[Bug libstdc++/92770] std::unordered_map requires both T and U to be fully declared

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92770

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
The relevant changes were r12-4258-g64acc43de1e336 and r12-4259-gd87105d697ced1