Monomorph confusion in function argument default values

[bendmorris]

If this is a code issue, provide a minimal code example:

struct MyStruct {
    var a: Int;
}

function f(x: Option[MyStruct] = None) {
    return x;
}

function main() {
    var s = f();
    if s.isNone() {
        puts("hi");
    }
}

This types correctly, but generates code for the wrong None monomorph.