https://gcc.gnu.org/bugzilla/show_bug.cgi?id=125371
--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> --- It *might* break some code which is relying on the forward declarations without explicitly including <iosfwd>, but we could document that in the gcc-17/porting_to.html notes. We've broken worse before ;-) I think we might be missing some tests to verify that <iostream> does include <ios> and therefore declares all the streambuf and stream types. Likewise for <bitset> which is required to include <iosfwd>. We do include it in <bitset>, but I don't think we have tests to verify that.
