propagate_const and forward declaration

Instantiating a standard library template with an incomplete type is generally prohibited.

std::map is not an exception to that rule.

Querying decltype(*std::declval<_Tp&>()) with _Tp = std::unique_ptr<std::map<int, FWD>> necessitates the instantiation of all associated classes of _Tp to look for potential friend operator* declarations.

Among those associated classes is std::map<int, FWD>.

Instantiation of std::map<int, FWD> invokes undefined behavior.