[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 #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Intentionally, but it's not guaranteed to keeps working in future. The standard
still says it's undefined.