Union of `Never` can't be used

Bug Report

Functions which optionally take a value of Never type have impossible requirements placed on usages of that optional value.

To Reproduce

https://gist.github.com/mypy-play/a49d72bdacc179ffda5f69d8b66f41ff

from typing import Never


def blah(s: Never | str) -> None:
    print(s + "hi")

Expected Behavior

The code type-checks. Since it is impossible for any variable to take a value of type Never, the type signature of blah is equivalent to str -> None. The function's body is valid when s is annotated with type str, so it should be valid when annotated with Never | str.

Actual Behavior

main.py:5: error: Unsupported left operand type for + ("Never")  [operator]
main.py:5: note: Left operand is of type "Never | str"
Found 1 error in 1 file (checked 1 source file)

Your Environment

• Python 3.12
• MyPy 1.15.0, invocation as performed in the Playground

Background

Pyright appears to do the right thing here: it accepts this code.

I ran into this when implementing this pattern, which is me placing types around something I found untyped in the wild:

T = TypeVar("T", contravariant=True)
class _MyProtocol(Protocol[T]):
  def _foo(self, arg: T | IO[str]) -> None: ...

try:
    import foo
    class _MyImpl(_MyProtocol[foo.Foo]):
      def _foo(self, arg: foo.Foo | IO[str]) -> None: 
        # do something here
        pass

    MyType = _MyImpl

except ImportError:
    class _MyImplNone(_MyProtocol[Never]):
      def _foo(self, arg: Never | IO[str]) -> None:
        # correctly assume arg is an IO[str] here
        pass

    MyType = _MyImplNone

That way, MyType has type _MyProtocol[foo.Foo] | _MyProtocol[Never], allowing a type-level discrimination of whether the foo functionality is present or not.

Commencez par le reproducteur minimal en Python présent dans l’issue ou dans l’exemple mypy-play lié, puis comparez l’erreur d’opérateur signalée au comportement attendu équivalent à str. Suivez la manière dont les unions contenant Never sont traitées lors de la vérification des types, puis ajoutez une couverture pour la fonction présentée et confirmez que mypy l’accepte sans l’erreur d’opérande Never.