Type Inference with for loop incorrect

[alanhdu]

Using mypy 0.761 on Python 3.6, the following code, running mypy --warn-unreachable on the following code fails:

from typing import Sequence, List


def remove_duplicates(xs: Sequence[int]) -> Sequence[int]:
    output: List[int] = []

    prev = None
    for x in xs:
        reveal_type(prev)

        if prev == x:
            continue
        elif prev is not None and prev != 999:
            output.append(prev)
        prev = x

        reveal_type(prev)
    reveal_type(prev)
    return output

with

$ mypy --warn-unreachable scratch.py 
scratch.py:9: note: Revealed type is 'None'
scratch.py:13: error: Right operand of 'and' is never evaluated
scratch.py:14: error: Statement is unreachable
scratch.py:17: note: Revealed type is 'builtins.int*'
scratch.py:18: note: Revealed type is 'Union[builtins.int*, None]'

This is clearly false -- it is possible for the prev != 999 to actually run. The reveal_types indicate that this has something to do with type inference: although mypy successfully infers that prev is Optional[int] after the for loop, it does not combine the int and None types during the for loop.

[msullivan]

Yeah this is a bad one. Duplicate of #5423. Thanks for the report!