c++: diagnose this specifier in requires expr [PR116798]

We don't detect an explicit object parameter in a requires expression.
We can get there by way of requires-expression -> requirement-parameter-list
-> parameter-declaration-clause -> ... -> parameter-declaration with
this[opt].  But [dcl.fct]/5 doesn't allow an explicit object parameter
in this context.  So let's fix it like r14-9033 and not like r14-8832.

	PR c++/116798

gcc/cp/ChangeLog:

	* parser.cc (cp_parser_parameter_declaration): Detect an explicit
	object parameter in a requires expression.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp23/explicit-obj-diagnostics12.C: New test.

Reviewed-by: Jason Merrill <jason@redhat.com>

1 files changed, 8 insertions, 3 deletions

diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
index 4dd9474..dbc6070 100644
--- a/gcc/cp/parser.cc
+++ b/gcc/cp/parser.cc
@@ -25982,10 +25982,15 @@ cp_parser_parameter_declaration (cp_parser *parser,
 
   bool xobj_param_p
     = decl_spec_seq_has_spec_p (&decl_specifiers, ds_this);
-  if (xobj_param_p && template_parm_p)
+  if (xobj_param_p
+      && (template_parm_p || current_binding_level->requires_expression))
     {
-      error_at (decl_specifiers.locations[ds_this],
-		"%<this%> specifier in template parameter declaration");
+      if (template_parm_p)
+	error_at (decl_specifiers.locations[ds_this],
+		  "%<this%> specifier in template parameter declaration");
+      else
+	error_at (decl_specifiers.locations[ds_this],
+		  "%<this%> specifier in a requires-expression parameter");
       xobj_param_p = false;
       decl_specifiers.locations[ds_this] = 0;
     }