[swift-evolution] Declaring a function that always returns nil

[Andrew Duncan]

It should be possible to declare a function that returns only nil, and have its return type be substitutable for any function that returns and Optional. This is something like having a bottom type but not really. What I mean is:

func returnsNil(errCode:Int) -> nil {
    logError(errCode)     // A side-effect. Not FP, sosumi.
    return nil
}

func returnAOptional() -> A? {
    // Bla bla. We discover an error so we decide to bail and return nil.
    return returnsNil(errCode) // Would be legal.
}
func returnsBOptional() -> B? {
    // Bla bla. We discover an error so we decide to bail and return nil.
    return returnsNil(errCode)  // Would also be legal.
}

I seek a return type that conforms to any Optional -- I think that implies it *must* (or correct me here) be nil.

Now perhaps this is already possible with wizardry from the Next Level. (There always is one.)