duplicates:

• SR-5016 Swift 4: Unable to satisfy associated type requirements implicitly

relates to:

• SR-5166 typealias prevents compilation of generic protocol implementation

Issue Description:

SE-0110 seems to be confusing the compiler in this situation:

• a protocol has an associated type

• the protocol requires a method whose argument is a function

• the argument of the methods's function is the associated type.

Under those conditions, the associated type seems to be inferred to be a "single element tuple" (which doesn't exist in Swift).

protocol A {
    associatedtype E
    func f(_: (E) -> Void)
}

// error: type 'B<E>' does not conform to protocol 'A'
// note: candidate has non-matching type '<E> ((E) -> Void) -> ()' [with E = (E)]
struct B<E>: A {
    func f(_: (E) -> Void) {
    }
}

// error: type 'C<V>' does not conform to protocol 'A'
// note: candidate has non-matching type '<V> ((V) -> Void) -> ()' [with E = (V)]
struct C<V>: A {
    func f(_: (V) -> Void) {
    }
}

Note that the compiler fix-it offers to create a stub for the function it thinks to be missing, which however causes another compile error:

    // error: invalid redeclaration of 'f'
    func f(_: ((E)) -> Void) {
    }

the workaround is to explicitly declare a typealias for the associated type:

// OK
struct D<V>: A {
    typealias E = V
    func f(_: (V) -> Void) {
    }
}