[SR-1581] Can not use protocol to fulfill associatedtype requirement where associatedType has protocol constraint

[swift-ci]

Previous ID	SR-1581
Radar	None
Original Reporter	vishal.panwar (JIRA User)
Type	Bug
Status	Resolved
Resolution	Duplicate

Additional Detail from JIRA

Votes	10
Component/s	Compiler
Labels	Bug
Assignee	None
Priority	Medium

md5: d2a9b745ccd1923da6ccec087b0df3f9

duplicates:

• SR-55 non-@objc protocol existentials do not conform to their own protocol type

is duplicated by:

• SR-11340 Compilation error when there shouldn't be one

relates to:

• SR-55 non-@objc protocol existentials do not conform to their own protocol type
• SR-2020 Protocol composition doesn't conform to associated type requirement

Issue Description:

The following code sample does not compile.

protocol Bar {   
}

protocol Buz: Bar {   
}

protocol Foo {
    associatedtype SomeType: Bar
    func someOperation(someType: SomeType)
}

struct Quack: Foo {
    func someOperation(someType: Buz) {
    }
}

It throws error:

Inferred type 'Buz' (by matching requirement 'someOperation') is invalid: does not conform to 'Bar'

I am not sure if we are only allowed to use a concrete type for associatedtype with protocol constraint. I don't see any reason for this restriction. If we remove the protocol constraint from associatedtype, then it compiles with no error.

Update 2016/12/12: I am using below workaround to tackle above bug. This seems more logical and makes me feel like its not a bug.

struct Quack<T>: Foo where T: Buz {
    func someOperation(someType: T) {
    }
}

// Usage
struct SomeBuz: Buz {}
let quack: Quack<SomeBuz> = Quack<SomeBuz>()
quack.someOperation(SomeBuz())

The reason this makes more sense to me is that the compiler now has more context on the actual type.

[belkadan]

Protocols don't always "conform to themselves", but this configuration seems safe. @jckarter?

[karwa]

+1 for this. The thing that we want to express is safe; if there are nuances with the current syntax we should devise a way to make it possible.

protocol A {}
protocol SubA : A {}

protocol B { associatedtype TypeOfA : A }

struct MyB : B {     // ERROR: `MyB` does not conform to `B`
    typealias TypeOfA = SubA
}