mypy reports an error with a function with generic argument.

[s17b2-voroneckij]

Bug Report

For a function that accepts objects of A | B | C type, mypy is not correctly inferring types for function calls from that object.

To Reproduce
https://mypy-play.net/?mypy=latest&python=3.12&gist=a090296a66d822acabcb0e92e53f5871

from typing import Self, reveal_type, Protocol


class A:
    def dup(self) -> list[Self]:
        return [self, self]


class B:
    def dup(self) -> list[Self]:
        return [self, self]


class C:
    def dup(self) -> list[Self]:
        return [self, self]


class HasDup(Protocol):
    def dup(self) -> list[Self]: ...


def process[T: A | B | C](elem: T) -> list[T]:
    reveal_type(elem.dup)
    duplicated: list[T] = elem.dup()
    return duplicated


process(A())
process(B())
process(C())

Expected Behavior
No error to be reported

Actual Behavior
main.py:24: note: Revealed type is "Union[def () -> builtins.list[main.A], def () -> builtins.list[main.B], def () -> builtins.list[main.C]]"
main.py:25: error: Incompatible types in assignment (expression has type "list[A] | list[B] | list[C]", variable has type "list[T]") [assignment]
Found 1 error in 1 file (checked 1 source file)

Your Environment

• Mypy version used: 1.14.1
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3,12

[A5rocks]

Note that list[A | B | C] is not list[A] | list[B] | list[C] because list is an invariant type. Here's some ideas for what you could do:

• use Sequence, which is covariant (generally just follow https://mypy.readthedocs.io/en/stable/common_issues.html#invariance-vs-covariance)
• don't accept A | B | C as an argument: instead, try T: (A, B, C) which constrains T to be a subclass of one of those three.

[s17b2-voroneckij]

@A5rocks, using Sequence doesn`t help in any way:
https://mypy-play.net/?mypy=latest&python=3.12&gist=8592bae5e6db2338470bf68cf73cfbcf

The issue I think is that mypy incorrectly detects the type of elem.dup. It should be () -> Sequence[T] and not Union[def () -> typing.Sequence[main.A], def () -> typing.Sequence[main.B], def () -> typing.Sequence[main.C]]

[A5rocks]

Good catch. I searched a bit and I think that's #18170 #16826.

My second recommendation should hopefully work though.

[s17b2-voroneckij]

@A5rocks , yes, the second one works indeed