Since there are no official fwd headers, we're quite lost here.

Probably the only thing one might then resort to is consuming a definitely most official/central other provider of such fwd header implementations - such as https://github.com/Philip-Trettner/cpp-std-fwd (but as its README prominently states, this firmly is UB land - and of course consuming one old version of that project - and not keeping it updated!? - is far less reliable still than always having supplied directly matching fwd headers to official STL headers, with your compiler installation footprint).

That's why IMHO it is very important that API providers do also supply official/central fwd.h headers for their (changing!?) types. Since it is not the consumer's job to be doing dangerous guesswork.