mypy plugin allows false positive error when exhaustively pattern matching on Result

示されている pattern_div.py の例と pyproject.toml の mypy 設定を使ってレポートを再現します。次に、returns.contrib.mypy.returns_plugin のエントリーポイントと、存在する場合は既存のプラグインテストを調査します。Success/Failure の網羅的なマッチングで missing-return エラーが発生しなくなり、その例を対象とするリグレッションテストが追加されていれば完了です。

Bug report

What's wrong

Again, this is a really cool project ❤️

Consider the following code:

from enum import Enum, auto
import math
from typing import TypeAlias

from returns.result import Failure, Result, Success


class MathError(Enum):
    DivisionByZero = auto()
    NonPositiveLogarithm = auto()


MathResult: TypeAlias = Result[float, MathError]


def div(x: float, y: float) -> MathResult:
    if y == 0.0:
        return Failure(MathError.DivisionByZero)

    return Success(x / y)


def ln(x: float) -> MathResult:
    if x <= 0.0:
        return Failure(MathError.NonPositiveLogarithm)

    return Success(math.log(x))


def op_(x: float, y: float) -> MathResult:
    z = div(x, y)
    match z:
        case Success(ratio):
            return ln(ratio)
        case Failure(_):
            return z

mypy configuration in pyproject.toml

[tool.mypy]
ignore_missing_imports = true
strict = true
plugins = [
    "returns.contrib.mypy.returns_plugin",
]

Running:

$ mypy pattern_div.py
pattern_div.py:32: error: Missing return statement

Related to #1090

How is that should be

As far as I can tell (new to returns), I'm matching exhaustively here, so I would not expect a mypy error.

mypy seems to understand exhaustive matching in general. mypy does not throw a type checking error in the following snippet.

def test(err: MathError) -> int:
    match err:
        case MathError.DivisionByZero:
            return 0
        case MathError.NonPositiveLogarithm:
            return 1

System information

• python version: 3.10.2
• returns version: 0.19.0
• mypy version: 0.942

$ pip list
Package           Version
----------------- -------
black             22.3.0
click             8.1.2
distlib           0.3.4
filelock          3.6.0
isort             5.10.1
mypy              0.942
mypy-extensions   0.4.3
packaging         21.3
pathspec          0.9.0
pep517            0.12.0
pip               22.0.4
pip-tools         6.6.0
platformdirs      2.5.1
pluggy            1.0.0
py                1.11.0
pyparsing         3.0.8
returns           0.19.0
setuptools        62.1.0
six               1.16.0
toml              0.10.2
tomli             2.0.1
tox               3.25.0
typing_extensions 4.1.1
virtualenv        20.14.1
wheel             0.37.1