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

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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.