[Bug c++/113929] New: GCC accepts template<this auto>

[Bug c++/113929] New: GCC accepts template<this auto>

[hewillk at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113929

            Bug ID: 113929
           Summary: GCC accepts template<this auto>
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: hewillk at gmail dot com
  Target Milestone: ---

This is the sibling of Bug 113788.

template<this auto> struct S {} ;
S<0> s{};

https://godbolt.org/z/GbdqW9zee

Thank you.

[Bug c++/113929] GCC accepts template<this auto>

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113929

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jakub at gcc dot gnu.org

--- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
This is IMHO different, because the grammar is:
template-parameter:
  type-parameter
  parameter-declaration 
parameter-declaration:
  attribute-specifier-seq[opt] this[opt] decl-specifier-seq declarator
  attribute-specifier-seq[opt] decl-specifier-seq declarator =
initializer-clause
  attribute-specifier-seq[opt] this[opt] decl-specifier-seq
abstract-declarator[opt]
  attribute-specifier-seq[opt] decl-specifier-seq abstract-declarator[opt] =
initializer-clause 
So, unlike the other cases, the grammar allows this specifier in there, just
https://eel.is/c++draft/dcl.fct#6 says it is invalid, because it is not the
first parameter-declaration of a parameter-declaration-list (because it is a
parameter-declaration of a template-parameter-list instead).

I'd go with (on top of PR113802 fix):
2024-02-15  Jakub Jelinek  <jakub@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 {};

[Bug c++/113929] GCC accepts template<this auto>

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113929

--- Comment #2 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jakub Jelinek <jakub@gcc.gnu.org>:

https://gcc.gnu.org/g:5286b0761b5dfac4348d1c5bfdcc162a66f338ee

commit r14-9033-g5286b0761b5dfac4348d1c5bfdcc162a66f338ee
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Fri Feb 16 17:42:32 2024 +0100

    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  <jakub@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.

[Bug c++/113929] GCC accepts template<this auto>

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113929

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |FIXED

--- Comment #3 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Fixed.