protocol P {
    associatedtype A: PA = Never where A.VA == B.A
    associatedtype B: PB
}

protocol PA {
    associatedtype VA = Self
    init(va: VA)
}
extension PA where VA == Self {
    init(va: VA) {
        self = va
    }
}
extension PA where VA == Never {
// note: when this is spelled
// init(va: VA) {
// the error does not occur
    init(va: Never) {
        switch va {}
    }
}
extension Never: PA {
    init(va: Never) {
        switch va {}
    }
}

protocol PB {
    associatedtype A = Void
}

struct S: P { // ERROR: 'P' requires the types 'Never' and 'SB.A' be equivalent
    typealias B = SB
    typealias A = B.A
}

struct SB: PB {
    enum A: PA {}
}