[Bug libstdc++/77395] [6/7 Regression] std::is_constructible is false for type constructible via implicit conversion operator affecting std::tuple

[ville.voutilainen at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77395

--- Comment #6 from Ville Voutilainen <ville.voutilainen at gmail dot com> ---
Fixed on trunk, backporting to the gcc-6 branch.