[s28l]

I don't think that's true. Neither C nor C++ would allow you to invoke a function with an incomplete type, regardless of whether it is a `struct` or an `enum`, so there's no ABI issues with forward declaring them, but you wouldn't be able to define or invoke a function that takes an incomplete type argument by value.

    struct A;
    enum B; // as you pointed out, not allowed by the C++ standard

    // fine to declare, define, and invoke
    void fooA(struct A*);
    void fooB(enum B*);

    // ok to forward declare, but you can't call them
    void barA(struct A);
    void barB(enum B);

[gpderetta]

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.