import language.experimental.captureChecking trait IO: def use(): Unit case class Boxed[+T](unbox: T) // `foo` is a function that unboxes its parameter // and uses the capability boxed inside the parameter. def foo(x: Boxed[IO^]): Unit = x.unbox.use() // error: local reach capability {x*} leaks // `bar` is a function that does the same thing in a // slightly different way. // But, no type error reported. def bar(x: Boxed[IO^]): Unit = val x1: Boxed[IO^] = x val io = x1.unbox io.use()

-- Error: issues/localreach.scala:9:4 ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- 9 | x.unbox.use() // error: local reach capability {x*} leaks | ^^^^^^^ | Local reach capability x* leaks into capture scope of method foo