[clang][sema] Provide better diagnostic for missing template arguments

Instead of just complaining that "x is not a class, namespace or
enumeration", mention that using x requires template arguments.

Differential Revision: https://reviews.llvm.org/D127638

Fixes https://github.com/llvm/llvm-project/issues/55962

clang/docs/ReleaseNotes.rst

@@ -264,7 +264,9 @@ Improvements to Clang's diagnostics
 - ``-Wshift-overflow`` will not warn for signed left shifts in C++20 mode
   (and newer), as it will always wrap and never overflow. This fixes
   `Issue 52873 <https://github.com/llvm/llvm-project/issues/52873>`_.
-
+- When using class templates without arguments, clang now tells developers
+  that template arguments are missing in certain contexts.
+  This fixes `Issue 55962 <https://github.com/llvm/llvm-project/issues/55962>`_.
 
 Non-comprehensive list of changes in this release
 -------------------------------------------------

clang/lib/Sema/SemaCXXScopeSpec.cpp

@@ -828,10 +828,14 @@ bool Sema::BuildCXXNestedNameSpecifier(Scope *S, NestedNameSpecInfo &IdInfo,
   }
 
   if (!Found.empty()) {
-    if (TypeDecl *TD = Found.getAsSingle<TypeDecl>())
+    if (TypeDecl *TD = Found.getAsSingle<TypeDecl>()) {
       Diag(IdInfo.IdentifierLoc, diag::err_expected_class_or_namespace)
           << Context.getTypeDeclType(TD) << getLangOpts().CPlusPlus;
-    else {
+    } else if (Found.getAsSingle<TemplateDecl>()) {
+      ParsedType SuggestedType;
+      DiagnoseUnknownTypeName(IdInfo.Identifier, IdInfo.IdentifierLoc, S, &SS,
+                              SuggestedType);
+    } else {
       Diag(IdInfo.IdentifierLoc, diag::err_expected_class_or_namespace)
           << IdInfo.Identifier << getLangOpts().CPlusPlus;
       if (NamedDecl *ND = Found.getAsSingle<NamedDecl>())

clang/test/SemaCXX/nested-name-spec.cpp

@@ -473,3 +473,10 @@ namespace DependentTemplateInTrivialNNSLoc {
     x: goto x;
   }
 }
+
+template <typename T>
+struct x; // expected-note {{template is declared here}}
+
+template <typename T>
+int issue55962 = x::a; // expected-error {{use of class template 'x' requires template arguments}} \
+                       // expected-warning {{variable templates are a C++14 extension}}