Clarification on how Typevars in a generic function should resolve

[kamalfarahani]

Hi, regarding the issue opened here:

astral-sh/ty#2255

Can you please provide clarification on how TypeVars in a function like func in the below code should be resolved?

class A:
    pass

class B(A):
    pass

class C(A):
    pass

def func[T: A](x: T, y: T) -> T:
    ...

func(B(), C()) # I think this should raise a type error by type checkers but no type checker (mypy, pyright, etc) raise an error for this

Please look at these comments:
astral-sh/ty#2255 (comment)
astral-sh/ty#2255 (comment)
astral-sh/ty#2255 (comment)
astral-sh/ty#2255 (comment)

[JelleZijlstra]

The spec does not prescribe exactly how this TypeVar should be solved, but it's expected that they would solve the TypeVar to some common supertype of B and C—that is, A, or B | C.

In https://discuss.python.org/t/support-function-with-multiple-generic-parameters-same-type/57931/6 I suggested a type system addition that would make it so code like yours produces an error. I am not going to pursue that further, but if you think this is important, you can work on trying to standardize this idea as an addition to the type system.