fix list.copy to use the expected type

[KotlinIsland]

support:

a: list[object] = list[int]().copy()

[srittau]

_S has no meaning, as it's only used once in this context. It's just interpreted as Any in this context.

[srittau]

I see what you're trying to do, but we should probably use:

Suggested change

	def copy(self) -> list[_S | _T]: ...
	# The new list can allow broader types that the old one. `Any` allows that.
	def copy(self) -> list[_T | Any]: ...

[KotlinIsland]

_S has no meaning, as it's only used once in this context. It's just interpreted as Any in this context.

that is demonstrably not true

[KotlinIsland]

Code sample in basedpyright playground

from __future__ import annotations
from typing import Any

class A[T]:
    t: T
    def copy1[Expected](self) -> A[Expected | T]:
        return A()
    def copy2[Expected](self) -> A[Any | T]:
        return A()
a: A[object] = A[int]().copy1()
a: A[object] = A[int]().copy2()

[JelleZijlstra]

basedpyright is incorrect there. A[int | Any] should be assignable to A[object].

[KotlinIsland]

consider this example:

def empty_list[T=Never]() -> list[T]:
    return []

a = empty_list()
b: list[int] = empty_list()

if we were to use Any here, then the type of a would be list[Any]

additionally, there would be absolutely no safety within empty_list to return the correct type. i'm not really sure where this idea that type variables only in return positions being invalid came from

this is literally the meaning of a type variable, to use Any is a mistake

[KotlinIsland]

basedpyright is incorrect there. A[int | Any] should be assignable to A[object].

yes: microsoft/pyright#10713 (comment)

i was a little hasty with the first example, i think it's clear though that this case is the intended use of a type variable, not Any

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