Typing narrowing fails depending on the order of if clauses

まず、mypy 1.4.1 を使って bug.py の再現を実行するか、リンクされている mypy-play の例を実行し、失敗する節の順序と動作する節の順序を比較します。if 文を通じて string、string_a、string_b の型の絞り込みを追跡します。報告されている arg-type エラーが、意図した動作を変更せずに解消されれば完了です。

Bug Report

Type narrowing fails in the code below.

To Reproduce

def takes_one_str(string: str) -> str:
    return string


# just the problematic case -> failing
def takes_two_optional_str0a(string: str | None, whatever: bool) -> str:
    if not (whatever and (string is None)) and whatever:
        return takes_one_str(string)
    return ""


# reformulated by reversing the order of if clauses -> working
def takes_two_optional_str0b(string: str | None, whatever: bool) -> str:
    if whatever and not (whatever and (string is None)):
        return takes_one_str(string)
    return ""


# what was I actually trying to achieve -> failing
def takes_two_optional_str1(string_a: str | None, string_b: str | None) -> str:
    if (string_a is None) and (string_b is None):
        return takes_one_str("")
    if string_a is None:
        # false-positive arg-type (same without return, with elif)
        # string_b cannot be None
        return takes_one_str(string_b)
    if string_b is None:
        return takes_one_str(string_a)
    return takes_one_str(string_a + string_b)


# workaround variant -> working
def takes_two_optional_str2(string_a: str | None, string_b: str | None) -> str:
    if (string_a is not None) and (string_b is not None):
        return takes_one_str(string_a + string_b)
    if string_a is not None:
        # false-positive arg-type (same without return, with elif)
        return takes_one_str(string_a)
    if string_b is not None:
        return takes_one_str(string_b)
    return takes_one_str("")

https://mypy-play.net/?mypy=latest&python=3.11&gist=3f3ce6b3021c1a71a4ecf31d35f62a98

Expected Behavior

No errors

Actual Behavior

bug.py:8: error: Argument 1 to "takes_one_str" has incompatible type "str | None"; expected "str" [arg-type]
bug.py:26: error: Argument 1 to "takes_one_str" has incompatible type "str | None"; expected "str" [arg-type]
Found 2 errors in 1 file (checked 1 source file)

Your Environment

• Mypy version used: mypy 1.4.1 (compiled: yes)
• Mypy command-line flags: None
• Mypy configuration options from mypy.ini (and other config files): None
• Python version used: 3.11.4