Inner functions lose type context

[psteele-wayfair]

MyPy can lose type information in an inner function deduced in an outer function.

Consider the following sample code:

from typing import Callable, Optional


def f(x: Optional[int]) -> Callable[[], int]:
    if x is None:
        x = 0
    reveal_type(x)

    def g() -> int:
        reveal_type(x)
        return x + 1

    return g

I would expect that MyPy accept this code without error, and for both reveal_type calls to return builtins.int. Running python -m mypy minimal.py we get

minimal.py:7: error: Revealed type is 'builtins.int'
minimal.py:10: error: Revealed type is 'Union[builtins.int, None]'
minimal.py:11: error: Unsupported operand types for + ("None" and "int")
minimal.py:11: note: Left operand is of type "Optional[int]"

The first reveal_type shows MyPy has correctly narrowed Optional[int] to int. However, inside the inner function we see that MyPy forgets this narrowing.

This was run using Python 3.6.0 and MyPy 0.660. The error persisted when running MyPy from the current master branch, currently hash f03ad7094e7a9bd174011e2ae3bc0c7204c95e22.

[ilevkivskyi]

Duplicate of #2608