Figure out whether we are mis-using instancetype in the Darwin APIs

[bzbarsky-apple]

See #23364 (comment) -- what does instancetype promise, exactly? When is it appropriate to use it?

@jtung-apple @ksperling-apple

[bzbarsky-apple]

Also see #23364 (comment)

[jtung-apple]

Quoting @ksperling-apple - first comment:

Not changed in this PR, but it looks to me like the return type should be MTRDevice * instead of instancetype? If you were to call this method on a sub-class of MTRDevice, would you be guaranteed to get an instance of that sub-class?

A subclass of MTRDevice would return an object of the subclass in "reasonable code", as in, using the class names explicitly.

If you play with a Class variable in a way that limits the compiler's ability to infer the actual class, then you may get an id which is assignable to anything (the runtime type will still be correct, if you ask the object for its class). But doing this is not good practice in ordinary code.

[bzbarsky-apple]

I think the issue is this: for a factory method as here, what happens? It can do one of two things. The impl could [[MTRDevice alloc] init] as I think we do, or it could [[self.class alloc] init], right? And it seems like we should return instancetype in the latter case and MTRDevice * in the former?

[jtung-apple]

Okay you're right about this. Given the implementation of the factory method asks MTRDeviceController for a MTRDevice object, the return type really can't be anything but MTRDevice *. And subclassing MTRDevice really doesn't make sense because of that.

[jtung-apple]

We should change the factory method to return MTRDevice *.