Unnecessary Cloning Operations

[DavePearce]

The following generates a clone unexpectedly:

function f(null|int x) -> (int r):
    if x is int:
        return x
    else:
        return 0

This generates the following JavaScript:

function f_u2NI(/*(null|int)*/ x)// -> (/*int*/ r)
{
    // function or method body
    if(typeof x === "number") {
            return Wy.copy(x);
        } else {
            return 0;
        }
}

Obviously, it's strange that we have Wy.copy(x) here. I'm guessing the reason for this is that isCopyable() doesn't think about union types properly.