Incompatible types in assignment for `dict[key] = tmp = value` and nested structures

I have Python2 code building some nested structures like lists/tuples/sets/dicts in dicts. For populating the nested structure I keep a temporary reference, which mypy fails to parse if the (untyped) temporary reference is not the first variable being assigned:

#!/usr/bin/python2.7
from typing import Dict, Set  # noqa
x = {}  # type: Dict[str, Set[str]]
z = x[''] = set()  # okay
x[''] = y = set()  # fail: Incompatible types in assignment (expression has type "Set[<nothing>]", target has type "Set[str]")

Start with the minimal Python 2.7 reproduction shown in the issue and compare the two chained-assignment forms. Trace how mypy infers the untyped temporary in each case; done means the second form is accepted consistently with the first without weakening the declared Dict[str, Set[str]] checking.