Well I misremembered it seems. You can use forward declared enum class or enum : <type> in C++ as per standard:

   enum class A; // defaults to int
   enum B :int;
   enum C;
   fooA(A);
   FooB(B);
   FooC(C);

   fooA((A)0);
   fooB((B)0);
   fooC((C)0); // error

but indeed GCC refuses plain enum. Forward declaring enums in C is an old GCC extension that still deosn't allow to pass them when incomplete;

Enums in GCC do always default to int or unsigned int, unless -fshot-enums is used which is ABI breaking.

Probably I was misremembering this combination of the int ABI and the forward declaring extension.