@_dynamicReplacement(for: selfRec(x: acc:))
func selfRec_r(x: Int, acc: Int) -> Int {
if x <= 0 {
return acc
}
// Normally, this will call selfRec(x: acc:)'s implementation.
// With the option, this will call to selfRec_r(x: acc:).
return selfRec(x: x - 1, acc: acc + 1)
}
rdar://51229650
