[Bug libstdc++/87704] [6/7/8/9 Regression] unique_ptr(nullptr_t) requires T to be complete

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87704

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
C++17 doesn't have the defect, it appears to have been fixed as an unintended
consequence of https://wg21.link/lwg2801