original.name="Contractive_Valid_2"
whiley.compile.ignore=true
WhileyCompiler.issue=1003
======
>>> main.whiley
type C1 is null | bool
type C2 is null | bool
type C3 is C1 | C2

method f(C1 x) -> C3:
    return x

method g(C2 x) -> C3:
    return x

method h(C3 x) -> C1:
    return x

method i(C3 x) -> C2:
    return x

public export method test() :
    C3 x = f(null)
    assume x == null

---