[Clang][Sema] Diagnose use of template keyword after declarative nested-name-specifiers (#78595)

According to [temp.names] p5:
> The keyword template shall not appear immediately after a declarative nested-name-specifier.

[expr.prim.id.qual] p2 defines a declarative nested-name-specifier as follows:
> A nested-name-specifier is declarative if it is part of
> - a class-head-name,
> - an enum-head-name,
> - a qualified-id that is the id-expression of a declarator-id, or
> - a declarative nested-name-specifier.

Note: I believe this definition is defective as it doesn't include _nested-name-specifiers_ appearing in _elaborated-type-specifiers_ that declare partial/explicit specializations and explicit instantiations. See my post to the core reflector. Minus a few bugs that are addressed by this PR, this is how we implement it.

This means that declarations like:
```
template<typename>
struct A
{
    template<typename> 
    struct B
   {
        void f();
   };
};

template<typename T>
template<typename U>
void A<T>::template B<U>::f() { } // error: 'template' cannot be used after a declarative nested name specifier
```
are ill-formed. This PR add diagnostics for such declarations. The name of the diagnostic group is `template-in-declaration-name`.

Regarding the aforementioned "few bugs that are addressed by this PR" in order to correctly implement this:
- `CheckClassTemplate` did not call `diagnoseQualifiedDeclaration` when the semantic context was dependent. This allowed for constructs like:
```
struct A
{
    template<typename T>
    struct B
    {
        template<typename U>
        struct C;
    };
};

template<typename T>
template<typename U>
struct decltype(A())::B<T>::C { };
```
- `ActOnClassTemplateSpecialization` did not call `diagnoseQualifiedDeclaration` at all, allowing for qualified partial/explicit specializations at class scope and other related nonsense
- `TreeTransform::TransformNestedNameSpecifierLoc` would rebuild a `NestedNameSpecifier::TypeSpecWithTemplate` as a `NestedNameSpecifier::TypeSpec`
- `TemplateSpecializationTypeLoc::initializeLocal` would set the `template` keyword `SourceLocation` to the provided `Loc` parameter, which would result in a `TemplateSpecializationTypeLoc` obtained via `ASTContext::getTrivialTypeSourceInfo` being displayed as always having a `template` prefix (since the presence of the keyword is not stored anywhere else).

Author: sdkrystian

clang/docs/ReleaseNotes.rst

@@ -149,6 +149,7 @@ Improvements to Clang's diagnostics
   prints.
 
 - Clang now diagnoses member template declarations with multiple declarators.
+- Clang now diagnoses use of the ``template`` keyword after declarative nested name specifiers.
 
 Improvements to Clang's time-trace
 ----------------------------------

clang/include/clang/AST/TypeLoc.h

@@ -189,6 +189,9 @@ class TypeLoc {
   /// pointer types, but not through decltype or typedefs.
   AutoTypeLoc getContainedAutoTypeLoc() const;
 
+  /// Get the SourceLocation of the template keyword (if any).
+  SourceLocation getTemplateKeywordLoc() const;
+
   /// Initializes this to state that every location in this
   /// type is the given location.
   ///
@@ -1691,7 +1694,7 @@ class TemplateSpecializationTypeLoc :
   }
 
   void initializeLocal(ASTContext &Context, SourceLocation Loc) {
-    setTemplateKeywordLoc(Loc);
+    setTemplateKeywordLoc(SourceLocation());
     setTemplateNameLoc(Loc);
     setLAngleLoc(Loc);
     setRAngleLoc(Loc);

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -8247,6 +8247,9 @@ def err_invalid_declarator_in_block : Error<
   "definition or redeclaration of %0 not allowed inside a block">;
 def err_not_tag_in_scope : Error<
   "no %select{struct|interface|union|class|enum}0 named %1 in %2">;
+def ext_template_after_declarative_nns : ExtWarn<
+    "'template' cannot be used after a declarative nested name specifier">,
+    InGroup<DiagGroup<"template-in-declaration-name">>;
 
 def err_no_typeid_with_fno_rtti : Error<
   "use of typeid requires -frtti">;

clang/include/clang/Sema/Sema.h

@@ -2960,7 +2960,8 @@ class Sema final {
   bool DiagnoseClassNameShadow(DeclContext *DC, DeclarationNameInfo Info);
   bool diagnoseQualifiedDeclaration(CXXScopeSpec &SS, DeclContext *DC,
                                     DeclarationName Name, SourceLocation Loc,
-                                    bool IsTemplateId);
+                                    TemplateIdAnnotation *TemplateId,
+                                    bool IsMemberSpecialization);
   void
   diagnoseIgnoredQualifiers(unsigned DiagID, unsigned Quals,
                             SourceLocation FallbackLoc,

