tested with pony 0.27.0-95d94ac [release] on Windows 10

horse_tranquilizer.pony

trait val Tr is Stringable
    fun string(): String iso ^

class val Y is Tr
    new val create() => None
    fun string(): String iso ^ => "Y".string()

// if this is only Tr, it won't crash
// it this is (Y | None), it won't crash
type Exp is (Tr | None)
//type Exp is (Y | None)

primitive Indirection
    fun apply(): Exp val => Y

actor Main
    new create(env: Env) =>
        // if this is set to Y, instead of Indirection(), it won't crash
        let testerino: Exp = Indirection()

        // if we dont run testerino.string(), it won't crash
        env.out.print(testerino.string())

        env.out.print("One would assume that you'll see this line")