[nupark2]

Cheers to the peer comment regarding HN discourse. Unfortunately (?) I have more :)

> This doesn't affect interchanging Objective-C types though. Yes, the arity and order are important, but the compiler doesn't enforce anything beyond that a pointer is passed for id types.

This is true prior to ARC: all ObjC pointers are the same size, and hence ABI-compatible given equivalent arity/order. It's theoretically possible that a future ABI could be incompatible between two methods returning void vs pointer return value, but currently, all supported ABIs return pointer-sized values in a register.

However, with ARC, this changes. The type system has been effectively extended to denote the required referencing behavior for calling code. This means that for a given arity/order, you must also have equivalent referencing attributes.