https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83294
--- Comment #6 from joseph at codesourcery dot com <joseph at codesourcery dot com> --- The response to C99 DR#315 says that for all the types not specifying "signed" or "unsigned" explicitly, if an implementation accepts them as bit-field types it's implementation-defined what the signedness is. That's compatible with C++ up to C++11 (C++14 removed the special-casing allowing plain int etc. bit-fields to be unsigned). C11 footnote 125 explicitly refers to "typedef-name defined as int", so the intent is clear even if that's not explicit in normative text.
