[swift-evolution] "AnyValue", "protocol MyProtocol : value" as counterpart to "AnyObject", ": class"

[Karl]

I was building a collection object recently, which stores an array of tuples: (Range<I>, T) - some generic type, and a range. 
Part of the workings of the collection involves splitting these tuples, which means we need to split the Range (easy enough) and duplicate T.

In practice, I was using value types for T (it was another Array), so I could duplicate by assignment; but what if somebody stuck an NSArray or other reference-type in instead?

My first thought was - I could allow it, create some “Copyable” protocol and require conformance. But there are other guarantees I get from value types as well. Let’s say I had a sorted Array of T; if T could be a reference-type, anybody who gets a T from the collection could mutate it and invalidate the sort order at any time! That could lead to unexpected behaviour, such as infinite loops in binary-search algorithms, and it might not be obvious to a user of the collection how that happened (or that it’s even their fault - how are they supposed to know this is kept in a sorted Array, especially if it’s wrapped by another type, and that this particular search algorithm could infinitely loop if the Array is not sorted?). I could return a copy from the getter, but at this point I’m basically re-inventing value-type semantics for classes while creating burdensome requirements for actual value-types.

Actually, this is a problem with the language in general. While we have a way to specify that a generic type or protocol must obey reference-type semantics (via “<T where T:AnyObject>”, “protocol MyProtocol : class”), we don’t have a value-type semantic counterpart.

So I’d like to see us provide an “AnyValue” and “value” equivalent. We should be able to require that generic types or protocols are satisfied by values so we know how to treat them.