Protocol conformance error

Using Swift
1

Hi,

Here is some sample code that gives a protocol conformance error in a
playground:

protocol A {}
protocol B: A {}

protocol C {
    func test(x: A)
}

class M: C {
    func test(x: B) {}
}

Is there a reason why the compiler doesn't infer that ((B) -> ())
matches ((A) -> ()) because of inheritance?

···

--
Warm regards
Roshan

2

Because it would break this:

func foo<T: C, U: A> (c: T, a: U) { c.test(a) }

Nevin

···

On Wed, Jan 17, 2018 at 2:29 AM, Roshan via swift-users < swift-users@swift.org> wrote:

Hi,

Here is some sample code that gives a protocol conformance error in a
playground:

protocol A {}
protocol B: A {}

protocol C {
    func test(x: A)
}

class M: C {
    func test(x: B) {}
}

Is there a reason why the compiler doesn't infer that ((B) -> ())
matches ((A) -> ()) because of inheritance?

--
Warm regards
Roshan
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https://lists.swift.org/mailman/listinfo/swift-users

3

In simpler terms: test(x: B) will work only for Bs, which makes it more restrictive compared to what C requires (all kinds of As).
Maybe you want something more like

protocol C {
  associatedtype T: A
  func test(x: T)
}

but still introduce associatedtypes only if strictly necessary :)