EDIT3: It's not std::unordered_map that's changed, it's std::pair. I suspect it's this commit that made this sort of incomplete container possible (yay concepts!), but would love for someone who speaks stdlib to confirm.

Original Post:

This compiles on GCC trunk:

#include <unordered_map>

struct Incomplete;

std::unordered_map<int, Incomplete> map;

struct Incomplete {};

int main() {};

Was this an intentional change or just a collateral bonus? It makes implementing certain types of recursive types convenient, and it's been true (though obviously not standards conforming) for std::map for awhile. Just wondering if there was some impetus for this change.

EDIT: To be clear, this is only supported on trunk. I'm not literally asking "since when?" I'm asking "How? And why now?"

EDIT2: This supported on clang trunk (and only clang trunk) as well, though that makes sense since it's probably a libstdc change not a compiler one.