[Bug c++/91592] `__is_assignable` fails for private assignment operators in certain contexts

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91592

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Using std::is_assignable<B&, B const&> instead works fine.