[Clang][Sema] Correctly look up primary template for variable template specializations (#80359)

Consider the following:
```
namespace N0 {
  namespace N1 {
    template<typename T>
    int x1 = 0;
  }
  using namespace N1;
}
template<>
int N0::x1<int>;
```

According to [dcl.meaning.general] p3.3:
> - If the _declarator_ declares an explicit instantiation or a partial
or explicit specialization, the _declarator_ does not bind a name. If it
declares a class member, the terminal name of the _declarator-id_ is not
looked up; otherwise, **only those lookup results that are nominable in
`S` are considered when identifying any function template specialization
being declared**.

In particular, the requirement for lookup results to be nominal in the
lookup context of the terminal name of the _declarator-id_ only applies
to function template specializations -- not variable template
specializations. We currently reject the above declaration, but we do
(correctly) accept it if the using-directive is replaced with a `using`
declaration naming `N0::N1::x1`. This patch makes it so the above
specialization is (correctly) accepted.

1 files changed, 6 insertions, 5 deletions

diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index fd1c470..de6067b 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -7727,7 +7727,7 @@ NamedDecl *Sema::ActOnVariableDeclarator(
               ? TemplateParamLists[0]->getTemplateLoc()
               : SourceLocation();
       DeclResult Res = ActOnVarTemplateSpecialization(
-          S, D, TInfo, TemplateKWLoc, TemplateParams, SC,
+          S, D, TInfo, Previous, TemplateKWLoc, TemplateParams, SC,
           IsPartialSpecialization);
       if (Res.isInvalid())
         return nullptr;
@@ -8070,8 +8070,8 @@ NamedDecl *Sema::ActOnVariableDeclarator(
     D.setRedeclaration(CheckVariableDeclaration(NewVD, Previous));
   } else {
     // If this is an explicit specialization of a static data member, check it.
-    if (IsMemberSpecialization && !NewVD->isInvalidDecl() &&
-        CheckMemberSpecialization(NewVD, Previous))
+    if (IsMemberSpecialization && !IsVariableTemplateSpecialization &&
+        !NewVD->isInvalidDecl() && CheckMemberSpecialization(NewVD, Previous))
       NewVD->setInvalidDecl();
 
     // Merge the decl with the existing one if appropriate.
@@ -8086,7 +8086,8 @@ NamedDecl *Sema::ActOnVariableDeclarator(
         Previous.clear();
         NewVD->setInvalidDecl();
       }
-    } else if (D.getCXXScopeSpec().isSet()) {
+    } else if (D.getCXXScopeSpec().isSet() &&
+               !IsVariableTemplateSpecialization) {
       // No previous declaration in the qualifying scope.
       Diag(D.getIdentifierLoc(), diag::err_no_member)
         << Name << computeDeclContext(D.getCXXScopeSpec(), true)
@@ -8094,7 +8095,7 @@ NamedDecl *Sema::ActOnVariableDeclarator(
       NewVD->setInvalidDecl();
     }
 
-    if (!IsVariableTemplateSpecialization && !IsPlaceholderVariable)
+    if (!IsPlaceholderVariable)
       D.setRedeclaration(CheckVariableDeclaration(NewVD, Previous));
 
     // CheckVariableDeclaration will set NewVD as invalid if something is in