The C standard does not specify whether the underlying type of an enum
is signed or unsigned, and until C23 there was no way to control this
explicitly. GCC appears to make enums unsigned unless there is a
negative value among cases of the enum, in which case it becomes signed.
MSCV appears to make enums signed by default.

A bitfield of an enum type (which is not specificied in the C standard
either) behaves as if it was an instance of a numeric type with a
reduced value range. Specifically, a 'signed int val : 2;' bitfield will
have the possible values of -2, -1, 0, and 1, with the usual wraparound
behavior for the values that don't fit (although this too is
implementation-defined).

This causes the following issue, if we have:

typedef enum
{
  GTK_ZERO,
  GTK_ONE,
  GTK_TWO
} GtkFoo;

struct _GtkBar
{
  GtkFoo foo : 2;
};

and then assign bar.foo = GTK_TWO and read it back, it will have the
expected value of 2 (aka GTK_TWO) on GCC, but a value of -2 (not
matching any of t...