Nested Function incorrect type inference

[alanhdu]

Running mypy --strict-optional on the following code:

from typing import Optional, Sequence

def f(timestamps: Optional[Sequence[float]] = None):
    if timestamps is None:
        timestamps = [0]

    reveal_type(timestamps)
    def index(i: int) -> float:
        return timestamps[i]
    return index(0)

fails with

test.py:7: error: Revealed type is 'typing.Sequence[builtins.float]'
test.py:9: error: Value of type "Optional[Sequence[float]]" is not indexable

This is pretty weird -- MyPy knows that timestamps isn't None at that point, but complains in the nested function.

[emmatyping]

It seems type narrowing is not being propagated to closures, but only in function defs (I checked the above with a lamba and the narrowing is propagated). The narrowing stays after the function definition, just not within.

[ilevkivskyi]

This is a duplicate of #2608