As pointed out in P2641R1, we can use GCC's __builtin_constant_p to
emulate the proposed std::is_active_member. This allows us to detect
which of the union members is active during constant evaluation, so we
don't need the extra bool data member. We still can't support constexpr
until C++20 though, as we need to change the active member during
constant evaluation.
libstdc++-v3/ChangeLog:
* include/experimental/internet (ip::basic_endpoint::_M_if_v6):
Revert change from member function to data member. Fix for
constant evaluation by detecting which union member is active.
(ip::basic_endpoint::resize): Revert changes to update _M_is_v6
flag.
Jonathan Wakely
10e573e86c