swtwsのexistentialの話、FixedWidthInteger.randomもそうだったなと思って調べたんですが https://github.com/apple/swift/blob/master/stdlib/public/core/Integers.swift.gyb#L2545-L2548

apple/swift

swift - The Swift Programming Language

3:56 PM

@swift-4.2.4

func testA(a: RandomNumberGenerator) { } func testB(a: inout RandomNumberGenerator) { print(a.next()) } var r: Random = Random.default testA(a: r) // OK testB(a: &r) // error: cannot pass immutable value as inout argument: implicit conversion from 'Random' to 'RandomNumberGenerator' requires a temporary