https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88731
--- Comment #2 from joseph at codesourcery dot com <joseph at codesourcery dot com> --- GCC makes the integer type of the bit-field in question "unsigned:4". See DR#315 (also, see the line of C90 DRs that led to the wording changes in C99 relating to types of bit-fields, references in <https://gcc.gnu.org/ml/gcc/2017-10/msg00192.html>).
