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

Reproduce el informe con el ejemplo mostrado de pattern_div.py y la configuración de mypy en pyproject.toml. Después, inspecciona el punto de entrada returns.contrib.mypy.returns_plugin y las pruebas existentes del plugin, si las hay. Se considera terminado cuando una correspondencia exhaustiva de Success/Failure ya no produce un error missing-return y hay una prueba de regresión que cubre el ejemplo.

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