[clang] require arg list in type specifiers using template kw

[evelez7]

Require a template argument list after a name prefixed by the template keyword in nested name specifiers. Addresses CWG 96 which was superseded by P1787.

Followup to #80801.

[llvmbot]

@llvm/pr-subscribers-clang

Author: Erick Velez (evelez7)

Changes

Require a template argument list after a name prefixed by the template keyword in nested name specifiers. Addresses [CWG 96](https://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#96) which was superseded by P1787.

Followup to #80801.

---

Full diff: https://github.com/llvm/llvm-project/pull/94674.diff

2 Files Affected:

• (modified) clang/lib/Parse/Parser.cpp (+11-1)
• (modified) clang/test/Parser/cxx2a-concepts-requires-expr.cpp (+1-1)

diff --git a/clang/lib/Parse/Parser.cpp b/clang/lib/Parse/Parser.cpp
index 6d0cf7b174e50..71b87147e9a5f 100644
--- a/clang/lib/Parse/Parser.cpp
+++ b/clang/lib/Parse/Parser.cpp
@@ -2060,9 +2060,19 @@ bool Parser::TryAnnotateTypeOrScopeToken(
       return true;
     }
 
+    bool TemplateKWPresent = false;
+    if (Tok.is(tok::kw_template)) {
+      ConsumeToken();
+      TemplateKWPresent = true;
+    }
+
     TypeResult Ty;
     if (Tok.is(tok::identifier)) {
-      // FIXME: check whether the next token is '<', first!
+      if (TemplateKWPresent && NextToken().isNot(tok::less)) {
+        Diag(Tok.getLocation(),
+             diag::missing_template_arg_list_after_template_kw);
+        return true;
+      }
       Ty = Actions.ActOnTypenameType(getCurScope(), TypenameLoc, SS,
                                      *Tok.getIdentifierInfo(),
                                      Tok.getLocation());
diff --git a/clang/test/Parser/cxx2a-concepts-requires-expr.cpp b/clang/test/Parser/cxx2a-concepts-requires-expr.cpp
index 971591afb08db..5755844a323d2 100644
--- a/clang/test/Parser/cxx2a-concepts-requires-expr.cpp
+++ b/clang/test/Parser/cxx2a-concepts-requires-expr.cpp
@@ -83,7 +83,7 @@ bool r23 = requires { typename identity<T>::temp<T>; };
 template<typename T>
 bool r24 = requires {
     typename identity<T>::template temp<T>;
-    typename identity<T>::template temp; // expected-error{{expected an identifier or template-id after '::'}}
+    typename identity<T>::template temp; // expected-error{{template argument list is expected after a name prefixed by the template keyword}}
 };
 
 bool r25 = requires { ; };

[evelez7]

I'm also preparing a patch for deprecating the use of template kw without an arg list for alias and class templates. Decided to split that up since a lot of tests require deprecation messages.

[AaronBallman]

LGTM -- no release note needed because this is a follow-up to work that already has a release note.