Block incorrectly treated as unreachable when strict optional disabled

[JukkaL]

This program doesn't generate an error when using --no-strict-optional, even though every line is reachable:

from typing import NamedTuple, Dict

class C(NamedTuple):
    a: int
    b: str

x: Dict[int, C]
y = x.get(int())
if not y:
    1 + ''  # No error (considered unreachable)

Apparently mypy thinks that the tuple object is always truthy, even though it could also be None when strict optional checking is disabled.

[ilevkivskyi]

@JukkaL isn't this essentially a duplicate of #3601?

[JukkaL]

Yeah, looks like a duplicate.