generics are removed from function signatures that are assigned to `Callable` types

from typing import TypeVar, Callable

T = TypeVar("T")

def asdf(fn: Callable[[], T]) -> Callable[[], T]: ...

@asdf
def foo() -> list[T]: ...

reveal_type(foo) # "def () -> list[Never]", should be "def [T] () -> list[T]"

playground

use case

i'm making a @NoInstance decorator that bans usages of classmethods on instances:

from typing import TypeVar, ParamSpec, Concatenate, Callable, Generic, overload, Never

out_T = TypeVar("out_T", covariant=True)
P = ParamSpec("P")
out_R = TypeVar("out_R", covariant=True)

class NoInstance(Generic[P, out_R]):

    def __init__(self, function: Callable[Concatenate[type[out_T], P], out_R]):
        ...

    @overload
    def __get__(self, instance: None, owner: type[object]) -> Callable[P, out_R]:
        ...
    @overload
    def __get__(self, instance: object, owner: type[object]) -> Never:
        ...
    
    def __get__(self, instance: object, owner: type[object]) -> object:
        ...     

class Foo:
    @NoInstance
    @classmethod
    def foo(cls):
        ...

Foo.foo() # allowed
Foo().foo() # not allowed

but the primary use case for class methods that should not be called on instances are constructor methods, which means the resulting signature only has the typevar on the return type (in this case, Self):

class Foo:
    @NoInstance
    @classmethod
    def from_int(cls) -> Self:
        ...

reveal_type(Foo.from_int) # `"def () -> Never"`, because the `Self` generic was removed by the `Concatenate`. should be `def () -> Foo` instead

Empieza ejecutando los ejemplos mínimos en el mypy playground enlazado y compara los tipos revelados con las firmas genéricas esperadas. Sigue la inferencia de tipos y el manejo de Callable/Concatenate implicados en esos ejemplos; se considera terminado cuando se conserva el T genérico o Self, en lugar de reducirse a Never.