https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70217
--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> --- dup of PR 70176 ? My guess is Cygwin uses a new version of newlib which has stricter C headers that don't define the C99 functions for __cplusplus < 201103L && __STRICT_ANSI__
