[Bug libstdc++/112591] variant allows for creating multiple empty objects at same address

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112591

--- Comment #8 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Tomasz Kaminski <tkaminsk@gcc.gnu.org>:

https://gcc.gnu.org/g:09bece00d0ec981d1b0b47ba93a67a745de3f062

commit r16-5961-g09bece00d0ec981d1b0b47ba93a67a745de3f062
Author: Tomasz KamiÅski <tkaminsk@redhat.com>
Date:   Mon Dec 8 08:49:45 2025 +0100

    libstdc++: Refactor _Variadic_union so _Unitialized<T, false> is not needed
[PR112591].

    The changes the _Variadic_union implementation, in a way that the
    _Unitialized<T, false> partial specialization for non-trivial types is not
    necessary.

    This is simply done by separating the specialization for
__trivially_destructible
    being true and false, and for the later defining an empty destructor
(similarly
    as it was done using concepts).

    We also reduce the number of specialization of _Variadic_union, so
specialization
    (int, int) is reused by (string, int, int) and (int, int). This is done by
    initialization __trivially_destructible with conjunction of
    is_trivially_destructible_v for remaining components. This is only
necessary
    for non-trivial (false) specialization, as if both _First and _Rest... are
    trivially destructible, then _Rest must also be.

    The above change does not regress the fix r14-7259-g2d55d94e5df389 for
    template depth, and both before and after the change template depth is 266.
    I have added dg-options to the 87619.cc to catch future regressions.

    This also add test for PR112591.

            PR libstdc++/112591

    libstdc++-v3/ChangeLog:

            * include/std/variant (_Variadic_union): Separate specializations
for
            for union of only trivially destructible types (true as first
template
            argument). Unconditionally define destructor for
_Variadic_union<false,
            _First, _Rest...>.
            * testsuite/20_util/variant/87619.cc: Add limit for the template
depth.
            * testsuite/20_util/variant/112591.cc: New test.

    Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
    Signed-off-by: Tomasz KamiÅski <tkaminsk@redhat.com>