Sequence of types

The following works in pyright, but not in pyrefly, ty, or (which is why I'm here!) mypy:

from __future__ import annotations

from dataclasses import dataclass

from typing_extensions import (
    Generic,
    Sequence,
    TypeVar,
    assert_type,
)

T = TypeVar("T")


@dataclass
class Agent(Generic[T]):
    output_type: Sequence[type[T]]


class Foo:
    pass


class Bar:
    pass


# pyright - works
# mypy - error: Expression is of type "Agent[object]", not "Agent[Foo | Bar]"  [assert-type]
# pyrefly - assert_type(Agent[Foo], Agent[Bar | Foo]) failed + Argument `list[type[Bar] | type[Foo]]` is not assignable to parameter `output_type` with type `Sequence[type[Foo]]` in function `Agent.__init__`
# ty - `Agent[Foo | Bar]` and `Agent[Unknown]` are not equivalent types
assert_type(Agent([Foo, Bar]), Agent[Foo | Bar])

# pyright - works
# mypy - error: Expression is of type "Agent[Never]", not "Agent[int | str]"  [assert-type]
# pyrefly - assert_type(Agent[int], Agent[str | int]) failed + Argument `list[type[str] | type[int]]` is not assignable to parameter `output_type` with type `Sequence[type[int]]` in function `Agent.__init__`
# ty - `Agent[int | str]` and `Agent[Unknown]` are not equivalent types
assert_type(Agent([int, str]), Agent[int | str])

# works
assert_type(Agent[Foo | Bar]([Foo, Bar]), Agent[Foo | Bar])

# works
assert_type(Agent[int | str]([int, str]), Agent[int | str])

It would be great to see this work in mypy, but if there's a good reason the other 2 out of 3 typecheckers also don't support this I'd love to understand why!

• This is related to a new PydanticAI feature, if you're curious check out https://github.com/pydantic/pydantic-ai/pull/1785#issuecomment-2905774110
• Issues for other type checkers:
  • https://github.com/facebook/pyrefly/issues/345
  • https://github.com/astral-sh/ty/issues/501

Start by running the minimal reproduction in the issue and compare mypy's inferred types with the explicitly parameterized cases. Investigate how generic constructor inference handles Sequence[type[T]] when the argument contains multiple types. Done means the unparameterized Agent([Foo, Bar]) and Agent([int, str]) assertions pass without regressing the existing cases.