Conditional expression with branches of type None and T should be Optional[T], not object

[gvanrossum]

Consider:

def f(flag; bool) -> Optional[int]:
  a = (None if flag else 0)
  return a

When run with --strict-optional I get the error

error: Incompatible return value type (got "object", expected "Optional[int]")

It looks like the unification code is not understanding what's going on and falling back on object.

[ddfisher]

Related to #1863. This is due to mypy's reluctance to make Union types.

[gnprice]

As discussed under #1863, we should go ahead and become willing to make Union types when one of the types to be combined is None. We might go back later and become less reluctant to make Union types more broadly, but there's more detailed discussion to be had there (see #1863.)