Allow enums in interface

[axic]

I believe this was raised previously a couple of times and it is handled as part of the inheritance discussion (#3729).

I'd however propose to make an exception and allow this simple change outside of concluding that discussion, right now.

Are there any reasons for disallowing it? I remember we disallowed it out of fear something might break (or rather lack of time to check everything) and probably enums weren't supported to be used from a foreign contract making it less useful.

However, as of today this should be working (granted bool TypeChecker::visit(EnumDefinition const& _enum) is removed in the type checker):

  enum X { A, B, C }
  function x() returns (I.X);
}

contract C is I {
  function x() returns (I.X) { return f(); }
  function f() returns (I.X) { return X.B; }
}

It also seems the output has the proper enum values after compilation.

[chriseth]

I think it is fine to allow both enums and structs in interfaces.

[chriseth]

I was never aware of any problem, we just wanted to restrict interfaces as much as possible.

[axic]

I think it was rather lack of time to properly evaluate the implications and to get a basic version of interfaces out.

[chriseth]

We should also allow structs.

[axic]

I am working on this now.

[chriseth]

Documentation change is missing.