Ambiguous Coercions

[DavePearce]

The compiler currently detects ambiguous coercions, such as the following:

type R1 is {real x, int y}
type R2 is {int x, real y}
type R3 is {int x, int y}

method f(R3 x) -> (R1|R2):
    return (R1|R2) x

However, I am concerned as to whether the algorithm is actually complete or not. For example, is it complete in the presence of recursive types?

This issue is relevant to the question as to whether or not to keep real types (#495).

[DavePearce]

Since #495 is now implemented, this is resolved.