd/CodeComplete.cpp

@@ -1467,7 +1467,6 @@ bool allowIndex(CodeCompletionContext &CC) {
     return true;
   case NestedNameSpecifier::Super:
   case NestedNameSpecifier::TypeSpec:
-  case NestedNameSpecifier::TypeSpecWithTemplate:
   // Unresolved inside a template.
   case NestedNameSpecifier::Identifier:
     return false;

clang-tools-extra/clangd/DumpAST.cpp

@@ -157,7 +157,6 @@ class DumpVisitor : public RecursiveASTVisitor<DumpVisitor> {
       NNS_KIND(Identifier);
       NNS_KIND(Namespace);
       NNS_KIND(TypeSpec);
-      NNS_KIND(TypeSpecWithTemplate);
       NNS_KIND(Global);
       NNS_KIND(Super);
       NNS_KIND(NamespaceAlias);

clang-tools-extra/clangd/FindTarget.cpp

@@ -500,7 +500,6 @@ struct TargetFinder {
       }
       return;
     case NestedNameSpecifier::TypeSpec:
-    case NestedNameSpecifier::TypeSpecWithTemplate:
       add(QualType(NNS->getAsType(), 0), Flags);
       return;
     case NestedNameSpecifier::Global:

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

@@ -144,7 +144,6 @@ class ASTWalker : public RecursiveASTVisitor<ASTWalker> {
     case NestedNameSpecifier::Global:
       return true;
     case NestedNameSpecifier::TypeSpec:
-    case NestedNameSpecifier::TypeSpecWithTemplate:
     case NestedNameSpecifier::Super:
     case NestedNameSpecifier::Identifier:
       return false;

clang/docs/ReleaseNotes.rst

@@ -275,6 +275,10 @@ Improvements to Clang's diagnostics
 - Diagnostics on chained comparisons (``a < b < c``) are now an error by default. This can be disabled with
   ``-Wno-error=parentheses``.
 - Clang now better preserves the sugared types of pointers to member.
+- Clang now better preserves the presence of the template keyword with dependent
+  prefixes.
+- When printing types for diagnostics, clang now doesn't suppress the scopes of
+  template arguments contained within nested names.
 - The ``-Wshift-bool`` warning has been added to warn about shifting a boolean. (#GH28334)
 - Fixed diagnostics adding a trailing ``::`` when printing some source code
   constructs, like base classes.

clang/include/clang/AST/ASTContext.h

@@ -1835,15 +1835,14 @@ class ASTContext : public RefCountedBase<ASTContext> {
                              TagDecl *OwnedTagDecl = nullptr) const;
   QualType getDependentNameType(ElaboratedTypeKeyword Keyword,
                                 NestedNameSpecifier *NNS,
-                                const IdentifierInfo *Name,
-                                QualType Canon = QualType()) const;
+                                const IdentifierInfo *Name) const;
 
   QualType getDependentTemplateSpecializationType(
-      ElaboratedTypeKeyword Keyword, NestedNameSpecifier *NNS,
-      const IdentifierInfo *Name, ArrayRef<TemplateArgumentLoc> Args) const;
+      ElaboratedTypeKeyword Keyword, const DependentTemplateStorage &Name,
+      ArrayRef<TemplateArgumentLoc> Args) const;
   QualType getDependentTemplateSpecializationType(
-      ElaboratedTypeKeyword Keyword, NestedNameSpecifier *NNS,
-      const IdentifierInfo *Name, ArrayRef<TemplateArgument> Args) const;
+      ElaboratedTypeKeyword Keyword, const DependentTemplateStorage &Name,
+      ArrayRef<TemplateArgument> Args, bool IsCanonical = false) const;
 
   TemplateArgument getInjectedTemplateArg(NamedDecl *ParamDecl) const;
 
@@ -2392,10 +2391,8 @@ class ASTContext : public RefCountedBase<ASTContext> {
                                         bool TemplateKeyword,
                                         TemplateName Template) const;
 
-  TemplateName getDependentTemplateName(NestedNameSpecifier *NNS,
-                                        const IdentifierInfo *Name) const;
-  TemplateName getDependentTemplateName(NestedNameSpecifier *NNS,
-                                        OverloadedOperatorKind Operator) const;
+  TemplateName
+  getDependentTemplateName(const DependentTemplateStorage &Name) const;
   TemplateName getSubstTemplateTemplateParm(TemplateName replacement,
                                             Decl *AssociatedDecl,
                                             unsigned Index,

clang/include/clang/AST/ASTImporter.h

@@ -446,6 +446,14 @@ class TypeSourceInfo;
     /// returns nullptr only if the FromId was nullptr.
     IdentifierInfo *Import(const IdentifierInfo *FromId);
 
+    /// Import the given identifier or overloaded operator from the "from"
+    /// context into the "to" context.
+    ///
+    /// \returns The equivalent identifier or overloaded operator in the "to"
+    /// context.
+    IdentifierOrOverloadedOperator
+    Import(IdentifierOrOverloadedOperator FromIO);
+
     /// Import the given Objective-C selector from the "from"
     /// context into the "to" context.
     ///

clang/include/clang/AST/ASTNodeTraverser.h

@@ -396,8 +396,7 @@ class ASTNodeTraverser
     // FIXME: Provide a NestedNameSpecifier visitor.
     NestedNameSpecifier *Qualifier = T->getQualifier();
     if (NestedNameSpecifier::SpecifierKind K = Qualifier->getKind();
-        K == NestedNameSpecifier::TypeSpec ||
-        K == NestedNameSpecifier::TypeSpecWithTemplate)
+        K == NestedNameSpecifier::TypeSpec)
       Visit(Qualifier->getAsType());
     if (T->isSugared())
       Visit(T->getMostRecentCXXRecordDecl()->getTypeForDecl());

clang/include/clang/AST/AbstractBasicReader.h

@@ -279,10 +279,8 @@ class DataStreamBasicReader : public BasicReaderBase<Impl> {
         continue;
 
       case NestedNameSpecifier::TypeSpec:
-      case NestedNameSpecifier::TypeSpecWithTemplate:
         cur = NestedNameSpecifier::Create(ctx, cur,
-                          kind == NestedNameSpecifier::TypeSpecWithTemplate,
-                          asImpl().readQualType().getTypePtr());
+                                          asImpl().readQualType().getTypePtr());
         continue;
 
       case NestedNameSpecifier::Global: