[SR-13296] Associated type inference through constraints on sibling associated type

[jeremyabannister]

Previous ID	SR-13296
Radar	None
Original Reporter	@jeremyabannister
Type	Bug

Additional Detail from JIRA

Votes	0
Component/s
Labels	Bug
Assignee	None
Priority	Medium

md5: 73808fb6010e083ce8ab513b19743c6a

Issue Description:

• • • UPDATE:

After thinking about this and experimenting with it a bit more I realized that the real thing I'm requesting is even simpler to demonstrate:

protocol A {
    associatedtype Foo
}


protocol B: A {
    var foo: Foo { get }
}


struct C: B {
    
    var foo: Int {
        0
    }
}

Shouldn't the compiler be able to infer the type of Foo based on the implementation of C?

— ORIGINAL POST:

Imagine I have a protocol A:

protocol A {
    
    associatedtype B
    
    associatedtype C: Collection where C.Element == B
    
    var c: C { get }
}

I was hoping to be able to conform to it like this:

struct X: A {
    
    var c: [Int] {
        [1, 2, 3]
    }
}

Is this manner of type inference supposed to work?

I understand that another approach in this case would be to remove B as an associated type and make it a typealias:

protocol A {
    
    associatedtype C: Collection
    
    var c: C { get }
}


extension A {
    typealias B = C.Element
}


struct X: A {
    
    var c: [Int] {
        [1, 2, 3]
    }
}

but this doesn't help me because in my real-world example the "sibling" associated type ("B") is actually inherited from the parent protocol, and must be satisfied. Here's a different version of my same request. I would like for this code to compile:

protocol A {
    associatedtype X
}

protocol B: A {

    associatedtype C: Collection where C.Element == X
    
    var c: C { get }
}

struct Y: B {

    var c: [Int] {
        [1, 2, 3]
    }
}