[swift-users] Existentials in Swift

[Toni Suter]

Hi everyone,

I am trying to understand existentials in Swift. I understand that in the
following code, P1 & P2 is an existential type, because it supports values
from all types that conform to both P1 and P2. 

protocol P1 {}
protocol P2 {}
var x: P1 & P2

However, this raises a few questions for me:

1. In the following code, is x also an existential?
protocol P {}
var x: P

The proposals / blog posts that I read always talk about the protocol composition syntax,
but to me, this is the same as the first example, just with only a single protocol requirement.

2. Are Any and AnyObject also existentials?
So far, my understanding is:
Any is an existential with no requirements
AnyObject is an existential with only a class requirement

3. Why can I not extend Any / AnyObject?
For example, AnyObject is a protocol defined in the standard library. Why is it not extensible?

Thanks and best regards,
Toni
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