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Subject: [Bug c++/99599] [11/12/13/14 Regression] Concepts requirement
 falsely reporting cyclic dependency, breaks tag_invoke pattern
Date: Fri, 08 Sep 2023 16:03:27 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D99599

--- Comment #18 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:2154bcd6d43cfd821ca70e1583880c4ed955355d

commit r14-3809-g2154bcd6d43cfd821ca70e1583880c4ed955355d
Author: Patrick Palka <ppalka@redhat.com>
Date:   Fri Sep 8 12:02:20 2023 -0400

    c++: refine CWG 2369 satisfaction vs non-dep convs [PR99599]

    As described in detail in the PR, the CWG 2369 resolution has the
    surprising consequence of introducing constraint recursion in seemingly
    valid and innocent code.

    This patch attempts to fix this surpising behavior for the majority of
    problematic cases.  Rather than checking satisfaction before _all_
    non-dependent conversions, as specified by the CWG resolution, this pat=
ch
    makes us first check "safe" non-dependent conversions, then satisfactio=
n,
    then followed by other non-dependent conversions.  A conversion is
    considered "safe" if computing it is guaranteed to not induce template
    instantiation, and we conservatively determine this by checking for
    user-declared constructors (resp. conversion functions) in the parm
    (resp. arg) class type, roughly.

            PR c++/99599

    gcc/cp/ChangeLog:

            * pt.cc (check_non_deducible_conversions): Add bool parameter
            passed down to check_non_deducible_conversion.
            (fn_type_unification): Call check_non_deducible_conversions
            an extra time before satisfaction with noninst_only_p=3Dtrue.
            (conversion_may_instantiate_p): Define.
            (check_non_deducible_conversion): Add bool parameter controlling
            whether to compute only conversions that are guaranteed to
            not induce template instantiation.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp2a/concepts-recursive-sat4.C: Make 'Int' non-aggreg=
ate
            in order to preserve intent of the testcase.
            * g++.dg/cpp2a/concepts-nondep4.C: New test.=