[Bug c++/94062] Cannot construct tuple from convertible types

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94062

--- Comment #16 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jonathan Wakely <redi@gcc.gnu.org>:

https://gcc.gnu.org/g:91e6226f880b048275a7ceedef716e159c7cefd9

commit r11-2720-g91e6226f880b048275a7ceedef716e159c7cefd9
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Fri Aug 7 17:13:56 2020 +0100

    libstdc++: Remove inheritance from elements in std::tuple

    This fixes a number of std::tuple bugs by no longer making use of the
    empty base-class optimization. By using the C++20 [[no_unique_address]]
    attribute we can always store the element as a data member, while still
    compressing the layout of tuples containing empty types.

    Since we no longer use inheritance we could also apply the compression
    optimization for final types and for tuples of tuples, but doing so
    would be an ABI break.

    Using [[no_unique_address]] more liberally for the unstable std::__8
    configuration is left for a later date. There may be reasons not to
    apply the attribute unconditionally, e.g. see the discussion about
    guaranteed elision in PR 94062.

    libstdc++-v3/ChangeLog:

            PR libstdc++/55713
            PR libstdc++/71096
            PR libstdc++/93147
            * include/std/tuple [__has_cpp_attribute(no_unique_address)]
            (_Head_base<Idx, Head, true>): New definition of the partial
            specialization, using [[no_unique_address]] instead of
            inheritance.
            * testsuite/libstdc++-prettyprinters/48362.cc: Adjust expected
            output.
            * testsuite/20_util/tuple/comparison_operators/93147.cc: New test.
            * testsuite/20_util/tuple/creation_functions/55713.cc: New test.
            * testsuite/20_util/tuple/element_access/71096.cc: New test.