Why must an enumeration's size be provided when it is forward declared?

I just can't see why the size of the enumeration is relevant for the compiler while the size of the class is not.

My code example:

class A;
enum E;   // must be enum E : int; in order to compile 

void f(const A & param);
void f(const E & param);

I'm talking here of standard C++ compilers. I know that MSVC let it compile and works fine. So the question is:

Why this has not been standardized?