Failure to narrow down type of elements of Tuple

• Are you reporting a bug, or opening a feature request?
  kind of both

• Please insert below the code you are checking with mypy,

from typing import Optional, Tuple, TypeVar

_T = TypeVar("_T")
_U = TypeVar("_U")


def promote_optional(tup: Tuple[Optional[_T], Optional[_U]]) -> Optional[Tuple[_T, _U]]:
    for val in tup:
        if val is None:
            return None
    return tup

def promote_optional2(tup: Tuple[Optional[_T], Optional[_U]]) -> Optional[Tuple[_T, _U]]:
    if tup[0] is None:
        return None
    if tup[1] is None:
        return None
    return tup

• What is the actual behavior/output?

/tmp/mwe.py:11: error: Incompatible return value type (got "Tuple[Optional[_T], Optional[_U]]", expected "Optional[Tuple[_T, _U]]")
/tmp/mwe.py:19: error: Incompatible return value type (got "Tuple[Optional[_T], Optional[_U]]", expected "Optional[Tuple[_T, _U]]")
Found 2 errors in 1 file (checked 1 source file)

• What is the behavior/output you expect?
  Mypy should accept the code.

• What are the versions of mypy and Python you are using?
  mypy 0.782

I want to turn a Tuple[Optional...] to an Optional[Tuple], because the whole tuple is worthless if it contains any None. Unfortunately Mypy seems to be unable to detect that I checked all elements of the tuple for being none.

/tmp/mwe.py の最小再現から始め、mypy に対して実行して現在のエラーを確認します。None チェック後に mypy がタプル要素の型をどのように絞り込むかを追跡します; 両方の promote_optional 関数が、互換性のない戻り値型のエラーなしで受け入れられれば完了です。