Pyright does not infer generic class parameter when specified as argument to class method

[superlopuh]

Here's a simple example, T is completely specified, yet Pyright complains:

from typing import Generic, TypeVar

T = TypeVar("T")

class GenericClass(Generic[T]):

    @classmethod
    def method(cls, param: T) -> T:
        return param

val = GenericClass.method(1)

Pyright errors:

pyright xdsl/dialects/bla.py
12:1 - error: Type of "val" is unknown (reportUnknownVariableType)
12:7 - error: Type of "method" is partially unknown
    Type of "method" is "(param: Unknown) -> Unknown" (reportUnknownMemberType)
2 errors, 0 warnings, 0 informations 

With pyright 1.1.384

[erictraut]

Pyright's behavior here is correct, so this isn't a bug.

You've defined a class-scoped type variable here, so the type argument is associated with the class (GenericClass), not with the method (method). Any class-scoped type variables that appear within a method's signature are specialized at the time you access the method through the class. In this case, the class is GenericClass[Unknown] because you haven't provided an explicit type argument. If you had provided a default value for the type variable (as per PEP 696), then GenericClass would be specialized with that default value. Since there was no default specified, the default is implicitly Unknown. That means the type of GenericClass.method is (param: Unknown) -> Unknown. There is no type variable to solve in this case, so it doesn't matter what arguments you pass.

There's only one case specified in the typing spec where class-scoped type variables can be inferred when calling a method, and that's in the case of a constructor call. This special case doesn't apply to class methods.

[superlopuh]

Thank you for the detailed response!