Reason given for disallowing non-concrete subtype assignment is unsound

Issue

According to the discussion https://github.com/python/typing/discussions/1305, “type checkers allow incompatible __init__ overrides, because flagging them would be too disruptive.”

Accepting above as de facto, the example given as the main reason for disallowing non-concrete subtype assignment in the following section of the spec is unsound:
https://github.com/python/typing/blob/e08290b70f58df509f998cbbe09a8e65abb57a9b/docs/spec/protocol.rst#type-and-class-objects-vs-protocols

class Proto(Protocol):
    @abstractmethod
    def meth(self) -> int:
        ...
class Concrete:
    def meth(self) -> int:
        return 42

def fun(cls: type[Proto]) -> int:
    return cls().meth() # ???
fun(Proto)              # Why should this error?
fun(Concrete)           # OK

var: Type[Proto]
var = Proto    # Why should this error?
var = Concrete # OK
var().meth()   # ???

(credit https://github.com/python/mypy/issues/4717#issuecomment-1978239641 for pointing out the contradiction)

Thoughts

One radical approach would be to remove the concreteness rule from the spec altogether. The type of Proto is type[Proto], and if the constructor compatibility is not checked, there is little reason to disallow it from being assigned to variables annotated as type[Proto].

If that is too radical, the spec can stay as is but the reasoning and associated examples should be clearly marked ‘historical’ and no longer valid. That way, we can avoid any immediate changes in practice, but at the same time encourage discussions towards appropriate future specs.

Comienza con la sección enlazada «type-and-class-objects-vs-protocols» en docs/spec/protocol.rst y, después, lee la typing discussion enlazada y el mypy issue para conocer la justificación relacionada. El issue presenta direcciones contrapuestas en lugar de un cambio definido; el trabajo estaría terminado cuando se haya acordado una decisión sobre la especificación y, posteriormente, se actualicen la justificación y los ejemplos asociados.