TypeVar bound not respected in Generic class method overloads

[jackmpcollins]

Bug Report

Mypy chooses the wrong overload of a generic class method in the below code. str | Sequence[int] would match Sequence[ItemT] if ItemT was not bounded or constrained, but since it is constrained to non-str the overload for str | Sequence[ItemT] should be matched instead. The issue only affects generic class methods and not regular functions.

To Reproduce

from collections.abc import Sequence
from typing import Generic, TypeVar, overload

from typing_extensions import assert_type


T = TypeVar("T", covariant=True)
ItemT = TypeVar("ItemT", bound=int)  # OR TypeVar("ItemT", int, float)


class MyGeneric(Generic[T]):
    @overload
    def transform(self: "MyGeneric[str]") -> "MyGeneric[str]": ...

    @overload
    def transform(self: "MyGeneric[Sequence[ItemT]]") -> "MyGeneric[Sequence[ItemT]]": ...

    @overload
    def transform(self: "MyGeneric[str | Sequence[ItemT]]") -> "MyGeneric[str | Sequence[ItemT]]": ...

    def transform(self: "MyGeneric[str | Sequence[ItemT]]") -> "MyGeneric[str | Sequence[ItemT]]": ...  # type: ignore


assert_type(MyGeneric[str]().transform(), MyGeneric[str])
assert_type(MyGeneric[Sequence[int]]().transform(), MyGeneric[Sequence[int]])
assert_type(MyGeneric[str | Sequence[int]]().transform(), MyGeneric[str | Sequence[int]])
# error: Expression is of type "MyGeneric[Sequence[object]]", not "MyGeneric[str | Sequence[int]]"

# ---
# The equivalent functions do not have the same issue

@overload
def transform(x: "MyGeneric[str]") -> "MyGeneric[str]": ...

@overload
def transform(x: "MyGeneric[Sequence[ItemT]]") -> "MyGeneric[Sequence[ItemT]]": ...

@overload
def transform(x: "MyGeneric[str | Sequence[ItemT]]") -> "MyGeneric[str | Sequence[ItemT]]": ...

def transform(x: "MyGeneric[str | Sequence[ItemT]]") -> "MyGeneric[str | Sequence[ItemT]]": ...  # type: ignore


assert_type(transform(MyGeneric[str]()), MyGeneric[str])
assert_type(transform(MyGeneric[Sequence[int]]()), MyGeneric[Sequence[int]])
assert_type(transform(MyGeneric[str | Sequence[int]]()), MyGeneric[str | Sequence[int]])

Your Environment

• Mypy version used: 1.14.1
• Mypy command-line flags:
• Mypy configuration options from mypy.ini (and other config files):
• Python version used: 3.13.0

[jackmpcollins]

This could be the same issue and is a simpler reproduction, and rules out overload as being the cause. The typevar StrIntT is constrained to str or int, so the Literal input should become a str output, but it remains Literal.

from typing import Generic, Literal, TypeVar

from typing_extensions import assert_type


T = TypeVar("T")
StrIntT = TypeVar("StrIntT", str, int)


class MyGeneric(Generic[T]):
    def transform(self: "MyGeneric[StrIntT]") -> "MyGeneric[StrIntT]":
        return self


assert_type(MyGeneric[str]().transform(), MyGeneric[str])
assert_type(MyGeneric[Literal["x"]]().transform(), MyGeneric[str])
# error: Expression is of type "MyGeneric[Literal['x']]", not "MyGeneric[str]"


# ---
# The equivalent functions do not have the same issue

def transform(x: MyGeneric[StrIntT]) -> MyGeneric[StrIntT]: ...  # type: ignore[empty-body]

assert_type(transform(MyGeneric[str]()), MyGeneric[str])