regression with Generic/ParamSpec between 1.4.1 and 1.5.0

Bug Report

With 1.5.0, some of our software has started to surface a var-annotated ("Need type annotation for") error that it did not surface with 1.4.1; I could not find anything in the 1.5.0 release notes that suggested that this was intentional.

To Reproduce

Our actual software is a bit domain-specific, but I've tried to extract the details; the concept involves a data descriptor that attaches a callable to a class and generates a new callable that inverts how the callable's constructor and runtime arguments are supplied. The solution depends on ParamSpec to preserve the argument signatures and uses Generic so that the same internals can be used for different functions.

#!/usr/bin/env python3
from dataclasses import dataclass
from typing import Generic, ParamSpec, Protocol, Self, TypeVar, overload


P = ParamSpec("P")
T = TypeVar("T")


class Transform(Protocol[P, T]):
    """Generic protocol to transform a value using inputs."""

    def __init__(self, *args: P.args, **kwargs: P.kwargs) -> None:
        raise NotImplementedError

    def __call__(self, value: T) -> T:
        raise NotImplementedError


class BoundTransform(Generic[P, T]):
    """Wrapper around a transform function that performs late-binding."""

    def __init__(
        self,
        value: T,
        transform_cls: type[Transform[P, T]],
    ) -> None:
        self.value: T = value
        self.transform_cls: type[Transform[P, T]] = transform_cls

    def __call__(
        self,
        *args: P.args,
        **kwargs: P.kwargs,
    ) -> T:
        return self.transform_cls(*args, **kwargs)(self.value)


class HasValue(Protocol[T]):
    value: T

class Transformer(Generic[P, T]):
    """Data descriptor that applies the bound transformer to something with a value."""

    def __init__(
        self,
        transform_cls: type[Transform[P, T]],
    ) -> None:
        self.transform_cls: type[Transform[P, T]] = transform_cls

    @overload
    def __get__(self, obj: None, objtype: type[HasValue[T]]) -> Self:
        ...

    @overload
    def __get__(self, obj: HasValue[T], objtype: type[HasValue[T]]) -> BoundTransform[P, T]:
        ...

    def __get__(self, obj: HasValue[T] | None, objtype: type[HasValue[T]]) -> Self | BoundTransform[P, T]:
        if obj is None:
            return self

        return BoundTransform(obj.value, self.transform_cls)


@dataclass(frozen=True)
class Add:
    """Transform implementation that adds two integers."""
    value: int

    def __call__(self, value: int) -> int:
        return value + self.value


@dataclass(frozen=True)
class HasNumber(HasValue[int]):
    """HasValue implementation that holds an integer."""
    value: int

    add = Transformer(Add)



if __name__ == "__main__":
    # Usage example
    foo = HasNumber(42)
    bar = foo.add(-42)
    assert bar == 0

Expected Behavior

No errors from mypy. This is true for 1.4.1.

Actual Behavior

Errors. This is true for 1.5.0

main.py:81: error: Need type annotation for "add"  [var-annotated]
Found 1 error in 1 file (checked 1 source file)

Your Environment

• Mypy version used: 1.4.1 and 1.5.0
• Mypy command-line flags: n/a
• Mypy configuration options from mypy.ini (and other config files): n/a
• Python version used: 3.11.4

Usa la riproduzione main.py fornita ed eseguila con mypy 1.4.1 e 1.5.0, concentrandoti sull’assegnazione add alla riga 81 e sulla relativa diagnostica var-annotated. Confronta il comportamento del descrittore Generic/ParamSpec tra le versioni; il lavoro è completato quando l’errore imprevisto è risolto e la regressione è coperta da un test appropriato.