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Subject: [Bug c++/102137] class template argument deduction with template
 template parameter allows explicit deduction guide
Date: Wed, 09 Mar 2022 13:44:18 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102137

--- Comment #3 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:e32869a17b788bee9ca782b174a546b1db17b5ea

commit r12-7563-ge32869a17b788bee9ca782b174a546b1db17b5ea
Author: Patrick Palka <ppalka@redhat.com>
Date:   Wed Mar 9 08:42:32 2022 -0500

    c++: detecting copy-init context during CTAD [PR102137]

    Here we're failing to communicate to cp_finish_decl from tsubst_expr
    that we're in a copy-initialization context (via the LOOKUP_ONLYCONVERT=
ING
    flag), which causes us to always consider explicit deduction guides when
    performing CTAD for a templated variable initializer.

    It turns out this bug also affects consideration of explicit conversion
    operators for the same reason.  But consideration of explicit construct=
ors
    seems unaffacted thanks to code in build_aggr_init that sets
    LOOKUP_ONLYCONVERTING when the initializer represents copy-initializati=
on.

    So this patch fixes this by making cp_finish_decl set LOOKUP_ONLYCONVER=
TING
    just like build_aggr_init does, by inspecting the initializer, so that
    callers don't need to explicitly pass this flag appropriately.

            PR c++/102137
            PR c++/87820

    gcc/cp/ChangeLog:

            * cp-tree.h (is_copy_initialization): Declare.
            * decl.cc (cp_finish_decl): Set LOOKUP_ONLYCONVERTING
            when is_copy_initialization is true.
            * init.cc (build_aggr_init): Split out copy-initialization
            check into ...
            (is_copy_initialization): ... here.
            * pt.cc (instantiate_decl): Pass 0 instead of
            LOOKUP_ONLYCONVERTING as flags to cp_finish_decl.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp0x/explicit15.C: New test.
            * g++.dg/cpp1z/class-deduction108.C: New test.=