overloaded method that has `NoReturn` for certain arguments not detected

[Dr-Irv]

Bug Report

If you have a method that has an overload for specific arguments that are not allowed, by having that overload return NoReturn, and a wider default that accepts Any, mypy does not detect the invalid argument.

To Reproduce

from typing import Any, NoReturn, overload


class Goo:
    @overload
    def __add__(self, other: str) -> NoReturn:
        ...

    @overload
    def __add__(self, other: Any) -> bool:
        ...

    def __add__(self, other: Any) -> bool:
        return True


y = Goo()
y + 4
y + "abc"

Expected Behavior

I expect that the line y + "abc" is reported as an invalid operation.

Actual Behavior

Success: no issues found in 1 source file

Your Environment

• Mypy version used: 1.4.1
• Mypy command-line flags: None
• Mypy configuration options from mypy.ini (and other config files): None
• Python version used: 3.9.16

[JelleZijlstra]

Mypy is correct here. NoReturn is not a way to make the type checker emit errors; it indicates that the code path never returns. The standard example is sys.exit(): it returns NoReturn, but type checkers should not emit an error for every use of NoReturn.

You are looking for python/typing#1043.

[erictraut]

I agree with Jelle's analysis. I just noticed that pyright contains a bug that causes it to (incorrectly) emit an error when a binary operator like __add__ returns NoReturn. It should not emit an error here, consistent with mypy.

[Dr-Irv]

I agree with Jelle's analysis. I just noticed that pyright contains a bug that causes it to (incorrectly) emit an error when a binary operator like __add__ returns NoReturn. It should not emit an error here, consistent with mypy.

Interesting. I liked the pyright behavior in this case! That’s why I reported the issue here.