[clang] Template Diagnostic Improvements (PR #99933)

[Braden Helmer via cfe-commits]

https://github.com/bradenhelmer created https://github.com/llvm/llvm-project/pull/99933

This is for #17959.

 It turns out `SemaTemplate` handles this type of diagnostic already, however when  template gets encountered, it never gets parsed as a possible statement or declaration, only as an expression. 

Would like some feedback as I am unsure if this is the right fix. Thanks

>From 1aaa6ff0f1843f10606064f6b285c7070aaa5b44 Mon Sep 17 00:00:00 2001
From: Braden Helmer <bradenhelmeraus at gmail.com>
Date: Mon, 22 Jul 2024 16:46:04 -0400
Subject: [PATCH] Fix template error message

---
 clang/lib/Parse/ParseStmt.cpp           | 9 +++++++++
 clang/test/Parser/cxx-template-decl.cpp | 4 ++++
 2 files changed, 13 insertions(+)

diff --git a/clang/lib/Parse/ParseStmt.cpp b/clang/lib/Parse/ParseStmt.cpp
index 22d38adc28ebe..bdb3fc051d0b3 100644
--- a/clang/lib/Parse/ParseStmt.cpp
+++ b/clang/lib/Parse/ParseStmt.cpp
@@ -299,6 +299,15 @@ StmtResult Parser::ParseStatementOrDeclarationAfterAttributes(
     goto Retry;
   }
 
+  case tok::kw_template: {
+    SourceLocation DeclEnd;
+    ParsedAttributes Attrs(AttrFactory);
+    ParseTemplateDeclarationOrSpecialization(DeclaratorContext::Block, DeclEnd,
+                                             Attrs,
+                                             getAccessSpecifierIfPresent());
+    return StmtError();
+  }
+
   case tok::kw_case:                // C99 6.8.1: labeled-statement
     return ParseCaseStatement(StmtCtx);
   case tok::kw_default:             // C99 6.8.1: labeled-statement
diff --git a/clang/test/Parser/cxx-template-decl.cpp b/clang/test/Parser/cxx-template-decl.cpp
index 734438069b9ae..5a2d1cec9ad31 100644
--- a/clang/test/Parser/cxx-template-decl.cpp
+++ b/clang/test/Parser/cxx-template-decl.cpp
@@ -297,3 +297,7 @@ namespace PR46231 {
   template<> int; // expected-error {{declaration does not declare anything}}
   template<int> int; // expected-error {{declaration does not declare anything}}
 }
+
+namespace NoTemplateInBlockScope {
+  void foo() { template <typename> int i; } // expected-error {{templates can only be declared in namespace or class scope}}
+}