[Bug c++/105268] New: type/value mismatch when using variadic concept

[Bug c++/105268] New: type/value mismatch when using variadic concept

[barry.revzin at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105268

            Bug ID: 105268
           Summary: type/value mismatch when using variadic concept
           Product: gcc
           Version: 11.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: barry.revzin at gmail dot com
  Target Milestone: ---

>From StackOverflow (https://stackoverflow.com/q/71864544/2069064):

template<typename> concept C_one = true;
template<typename...> concept C_many = true;

template<bool B> struct S { };

template<typename T = S<C_one<int>>> void f();  // ok
template<typename T = S<C_many<int>>> void g(); // error

gcc rejects the declaration of g with:


<source>:7:35: error: type/value mismatch at argument 1 in template parameter
list for 'template<bool B> struct S'
    7 | template<typename T = S<C_many<int>>> void g();
      |                                   ^~
<source>:7:35: note:   expected a constant of type 'bool', got
'<template-parameter-1-1>'

But C_many<int> is a bool, so this should be fine. And template-parameter-1-1
isn't a very useful diagnostic anyway.

[Bug c++/105268] type/value mismatch when using variadic concept

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105268

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |rejects-valid
                 CC|                            |mpolacek at gcc dot gnu.org
   Last reconfirmed|                            |2022-04-13
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1

--- Comment #1 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Confirmed.

[Bug c++/105268] type/value mismatch when using variadic concept

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105268

--- Comment #2 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Looks like we mistakenly parse C_many<int> as a placeholder-type-specifier, and
things go wrong from there.

For C_one<int> cp_parser_placeholder_type_specifier -> finish_type_constraints
-> build_type_constraint -> build_concept_check -> build_standard_check ->
coerce_template_parms -> coerce_template_parms fails here:

 8916   nargs = inner_args ? NUM_TMPL_ARGS (inner_args) : 0;
 8917   if ((nargs - variadic_args_p > nparms && !variadic_p)
 8918       || (nargs < nparms - variadic_p
 8919           && require_all_args
 8920           && !variadic_args_p
 8921           && (!use_default_args
 8922               || (TREE_VEC_ELT (parms, nargs) != error_mark_node
 8923                   && !TREE_PURPOSE (TREE_VEC_ELT (parms, nargs))))))
 8924     {
 8925     bad_nargs:

but for C_many<int> variadic_p is true so we don't return error_mark_node but
<type_argument_pack>.

[Bug c++/105268] type/value mismatch when using variadic concept

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105268

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Assignee|unassigned at gcc dot gnu.org      |mpolacek at gcc dot gnu.org
             Status|NEW                         |ASSIGNED

[Bug c++/105268] type/value mismatch when using variadic concept

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105268

--- Comment #3 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The trunk branch has been updated by Marek Polacek <mpolacek@gcc.gnu.org>:

https://gcc.gnu.org/g:ccae4443c8322d9d82a19a1e7c689413a122a478

commit r12-8176-gccae4443c8322d9d82a19a1e7c689413a122a478
Author: Marek Polacek <polacek@redhat.com>
Date:   Thu Apr 14 13:48:15 2022 -0400

    c++: wrong error with variadic concept [PR105268]

    Here we issue a wrong error for the

      template<typename T = S<C_many<int>>> void g();

    line in the testcase.  I surmise that's because we mistakenly parse
    C_many<int> as a placeholder-type-specifier, and things go wrong from
    there.  We are in a default argument so we should reject parsing
C_many<int>
    as a placeholder-type-specifier, which would mean creating a new parameter.
    We want C_many<int> to be a concept-id instead.

    It's interesting to see why the same problem didn't occur for C_one<int>.
    In that case, cp_parser_placeholder_type_specifier ->
finish_type_constraints
    -> build_type_constraint -> build_concept_check -> build_standard_check ->
    coerce_template_parms fails the parse here:

     8916   nargs = inner_args ? NUM_TMPL_ARGS (inner_args) : 0;
     8917   if ((nargs - variadic_args_p > nparms && !variadic_p)
     8918       || (nargs < nparms - variadic_p
     8919           && require_all_args
     8920           && !variadic_args_p
     8921           && (!use_default_args
     8922               || (TREE_VEC_ELT (parms, nargs) != error_mark_node
     8923                   && !TREE_PURPOSE (TREE_VEC_ELT (parms, nargs))))))
     8924     {
     8925     bad_nargs:
     ...
     8943       return error_mark_node;

    because nargs is 2 (the targs are <WILDCARD_DECL, int>) while nparms is
    1 (for the one 'typename' in the tparam list of C_one).  But for
    C_many<int> variadic_p is true so we don't return error_mark_node but
    <type_argument_pack>.

    This patch does not issue any error for the !tentative case because
    I didn't figure out how to trigger that.  So it adds an assert instead.

            PR c++/105268

    gcc/cp/ChangeLog:

            * parser.cc (cp_parser_placeholder_type_specifier): Return
            error_mark_node when trying to build up a constrained parameter in
            a default argument.

    gcc/testsuite/ChangeLog:

            * g++.dg/concepts/variadic6.C: New test.

[Bug c++/105268] type/value mismatch when using variadic concept

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105268

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

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

--- Comment #4 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Fixed for GCC 12.