Cannot anotate type of generic functions.

先从泛型函数示例和链接的 Pyright issue 开始，然后将它们与当前的类型变量作用域规则进行比较。当 typing 规范就如何标注泛型函数变量和类实例变量达成一致的解决方案时，即视为完成；该 issue 不涉及任何仓库文件或测试。

The current scoping rules for type variables seems to deny type annotations over generic functions.

The issue

define two functions as follows:

def fx_1(f: Callable[[bool], int]) -> int:
    return f(True)

def fx_2[T](f: Callable[[bool], T]) -> T:
    return f(True)

If a function alias is required, these functions can be assigned to variables. However, if type annotations are used, fx_2 cannot be annotated using compliant type checkers (i.e. pyright).

T = TypeVar("T")
fx_1_alias: Callable[[Callable[[bool], int]], int] = fx_1  # OK!
fx_2_alias: Callable[[Callable[[bool], T]], T] = fx_2  # complains that T has no meaning

Justification

• All variables should be able to be type annotated, even when those variables are functions or the annotation is not required.
• def is a syntactic sugar equivalent to a variable assignation over a lambda function. As a result, it should be possible to define generic functions without def statements e.g.

fx_2_alternative: Callable[[Callable[[bool], T]], T] =  lambda f: f(True)

• Generics are the first ladder of what is called a "dependent type". In the case of functions, this corresponds to a Π type where the input itself is the concrete type that will be replaced on the type var of a generic type. Here type-vars scope is over the function type itself rather than some outer entity (class, function, method).
• By these definitions of Π type, it cannot exist an unbound type variable when function/callable types are involved, since in python, the type argument is implicit and supplied when referring a TypeVar.
  e.g.

T = TypeVar("T")
fx_2_alias: Callable[[Callable[[bool], T]], T] = fx_2
fx_2_alias_2: Callable[[Callable[[bool], T]], T] = fx_2

Even when using the same type var fx_2_alias and fx_2_alias_2 each var is an independent one.
A more clear syntax (like the introduced on def statements on python312) could be

fx_2_alias: Callable[X][[Callable[[bool], X]], X] = fx_2
fx_2_alias_2: Callable[X][[Callable[[bool], X]], X] = fx_2

This marks explicitly that the scope of the var is over the type itself. However, this issue does not aim to propose a new syntax, but rather aims to adjust the scoping rules of type vars.

Motivation

This issue was derived from a pyright issue where generic functions as class instance variables are denied by the checker because of the same scope issues described before.