This would need to go through the Swift Evolution Process.

Comment by Stephane Magne (JIRA)

A slight variation on this that I think would work would be to make all references weak by default, while requiring the user to enforce a strong reference.

func superFunc(session: AnyObject, identifier: Int) {
    preProcess(completion: { [strong self] in 
        session?.validate(self, with: identifier)     
    })
} 

In this case, self is explicitly held, and another developer coming across this code can see clear intent. Further, any implicitly made weak reference, such as session in this scenario, would also have the explicit ? enforced by the compiler.

In that sense, there is a cue to reader in all cases what is happening, and helps the code become more self-documenting, and likely reducing rates of accidental retain cycles.