clang/lib/AST/TypeLoc.cpp

@@ -738,3 +738,12 @@ AutoTypeLoc TypeLoc::getContainedAutoTypeLoc() const {
     return AutoTypeLoc();
   return Res.getAs<AutoTypeLoc>();
 }
+
+SourceLocation TypeLoc::getTemplateKeywordLoc() const {
+  if (const auto TSTL = getAsAdjusted<TemplateSpecializationTypeLoc>())
+    return TSTL.getTemplateKeywordLoc();
+  if (const auto DTSTL =
+          getAsAdjusted<DependentTemplateSpecializationTypeLoc>())
+    return DTSTL.getTemplateKeywordLoc();
+  return SourceLocation();
+}

clang/lib/Parse/ParseDecl.cpp

@@ -6663,12 +6663,14 @@ void Parser::ParseDirectDeclarator(Declarator &D) {
       }
 
       bool HadScope = D.getCXXScopeSpec().isValid();
+      SourceLocation TemplateKWLoc;
       if (ParseUnqualifiedId(D.getCXXScopeSpec(),
                              /*ObjectType=*/nullptr,
                              /*ObjectHadErrors=*/false,
                              /*EnteringContext=*/true,
                              /*AllowDestructorName=*/true, AllowConstructorName,
-                             AllowDeductionGuide, nullptr, D.getName()) ||
+                             AllowDeductionGuide, &TemplateKWLoc,
+                             D.getName()) ||
           // Once we're past the identifier, if the scope was bad, mark the
           // whole declarator bad.
           D.getCXXScopeSpec().isInvalid()) {

clang/lib/Sema/SemaDecl.cpp

@@ -6196,13 +6196,17 @@ bool Sema::DiagnoseClassNameShadow(DeclContext *DC,
 ///
 /// \param Loc The location of the name of the entity being declared.
 ///
-/// \param IsTemplateId Whether the name is a (simple-)template-id, and thus
-/// we're declaring an explicit / partial specialization / instantiation.
+/// \param IsMemberSpecialization Whether we are declaring a member
+/// specialization.
+///
+/// \param TemplateId The template-id, if any.
 ///
 /// \returns true if we cannot safely recover from this error, false otherwise.
 bool Sema::diagnoseQualifiedDeclaration(CXXScopeSpec &SS, DeclContext *DC,
                                         DeclarationName Name,
-                                        SourceLocation Loc, bool IsTemplateId) {
+                                        SourceLocation Loc,
+                                        TemplateIdAnnotation *TemplateId,
+                                        bool IsMemberSpecialization) {
   DeclContext *Cur = CurContext;
   while (isa<LinkageSpecDecl>(Cur) || isa<CapturedDecl>(Cur))
     Cur = Cur->getParent();
@@ -6231,7 +6235,7 @@ bool Sema::diagnoseQualifiedDeclaration(CXXScopeSpec &SS, DeclContext *DC,
   // Check whether the qualifying scope encloses the scope of the original
   // declaration. For a template-id, we perform the checks in
   // CheckTemplateSpecializationScope.
-  if (!Cur->Encloses(DC) && !IsTemplateId) {
+  if (!Cur->Encloses(DC) && !(TemplateId || IsMemberSpecialization)) {
     if (Cur->isRecord())
       Diag(Loc, diag::err_member_qualification)
         << Name << SS.getRange();
@@ -6277,12 +6281,32 @@ bool Sema::diagnoseQualifiedDeclaration(CXXScopeSpec &SS, DeclContext *DC,
     return false;
   }
 
+  // C++23 [temp.names]p5:
+  //   The keyword template shall not appear immediately after a declarative
+  //   nested-name-specifier.
+  //
+  // First check the template-id (if any), and then check each component of the
+  // nested-name-specifier in reverse order.
+  //
+  // FIXME: nested-name-specifiers in friend declarations are declarative,
+  // but we don't call diagnoseQualifiedDeclaration for them. We should.
+  if (TemplateId && TemplateId->TemplateKWLoc.isValid())
+    Diag(Loc, diag::ext_template_after_declarative_nns)
+        << FixItHint::CreateRemoval(TemplateId->TemplateKWLoc);
+
+  NestedNameSpecifierLoc SpecLoc(SS.getScopeRep(), SS.location_data());
+  while (SpecLoc.getPrefix()) {
+    if (SpecLoc.getNestedNameSpecifier()->getKind() ==
+        NestedNameSpecifier::TypeSpecWithTemplate)
+      Diag(Loc, diag::ext_template_after_declarative_nns)
+          << FixItHint::CreateRemoval(
+                 SpecLoc.getTypeLoc().getTemplateKeywordLoc());
+
+    SpecLoc = SpecLoc.getPrefix();
+  }
   // C++11 [dcl.meaning]p1:
   //   [...] "The nested-name-specifier of the qualified declarator-id shall
   //   not begin with a decltype-specifer"
-  NestedNameSpecifierLoc SpecLoc(SS.getScopeRep(), SS.location_data());
-  while (SpecLoc.getPrefix())
-    SpecLoc = SpecLoc.getPrefix();
   if (isa_and_nonnull<DecltypeType>(
           SpecLoc.getNestedNameSpecifier()->getAsType()))
     Diag(Loc, diag::err_decltype_in_declarator)
@@ -6350,9 +6374,13 @@ NamedDecl *Sema::HandleDeclarator(Scope *S, Declarator &D,
       return nullptr;
     }
     if (!D.getDeclSpec().isFriendSpecified()) {
-      if (diagnoseQualifiedDeclaration(
-              D.getCXXScopeSpec(), DC, Name, D.getIdentifierLoc(),
-              D.getName().getKind() == UnqualifiedIdKind::IK_TemplateId)) {
+      TemplateIdAnnotation *TemplateId =
+          D.getName().getKind() == UnqualifiedIdKind::IK_TemplateId
+              ? D.getName().TemplateId
+              : nullptr;
+      if (diagnoseQualifiedDeclaration(D.getCXXScopeSpec(), DC, Name,
+                                       D.getIdentifierLoc(), TemplateId,
+                                       /*IsMemberSpecialization=*/false)) {
         if (DC->isRecord())
           return nullptr;
 
