What's the difference between a generic constraint of <T: Any> and no constraint <T>

Question

I've seen some code that uses constraints like <T: Any> and can't find the difference in the docs between that and not specifying a constraint. Is there any difference at all, like restricting to non optionals?

Answer 1

The constraint is indeed redundant, as all types are subtypes of Any (including Optional).

Internally, the compiler actually models the type Any as being a protocol composition type
(e.g P1 & P2) consisting of zero protocols. There's no way to spell this in the language, which is why Any isn't defined in the standard library anymore¹, it's just a keyword that's parsed as a type.

So the constraint T : Any is literally interpreted as "T must conform to all of the protocols in this empty list of protocols", which is quite evidently a redundant constraint. Really the compiler ought to warn on it (I have actually started working on a patch to do so – aiming to open a pull request sometime this week all things going well).

_{1. Any used to be defined in the standard library as a typealias for protocol<>, when protocol compositions were spelt protocol<P1, P2> rather than P1 & P2.}