[Bug c++/103879] error: accessing value of variant::_Copy_ctor_base through a 'const variant<string>' glvalue in a constant expression

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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103879

--- Comment #11 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:ab36b554bd90e8db279d13b133369118814f13fb

commit r12-6425-gab36b554bd90e8db279d13b133369118814f13fb
Author: Patrick Palka <ppalka@redhat.com>
Date:   Mon Jan 10 14:57:54 2022 -0500

    c++: constexpr base-to-derived conversion with offset 0 [PR103879]

    r12-136 made us canonicalize an object/offset pair with negative offset
    into one with a nonnegative offset, by iteratively absorbing the
    innermost component into the offset and stopping as soon as the offset
    becomes nonnegative.

    This patch strengthens this transformation by making it keep on absorbing
    even if the offset is already 0 as long as the innermost component is at
    position 0 (and thus absorbing doesn't change the offset).  This lets us
    accept the two constexpr testcases below, which we'd previously reject
    essentially because cxx_fold_indirect_ref would be unable to resolve
    *(B*)&b.D123 (where D123 is the base A subobject at position 0) to just b.

            PR c++/103879

    gcc/cp/ChangeLog:

            * constexpr.c (cxx_fold_indirect_ref): Split out object/offset
            canonicalization step into a local lambda.  Strengthen it to
            absorb more components at position 0.  Use it before both calls
            to cxx_fold_indirect_ref_1.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp1y/constexpr-base2.C: New test.
            * g++.dg/cpp1y/constexpr-base2a.C: New test.