Allow yield to return type by convention

[NN---]

Currently yield requires you to return IEnumerable or IEnumerator.
It is not possible to return a derived Interface or return my own type.

Use case,
interface IRefEnumerable { ref T Current{get;} bool MoveNext(); }

I could yield return with this type if it was allowed.

[YairHalberstadt]

How would the compiler know how to generate code for these types?