c - What is the type of a bitfield?

Question

I can't find anywhere in the C standard where this is specified. For example, in

struct { signed int x:1; } foo;

is foo.x an lvalue of type int, or something else? It seems unnatural for it to be an lvalue of type int since you cannot store any value of type int in it, only 0 or -1, but I can't find any language that would assign it a different type. Of course, used in most expressions, it would get promoted to int anyway, but the actual type makes a difference in C11 with _Generic, and I can't find any language in the standard about how bitfields interact with _Generic either.

Answer 1

As Jonathan already cited, p5 clearly states what the type a bit-field has.

What you should have also in mind is that there is a special rule for bit-field arithmetic conversions in 6.3.1.1, basically stating that if an int can represent all values such a bit-field converts to an int in most expressions.

What the type would be in a _Generic should be the declared type (modulo the sign glitch), since it seems to be consensus that arithmetic conversions don't apply, there. So

_Generic((X), int: toto, unsigned: tutu)
_Generic(+(X), int: toto, unsigned: tutu)

could give you different results if X is an unsigned bit-field with a width that has all values fit into an int.