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

[Jakub Jelinek]

Hi!

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.

Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?

2024-02-16  Jakub Jelinek  <ja...@redhat.com>

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

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

--- gcc/cp/parser.cc.jj 2024-02-15 17:33:11.641453437 +0100
+++ gcc/cp/parser.cc    2024-02-15 17:40:29.592447265 +0100
@@ -25724,8 +25724,15 @@ cp_parser_parameter_declaration (cp_pars
      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;
+    }
 
   bool diag_xobj_parameter_pack = false;
   if (xobj_param_p && (declarator && declarator->parameter_pack_p))
--- gcc/testsuite/g++.dg/parse/pr113929.C.jj    2024-02-15 17:43:18.500129688 
+0100
+++ gcc/testsuite/g++.dg/parse/pr113929.C       2024-02-15 17:42:54.564458109 
+0100
@@ -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 {};

        Jakub