[Bug libstdc++/89510] [9 Regression] new_allocator::construct needs to be constrained

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89510

--- Comment #4 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-9 branch has been updated by Jonathan Wakely
<redi@gcc.gnu.org>:

https://gcc.gnu.org/g:7f84245366067259bc5604f5abf8b1e0c8e16b71

commit r9-8845-g7f84245366067259bc5604f5abf8b1e0c8e16b71
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Apr 30 15:47:52 2020 +0100

    libstdc++: Avoid errors in allocator's noexcept-specifier (PR 89510)

    This fixes a regression due to the conditional noexcept-specifier on
    std::allocator::construct and std::allocator::destroy, as well as the
    corresponding members of new_allocator, malloc_allocator, and
    allocator_traits. Those noexcept-specifiers were using expressions which
    might be ill-formed, which caused errors outside the immediate context
    when checking for the presence of construct and destroy in SFINAE
    contexts.

    The fix is to use the is_nothrow_constructible and
    is_nothrow_destructible type traits instead, because those traits are
    safe to use even when the construction/destruction itself is not valid.

    The is_nothrow_constructible trait will be false for a type that is not
    also nothrow-destructible, even if the new-expression used in the
    construct function body is actually noexcept. That's not the correct
    answer, but isn't a problem because providing a noexcept-specifier on
    these functions is not required by the standard anyway. If the answer is
    false when it should be true, that's suboptimal but OK (unlike giving
    errors for valid code, or giving a true answer when it should be false).

            PR libstdc++/89510
            * include/bits/alloc_traits.h (allocator_traits::_S_construct)
            (allocator_traits::_S_destroy)
            (allocator_traits<allocator<T>>::construct): Use traits in
            noexcept-specifiers.
            * include/bits/allocator.h (allocator<void>::construct)
            (allocator<void>::destroy): Likewise.
            * include/ext/malloc_allocator.h (malloc_allocator::construct)
            (malloc_allocator::destroy): Likewise.
            * include/ext/new_allocator.h (new_allocator::construct)
            (new_allocator::destroy): Likewise.
            * testsuite/20_util/allocator/89510.cc: New test.
            * testsuite/ext/malloc_allocator/89510.cc: New test.
            * testsuite/ext/new_allocator/89510.cc: New test.

    (cherry picked from commit b1983f4582bbe060b7da83578acb9ed653681fc8)