Proposed resolution:

Change [thread.req.exception] p.2:

-2- The error_category (19.5.1.1) of the error_code reported by such an exception's code() member function is as specified in the error condition Clause. The error_code reported by such an exception's code() member function shall compare equal to one of the conditions specified in the function's error condition Clause. [Example: When the thread constructor fails:

ec.category() == implementation-defined // probably system_category
ec == errc::resource_unavailable_try_again // holds true

— end example]