Doug Gregor's demonstration of conformance ambiguity from https://forums.swift.org/t/ergonomics-generic-types-conforming-in-more-than-one-way/34589/59

DougDemo.swift

// (A) Definition
protocol P {
    static var isEquatable: Bool { get }
}

// (B) Default implementation
extension P {
    static var isEquatable: Bool { false }
}

// (C) More-constrained default implementation
extension P where Self : Equatable {
    static var isEquatable: Bool { true }
}

// Phases of Doug's explanation, in order.
// Replace one false with true to experience earlier phases.
#if false
  // (D) Conformance
  extension Array : P {}
#elseif false
  // (D') Alternative, conditional conformance
  extension Array : P where Element: Equatable {}
#elseif false
  extension Array : P where Element: Equatable {}
  struct X: Equatable, Q {}
  let a = Array<X>() // Array<X>'s conformance to P should be ambiguous
#else
  // (D'') conformance with an ambiguity
  extension Array : P where Element: Equatable, Element: Q {}
#endif

protocol Q { }

// (E) Another constrained implementation
extension P where Self : Q {
    static var isEquatable: Bool { true }
}

// (F) Array conditionally conforms to Q
extension Array: Q where Element: Q { }