When we do subtype/assignability checks within a generic scope, we don't consider two different typevars to be equivalent, even if they have the same bounds/constraints, because they could specialize to two different types.

But when we compare two unspecialized generic signatures for assignability/subtyping, this consideration does not apply. Two identical signatures, except that they use a different typevar (with the same bounds/constraints), are the same signature; typevar identity is irrelevant. In this example, all six assertions should pass; currently only the first and third and fifth pass:

from typing import TypeVar, Callable

from ty_extensions import is_assignable_to, is_subtype_of, static_assert

T = TypeVar("T")
U = TypeVar("U")

static_assert(is_assignable_to(Callable[[T], T], Callable[[T], T]))
static_assert(is_assignable_to(Callable[[T], T], Callable[[U], U]))

static_assert(is_subtype_of(Callable[[T], T], Callable[[T], T]))
static_assert(is_subtype_of(Callable[[T], T], Callable[[U], U]))

static_assert(is_equivalent_to(Callable[[T], T], Callable[[T], T]))
static_assert(is_equivalent_to(Callable[[T], T], Callable[[U], U]))