Union of Never can't be used

[pleaseletmesearch]

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.

[jorenham]

According to the typing spec:

If B is a subtype of A, B | A is equivalent to A.

And since Never <: T for all types T, Never | T is indeed equivalent to T.

[sterliakov]

A safe casting (no Any, no cast) allows your code:

from typing import Never


def rejected(y: Never | str) -> None:
    y.startswith("")  # E: Item "Never" of "Never | str" has no attribute "startswith"  [union-attr]


def accepted(y: Never | str) -> None:
    x: str = y
    x.startswith("")

This means mypy models assignability rules correctly, but does not attempt filtering Never out of a union.

(I can also recommend the code above as a workaround for now: it's definitely safe and won't mask any future error)

[A5rocks]

Technically #18707 fixes this (I introduced a bunch of changes around Never) but maybe mypy should more aggressively simplify unions too.