From abf4a8cb15d4faf04ee0da14e37d7349d3bde9a1 Mon Sep 17 00:00:00 2001
From: Corentin Jabot <corentinjabot@gmail.com>
Date: Thu, 23 Mar 2023 14:26:45 +0100
Subject: [Clang] Improve diagnostics when using a concept as template argument

When using the name of a template variable or concept in places
where an expression was expected, Clang would drop the cxxscope token
preceeding it, if any.

This leads to subpar diagnostics - complaining about the
identifier being undeclared as clang would not know to look into a
non-global scope.

We make sure the scope is preserved.

When encountering `ns::Concept foo x;`, Clang would also fail
to provide the same quality as it does at global scope.

Reviewed By: aaron.ballman, erichkeane

Differential Revision: https://reviews.llvm.org/D146719
---
 clang/lib/Parse/Parser.cpp | 26 ++++++++++----------------
 1 file changed, 10 insertions(+), 16 deletions(-)

(limited to 'clang/lib/Parse/Parser.cpp')

diff --git a/clang/lib/Parse/Parser.cpp b/clang/lib/Parse/Parser.cpp
index e6b1d3f..c0f4556 100644
--- a/clang/lib/Parse/Parser.cpp
+++ b/clang/lib/Parse/Parser.cpp
@@ -1883,31 +1883,25 @@ Parser::TryAnnotateName(CorrectionCandidateCallback *CCC,
       return ANK_TemplateName;
     }
     [[fallthrough]];
+  case Sema::NC_Concept:
   case Sema::NC_VarTemplate:
   case Sema::NC_FunctionTemplate:
   case Sema::NC_UndeclaredTemplate: {
-    // We have a type, variable or function template followed by '<'.
-    ConsumeToken();
-    UnqualifiedId Id;
-    Id.setIdentifier(Name, NameLoc);
-    if (AnnotateTemplateIdToken(
-            TemplateTy::make(Classification.getTemplateName()),
-            Classification.getTemplateNameKind(), SS, SourceLocation(), Id))
-      return ANK_Error;
-    return ANK_Success;
-  }
-  case Sema::NC_Concept: {
-    UnqualifiedId Id;
-    Id.setIdentifier(Name, NameLoc);
+    bool IsConceptName = Classification.getKind() == Sema::NC_Concept;
+    // We have a template name followed by '<'. Consume the identifier token so
+    // we reach the '<' and annotate it.
     if (Next.is(tok::less))
-      // We have a concept name followed by '<'. Consume the identifier token so
-      // we reach the '<' and annotate it.
       ConsumeToken();
+    UnqualifiedId Id;
+    Id.setIdentifier(Name, NameLoc);
     if (AnnotateTemplateIdToken(
             TemplateTy::make(Classification.getTemplateName()),
             Classification.getTemplateNameKind(), SS, SourceLocation(), Id,
-            /*AllowTypeAnnotation=*/false, /*TypeConstraint=*/true))
+            /*AllowTypeAnnotation=*/!IsConceptName,
+            /*TypeConstraint=*/IsConceptName))
       return ANK_Error;
+    if (SS.isNotEmpty())
+      AnnotateScopeToken(SS, !WasScopeAnnotation);
     return ANK_Success;
   }
   }
-- 
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