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Subject: [Bug c++/101174] [12 Regression] CTAD causes instantiation of
 invalid class specialization since r12-926
Date: Wed, 23 Jun 2021 21:27:20 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D101174

--- 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:7da4eae3dcef6fd5d955eb2c80c453aa52368004

commit r12-1762-g7da4eae3dcef6fd5d955eb2c80c453aa52368004
Author: Patrick Palka <ppalka@redhat.com>
Date:   Wed Jun 23 17:23:39 2021 -0400

    c++: excessive instantiation during CTAD [PR101174]

    We set DECL_CONTEXT on implicitly generated deduction guides so that
    their access is consistent with that of the constructor.  But this
    apparently leads to excessive instantiation in some cases, ultimately
    because instantiation of a deduction guide should be independent of
    instantiation of the resulting class specialization, but setting the
    DECL_CONTEXT of the former to the latter breaks this independence.

    To fix this, this patch makes push_access_scope handle artificial
    deduction guides specifically rather than setting their DECL_CONTEXT
    in build_deduction_guide.  We could alternatively make the class
    befriend the guide via DECL_BEFRIENDING_CLASSES, but that wouldn't
    be a complete fix and would break class-deduction-access3.C below
    since friendship isn't transitive.

            PR c++/101174

    gcc/cp/ChangeLog:

            * pt.c (push_access_scope): For artificial deduction guides,
            set the access scope to that of the constructor.
            (pop_access_scope): Likewise.
            (build_deduction_guide): Don't set DECL_CONTEXT on the guide.

    libstdc++-v3/ChangeLog:

            * testsuite/23_containers/multiset/cons/deduction.cc:
            Uncomment CTAD example that was rejected by this bug.
            * testsuite/23_containers/set/cons/deduction.cc: Likewise.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp1z/class-deduction-access3.C: New test.
            * g++.dg/cpp1z/class-deduction91.C: New test.=