c++: Diagnose this specifier on template parameters [PR113929]

For template parameters, the optional this specifier is in the grammar
template-parameter-list -> template-parameter -> parameter-declaration,
just [dcl.fct/6] says that it is only valid in parameter-list of certain
functions.  So, unlike the case of decl-specifier-seq used in non-terminals
other than parameter-declaration, I think it is better not to fix this
by
   cp_parser_decl_specifier_seq (parser,
-                                flags | CP_PARSER_FLAGS_PARAMETER,
+                                flags | (template_parameter_p ? 0
+                                         : CP_PARSER_FLAGS_PARAMETER),
                                 &decl_specifiers,
                                 &declares_class_or_enum);
which would be pretending it isn't in the grammar, but by diagnosing it
separately, which is what the following patch does.

2024-02-16  Jakub Jelinek  <[email protected]>

	PR c++/113929
	* parser.cc (cp_parser_parameter_declaration): Diagnose this specifier
	on template parameter declaration.

	* g++.dg/parse/pr113929.C: New test.

Author: jakubjelinek

gcc/cp/parser.cc

@@ -25724,8 +25724,15 @@ cp_parser_parameter_declaration (cp_parser *parser,
      for a C-style variadic function. */
   token = cp_lexer_peek_token (parser->lexer);
 
-  bool const xobj_param_p
+  bool xobj_param_p
     = decl_spec_seq_has_spec_p (&decl_specifiers, ds_this);
+  if (xobj_param_p && template_parm_p)
+    {
+      error_at (decl_specifiers.locations[ds_this],
+		"%<this%> specifier in template parameter declaration");
+      xobj_param_p = false;
+      decl_specifiers.locations[ds_this] = 0;
+    }
 
   if (xobj_param_p
       && ((declarator && declarator->parameter_pack_p)

gcc/testsuite/g++.dg/parse/pr113929.C

@@ -0,0 +1,7 @@
+// PR c++/113929
+// { dg-do compile }
+
+template <this int C>		// { dg-error "'this' specifier in template parameter declaration" }
+struct S {};
+template <int N, this int C>	// { dg-error "'this' specifier in template parameter declaration" }
+struct T {};