Open Radar

 
Summary:
In Swift, it is possible to adopt a protocol on a structure as follows:

````
protocol Sig {
    typealias A
    let value : A
}

struct X : Sig {
    typealias A = String
    let value = "hello, world"
}
````

It is not possible, however, to erase all the ways in which a structure extends a signature. So, for instance, we have the definitional equality `X.A == String`; it is important for abstraction & modularity to be able to erase this information when desirable.

Steps to Reproduce:
In the following (hypothetical) syntax, we can get this à la Standard ML:

````
struct X :< Sig {
    typealias A = String
    let value = "hello, world"
}
````


Expected Results:
Now, for any `x:X`, we don't have `x.value : String` but rather only `x.value : X.A`. This becomes particularly useful when you have further constraints in the protocol on the abstract type member (like `Equatable` or something).

Actual Results:
Lacking an opaque protocol ascription feature, this kind of abstraction may only be got from upcasting `x:X` to `x:Sig`. It would be ideal if we could enforce abstraction at the level of structures as well.

For a good explanation of how this is useful, Bob Harper's Introduction to Standard ML is a very good text.