enumerate(Union[Sequence, Sequence])

• Are you reporting a bug, or opening a feature request?
  Bug
• Please insert below the code you are checking with mypy.

from typing import Sequence, Union

def baz(idx: int, arg: Union[int, str]) -> None:
    pass

def foo(the_list: Union[Sequence[int], Sequence[str]]) -> None:
    for idx, val in enumerate(the_list):
        reveal_type(idx)
        reveal_type(val)
        baz(idx, val)

• What is the actual behavior/output?

foo.py:8: note: Revealed type is 'builtins.int'
foo.py:9: note: Revealed type is 'builtins.object*'
foo.py:10: error: Argument 2 to "baz" has incompatible type "object"; expected "Union[int, str]"

• What is the behavior/output you expect?

foo.py:8: note: Revealed type is 'builtins.int'
foo.py:9: note: Revealed type is 'Union[int, str]'

• What are the versions of mypy and Python you are using?
  Do you see the same issue after installing mypy from Git master?
  0.761, 0.770 and master all have the same result.
• What are the mypy flags you are using? (For example --strict-optional)
  --strict

foo.py の例を mypy と報告されている --strict フラグで再現し、その後、シーケンスの Union に対する enumerate の型推論パスを調べます。reveal_type(val) が Union[int, str] を報告し、関連しないチェックを弱めることなく baz の呼び出しが受け入れられれば完了です。