Compiler doesn't allow class Member to be a specific alternative subtype

[braxtonmckee]

For instance,

T = Alternative("T", X=dict(x=int), Y=dict(y=float))

class C(Class):
    anX = Member(T.X)

    def __init__(self):
        self.anX = T.X()

Doesn't work, both because the call to T.X() knows the result as T, not T.X (which is just imprecise) and because we don't have appropriate converters set up.