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Subject: [Bug c++/102412] Template argument deduction fails when using
 concept as defaulted non-type template parameter
Date: Wed, 22 Sep 2021 15:18:11 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102412

--- Comment #2 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Patrick Palka <ppalka@gcc.gnu.org>:

https://gcc.gnu.org/g:9329344a6d81a6a5e3bd171167ebc7b158bb44f4

commit r12-3822-g9329344a6d81a6a5e3bd171167ebc7b158bb44f4
Author: Patrick Palka <ppalka@redhat.com>
Date:   Wed Sep 22 11:16:53 2021 -0400

    c++: concept-ids and value-dependence [PR102412]

    The problem here is that uses_template_parms returns true for all
    concept-ids (even those with non-dependent arguments), so when a concep=
t-id
    is used as a default template argument then during deduction the default
    argument is considered dependent even after substituting into it, which
    leads to deduction failure (from type_unification_real).

    This patch fixes this by implementing the resolution of CWG 2446 which
    says a concept-id is dependent only if its arguments are.

            DR 2446
            PR c++/102412

    gcc/cp/ChangeLog:

            * constexpr.c (cxx_eval_constant_expression)
            <case TEMPLATE_ID_EXPR>: Check value_dependent_expression_p
            instead of processing_template_decl.
            * pt.c (value_dependent_expression_p) <case TEMPLATE_ID_EXPR>:
            Return true only if any_dependent_template_arguments_p.
            (instantiation_dependent_r) <case CALL_EXPR>: Remove this case.
            <case TEMPLATE_ID_EXPR>: Likewise.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp2a/concepts-nondep2.C: New test.
            * g++.dg/cpp2a/concepts-nondep3.C: New test.=