Pyright unable to infer the type of Generic classmethod

[sunfkny]

Describe the bug
Since version 1.1.354, Pyright unable to infer the type of a generic class method in certain scenarios.

Code or Screenshots

from typing import Generic, TypeVar, reveal_type

T = TypeVar("T")

class Option(Generic[T]):
    def __init__(self, value: T):
        self._val = value

    @classmethod
    def Some(cls, val: T) -> "Option[T]":
        return cls(val)

reveal_type(Option(1))           # Pyright correctly infers Option[int]
reveal_type(Option.Some(2))      # Pyright incorrectly infers Option[Unknown]

In this code, Pyright correctly infers the type of Option(1) as Option[int], but since version 1.1.354, it incorrectly infers the type of Option.Some(2) as Option[Unknown] Pyright Playground. Prior to version 1.1.354, Pyright inferred the correct type for both cases Pyright Playground.

VS Code extension or command-line
ms-python.vscode-pylance v2024.4.1

[erictraut]

This is by design, not a bug. This behavior was previously somewhat ambiguous in the typing spec, but the recent addition of PEP 696 (which introduces TypeVar default values) clarified how this must work.

When you call a constructor of a generic class, the specialized type of the class is not yet established and can be inferred from the arguments passed to the constructor.

By contrast, when you call a class method, the specialized type of the class is already established at the time that the method is bound to the class — prior to the call. If you don't explicitly provide type argument values, they take on their default values (as explained in PEP 696). If no default is specified, it implicitly becomes Any (i.e. Unknown).

In your example above, if you want to invoke a class method Some on Option[int], you'd need to write Option[int].Some(2). Or if you use PEP 696 to specify a default type argument for T, you could do the following:

Code sample in pyright playground

from typing import Generic, reveal_type
from typing_extensions import TypeVar

T = TypeVar("T", default=int)

class Option(Generic[T]):
    def __init__(self, value: T):
        self._val = value

    @classmethod
    def Some(cls, val: T) -> "Option[T]":
        return cls(val)

reveal_type(Option.Some(2)) # Option[int]