[gcc r16-9508] testsuite: Clarify the std module export plugin diagnostics [PR125200]

https://gcc.gnu.org/g:742ca4610d51e156c287daebc8e293f155725c0b

commit r16-9508-g742ca4610d51e156c287daebc8e293f155725c0b
Author: Jakub Jelinek <[email protected]>
Date:   Thu Aug 6 11:03:43 2026 +0200

    testsuite: Clarify the std module export plugin diagnostics [PR125200]
    
    The following patch attempts to clarify the diagnostics, so that one e.g.
    doesn't look for import std::whatever; in the headers or something similar.
    
    2026-08-06  Jakub Jelinek  <[email protected]>
    
            PR libstdc++/125200
            * g++.dg/plugin/std_module_exports_plugin.cc (plugin_dump_decl): Use
            %< and %>.  Clarify the error message and add a note.
    
    Reviewed-by: Jason Merrill <[email protected]>
    (cherry picked from commit 19a304d3680a1aeeb8ca412a6fe108a3f3b9d305)

Diff:
---
 gcc/testsuite/g++.dg/plugin/std_module_exports_plugin.cc | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/gcc/testsuite/g++.dg/plugin/std_module_exports_plugin.cc b/gcc/testsuite/g++.dg/plugin/std_module_exports_plugin.cc
index e81eb6d3fb6c..21a765041f6e 100644
--- a/gcc/testsuite/g++.dg/plugin/std_module_exports_plugin.cc
+++ b/gcc/testsuite/g++.dg/plugin/std_module_exports_plugin.cc
@@ -106,13 +106,19 @@ plugin_dump_decl (tree decl, char *scope, hash_set<tree> **exported_usings)
 	if (((int) whitelist[i].enabled_in & (int) this_std) != 0)
 	  {
 	    inform (DECL_SOURCE_LOCATION (decl),
-		    "missing using %s%D; whitelisted", scope, name);
+		    "missing %<using %s%D;%> whitelisted", scope, name);
 	    return;
 	  }
 	break;
       }
 
-  error_at (DECL_SOURCE_LOCATION (decl), "missing using %s%D;", scope, name);
+  auto_diagnostic_group d;
+  error_at (DECL_SOURCE_LOCATION (decl),
+	    "missing %<using %s%D;%> in libstdc++-v3/src/c++23/std*.cc.in",
+	    scope, name);
+  inform (DECL_SOURCE_LOCATION (decl),
+	  "%<%s%D%> found in std namespace but not exported from std module",
+	  scope, name);
 }
 
 void