I’m running into trouble using protocols as types when they have an associated type. I understand the restriction of using a protocol with an associated type only as a generic constraint, since our down-casting syntax has no way of checking the associated types. However, if I’m considering whether an instance conforms to a sub protocol, which introduces no additional associated types itself, when the instance is already known to conform to the super-protocol which introduces the associated type, I believe such behavior is well-defined, and should be supported. So, I’m asking for help to put together a proposal to get this added.
Here’s an example:
3 protocols, one base Protocol which declares an associated type.
And 2 sub-protocols.
protocol A {
associatedType V
}
protocol B : A {
func someFunction<V>(V?)->String
}
protocol C : B {
func someOtherFunction<V>()->V?
}
Note that the sub protocols do not add additional associated types.
When I want to use these protocols, using the base protocol as a generic constraint works fine.
class M {
func doSomething<P, V>(a:P, completion:@escaping(V?)->()) where P : A, V==P.V {
if let c = a as? C {
c.someOtherFunction(…)
… more code
} else if let b = a as? B {
b.someFunction(…)
… more code
}
}
}
However, when I try to check whether the instance conforms to either of the sub-protocols, I get an error, saying protocols with associated types can only be used as generic constraints.
I believe that in this case, since the sub-protocols do not add any additional associated types, this code should be well-defined, and compilable.