Strict optional: Any narrowing to None in a conditional gets "stuck" at None #3338
Description
I think a code sample will explain:
from typing import Any, Optional
def narrow_any_to_none() -> None:
"""Broken case: If 'v' starts with type Any, then it gets "stuck" with type
None after the if."""
v = 1 # type: Any
if v is None:
reveal_type(v) # E: Revealed type is 'builtins.None'
v = 2
reveal_type(v) # E: Revealed type is 'builtins.None'
v + 3 # E: Unsupported operand types for + (None and "int")
def narrow_optional_to_none() -> None:
"""Working as expected: If 'v' starts with an Optional type, then you can
re-assign it no problem after the if."""
v = 1 # type: Optional[int]
if v is None:
reveal_type(v) # E: Revealed type is 'builtins.None'
v = 2
reveal_type(v) # E: Revealed type is 'builtins.int'
v + 3
In v0.510 and on the latest master, mypy --strict-optional on the above code produces the above errors. I expected the 2nd reveal_type to reveal 'builtins.int' and for the v + 3 line to pass type checking. (Without strict optional, everything works as expected.)