'Type parameter "T@Bar" is covariant, but "T@Bar" is not a subtype of "Jee"'. But T: Jee...?

[qexat]

The following snippet:

from __future__ import annotations

import dataclasses


class Foo:
    def bark(self) -> Jee:
        return self


@dataclasses.dataclass(slots=True, frozen=True)
class Bar[T: Jee]:  # T is clearly a Jee.
    meow: T

    def bark(self) -> Jee:
        return self  # error


type Jee = Foo | Bar[Jee]

causes Pyright to report the following:

  repro.py:16:16 - error: Type "Self@Bar[T@Bar]" is not assignable to return type "Jee"
    Type "Self@Bar[T@Bar]" is not assignable to type "Jee"
      "Bar[T@Bar]*" is not assignable to "Foo"
      "Bar[T@Bar]*" is not assignable to "Bar[Jee]"
        Type parameter "T@Bar" is covariant, but "T@Bar" is not a subtype of "Jee"
          Type "T@Bar" is not assignable to type "Jee"
            Type "T@Bar" is not assignable to type "Foo"
            Type "T@Bar" is not assignable to type "Bar[Jee]" (reportReturnType)
1 error, 0 warnings, 0 informations

I am confused. Am I missing something?

[erictraut]

Pyright's behavior here is correct. (Not surprisingly, mypy generates the same error in this case.)

You haven't provided a type annotation for the self parameter in the bark method, so its type is implicitly typing.Self. If your intent is for it to be Jee, you can annotate it as such, but that means this method won't be callable for certain specializations of Bar. If you want bark to work with all parameterizations of Bar, then you'd need to change the return type of bark to Self.

[qexat]

Pyright's behavior here is correct. (Not surprisingly, mypy generates the same error in this case.)

You haven't provided a type annotation for the self parameter in the bark method, so its type is implicitly typing.Self. If your intent is for it to be Jee, you can annotate it as such, but that means this method won't be callable for certain specializations of Bar. If you want bark to work with all parameterizations of Bar, then you'd need to change the return type of bark to Self.

Oh oops I guess I should have slept more :') Thank you for the answer!