// *** before: ***
// either the exact type was spelled out, and the implementation leaked
// into the signature and into every caller
func makeOld() -> LazyMapSequence<Range<Int>, Int> {
    return (0..<3).lazy.map { $0 * 2 }
}
// or a protocol was returned, and then Self requirements were forbidden:
// a function could not return Equatable or Collection at all

// *** in version 5.1: ***
func make() -> some Collection {
    return (0..<3).lazy.map { $0 * 2 }
}

protocol Shape { func area() -> Double }
struct Circle: Shape { func area() -> Double { 3.14 } }

func shape() -> some Shape { Circle() }

// the caller knows only "some Shape", the compiler knows the exact type:
// no box, no dynamic dispatch, and Self requirements are allowed.
// The whole of SwiftUI stands on this: var body: some View