Deny free type variables in instance attributes coming from constructor args

Feature

Detect free type variables as instance attribute annotations, when they appear from constructor with TypeVar in signature without Generic.

Pitch

It would be especially useful for beginners. People often do not understand Generic necessity under some circumstances, though more experienced user may also accidentally forget about this.

This does not deny type variables in constructor arguments, if they do not become part of attributes annotations (say, __init__(self, x: _T, y: _T): do_something(x, y); super().__init__()).

Simple example (playground link):

from typing import TypeVar

_T = TypeVar('_T')

class Foo:
    def __init__(self, foo: _T, bar: _T):
        self.foo = foo
        self.bar = bar

f = Foo(1, 1)
reveal_type(f.foo)  # N: Revealed type is "_T`-1"

mypy already denies another scenario, which is essentially the same:

...
class Bar:
    x: _T  # E: Type variable "__main__._T" is unbound  [valid-type]

results in

main.py:15: error: Type variable "__main__._T" is unbound  [valid-type]
main.py:15: note: (Hint: Use "Generic[_T]" or "Protocol[_T]" base class to bind "_T" inside a class)
main.py:15: note: (Hint: Use "_T" in function signature to bind "_T" inside a function)

Inizia con il reproducer mypy-play collegato e confrontalo con la diagnostica esistente per i TypeVar non associati negli attributi di classe. Traccia come le assegnazioni nel costruttore diventano annotazioni degli attributi di istanza; il lavoro è completo quando lì vengono segnalati i TypeVar non associati, consentendo al contempo i TypeVar usati solo negli argomenti del costruttore.