Infer Union for ternary expressions

[smallnamespace]

Ternary expressions currently infer to type object, when they should at least infer a Union of both branch's types (if not more, in the case where the predicate is known to be constant).

from typing import Union

u = 0 if True else ''  # type: Union[int, str]

currently results in

error: Incompatible types in assignment (expression has type "object", variable has type "Union[int, str]")

[gvanrossum]

Indeed! This is a duplicate of #622.

[smallnamespace]

Thanks! I'd searched for existing issues, but wasn't aware that Python calls them conditional expressions.