I just came across issue 1204 -- Global permission to move, which seems to address the concern raised by the example in c++std-lib-25030.

IIUC, the example violates the permission to assume that arguments bound to rvalue references are unnamed temporaries granted to implementations by the resolution of issue 1204 - Global permission to move.

I.e., the ostringstream(ostringstream &&rhs) ctor can leave the rhs pointers pointing to the newly constructed object's buffer just as long as the dtor doesn't change or invalidate the buffer. The caller may not make any assumptions about rhs after the move beyond it being safe to destroy or reassign.

So unless I misunderstood something, I still think the basic_stringbuf move ctor is overspecified. Specifically, I think the third sentence in the Effects clause and the last 6 bullets in the Postconditions clause can, and IMO should, be stricken.