[Bug libstdc++/87431] valueless_by_exception() should unconditionally return false if all the constructors are noexcept

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87431

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Author: redi
Date: Tue Sep 25 14:59:16 2018
New Revision: 264574

URL: https://gcc.gnu.org/viewcvs?rev=264574&root=gcc&view=rev
Log:
PR libstdc++/87431 optimise valueless_by_exception()

If a std::variant can never get into valueless state then we don't need
to do a runtime check for a valid alternative.

        PR libstdc++/87431
        * include/std/variant (_Variant_storage<true, _Types...>::_M_valid):
        Avoid runtime test when all alternatives are scalars and so cannot
        throw during initialization.

Modified:
    trunk/libstdc++-v3/ChangeLog
    trunk/libstdc++-v3/include/std/variant