Is it safe to use an enum in a bit field?

It’s allowed in all C++ compilers, that supports standard.

C++03 standard 9.6/3

A bit-field shall have integral or enumeration type (3.9.1). It is
implementation-defined whether a plain (neither explicitly signed nor unsigned) char, short, int or
long bit-field is signed or unsigned.

C++03 standard 9.6/4

If the value of an enu-
merator is stored into a bit-field of the same enumeration type and the number of bits in the bit-field is large
enough to hold all the values of that enumeration type, the original enumerator value and the value of the bit-field shall compare equal.

example

enum BOOL { f=0, t=1 };

struct A {
    BOOL b:1;
};

void f() {
    A a;
    a.b = t;
    a.b == t // shall yield true
}

But you can’t consider that enum has unsigned underlying type.

C++03 standard 7.2/5

The underlying type of an enumeration is an integral type that can represent all the enumerator values
defined in the enumeration. It is implementation-defined which integral type is used as the underlying type
for an enumeration except that the underlying type shall not be larger than int unless the value of an enu-
merator cannot fit in an int or unsigned int