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radical, the spec can stay as is but the reasoning and associated examples should be clearly marked ‘historical’ and no longer valid.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.

Inizia dalla sezione collegata «type-and-class-objects-vs-protocols» in docs/spec/protocol.rst, quindi leggi la typing discussion collegata e il mypy issue per comprendere la motivazione circostante. L’issue presenta direzioni alternative anziché una modifica definita; il lavoro sarebbe completato solo dopo aver concordato una decisione sulla specifica, seguita dagli aggiornamenti della motivazione e degli esempi associati.