@@ -17957,6 +17985,7 @@ Sema::ActOnTag(Scope *S, unsigned TagSpec, TagUseKind TUK, SourceLocation KWLoc,
       // nested-name-specifier against the current context.
       if ((TUK == TUK_Definition || TUK == TUK_Declaration) &&
           diagnoseQualifiedDeclaration(SS, DC, OrigName, Loc,
+                                       /*TemplateId=*/nullptr,
                                        isMemberSpecialization))
         Invalid = true;
 

clang/lib/Sema/SemaDeclCXX.cpp

@@ -3621,14 +3621,18 @@ Sema::ActOnCXXMemberDeclarator(Scope *S, AccessSpecifier AS, Declarator &D,
       // class X {
       //   int X::member;
       // };
-      if (DeclContext *DC = computeDeclContext(SS, false))
+      if (DeclContext *DC = computeDeclContext(SS, false)) {
+        TemplateIdAnnotation *TemplateId =
+            D.getName().getKind() == UnqualifiedIdKind::IK_TemplateId
+                ? D.getName().TemplateId
+                : nullptr;
         diagnoseQualifiedDeclaration(SS, DC, Name, D.getIdentifierLoc(),
-                                     D.getName().getKind() ==
-                                         UnqualifiedIdKind::IK_TemplateId);
-      else
+                                     TemplateId,
+                                     /*IsMemberSpecialization=*/false);
+      } else {
         Diag(D.getIdentifierLoc(), diag::err_member_qualification)
           << Name << SS.getRange();
-
+      }
       SS.clear();
     }
 

clang/lib/Sema/SemaTemplate.cpp

@@ -1890,8 +1890,12 @@ DeclResult Sema::CheckClassTemplate(
       ContextRAII SavedContext(*this, SemanticContext);
       if (RebuildTemplateParamsInCurrentInstantiation(TemplateParams))
         Invalid = true;
-    } else if (TUK != TUK_Friend && TUK != TUK_Reference)
-      diagnoseQualifiedDeclaration(SS, SemanticContext, Name, NameLoc, false);
+    }
+
+    if (TUK != TUK_Friend && TUK != TUK_Reference)
+      diagnoseQualifiedDeclaration(SS, SemanticContext, Name, NameLoc,
+                                   /*TemplateId-*/ nullptr,
+                                   /*IsMemberSpecialization*/ false);
 
     LookupQualifiedName(Previous, SemanticContext);
   } else {
@@ -8826,6 +8830,15 @@ DeclResult Sema::ActOnClassTemplateSpecialization(
   bool isMemberSpecialization = false;
   bool isPartialSpecialization = false;
 
+  if (SS.isSet()) {
+    if (TUK != TUK_Reference && TUK != TUK_Friend &&
+        diagnoseQualifiedDeclaration(SS, ClassTemplate->getDeclContext(),
+                                     ClassTemplate->getDeclName(),
+                                     TemplateNameLoc, &TemplateId,
+                                     /*IsMemberSpecialization=*/false))
+      return true;
+  }
+
   // Check the validity of the template headers that introduce this
   // template.
   // FIXME: We probably shouldn't complain about these headers for

clang/lib/Sema/TreeTransform.h

@@ -4394,7 +4394,8 @@ NestedNameSpecifierLoc TreeTransform<Derived>::TransformNestedNameSpecifierLoc(
           SS.Adopt(ETL.getQualifierLoc());
           TL = ETL.getNamedTypeLoc();
         }
-        SS.Extend(SemaRef.Context, /*FIXME:*/ SourceLocation(), TL,
+
+        SS.Extend(SemaRef.Context, TL.getTemplateKeywordLoc(), TL,
                   Q.getLocalEndLoc());
         break;
       }