[Bug libstdc++/94033] [10 Regression] is_trivially_copy_constructible<> fails with compiler error on complicated object with private default constructor

["cvs-commit at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94033

--- Comment #21 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-8 branch has been updated by Jonathan Wakely
<redi@gcc.gnu.org>:

https://gcc.gnu.org/g:1521433e2b6bff583b227fe391fe9dcf02f6000e

commit r8-10445-g1521433e2b6bff583b227fe391fe9dcf02f6000e
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Mar 18 23:19:12 2020 +0000

    libstdc++: Fix is_trivially_constructible (PR 94033)

    This attempts to make is_nothrow_constructible more robust (and
    efficient to compile) by not depending on is_constructible. Instead the
    __is_constructible intrinsic is used directly. The helper class
    __is_nt_constructible_impl which checks whether the construction is
    non-throwing now takes a bool template parameter that is substituted by
    the result of the instrinsic. This fixes the reported bug by not using
    the already-instantiated (and incorrect) value of std::is_constructible.
    I don't think it really fixes the problem in general, because
    std::is_nothrow_constructible itself could already have been
    instantiated in a context where it gives the wrong result. A proper fix
    needs to be done in the compiler.

    Backported to the gcc-8 and gcc-9 branches to fix PR 96999.

            PR libstdc++/94033
            * include/std/type_traits (__is_nt_default_constructible_atom):
Remove.
            (__is_nt_default_constructible_impl): Remove.
            (__is_nothrow_default_constructible_impl): Remove.
            (__is_nt_constructible_impl): Add bool template parameter. Adjust
            partial specializations.
            (__is_nothrow_constructible_impl): Replace class template with
alias
            template.
            (is_nothrow_default_constructible): Derive from alias template
            __is_nothrow_constructible_impl instead of
            __is_nothrow_default_constructible_impl.
            * testsuite/20_util/is_nothrow_constructible/94003.cc: New test.

    (cherry picked from commit b3341826531e80e02f194460b4fbe1b0541c0463)