----- Mail original -----
> De: "mark" <m...@io7m.com>
> À: "amber-spec-experts" <amber-spec-experts@openjdk.java.net>
> Envoyé: Dimanche 27 Décembre 2020 22:04:43
> Objet: Unexpected compilation error with generic sealed interface
> Hello!
>
> A friend of mine handed me this example that fails to compile using
> JDK 17 EA 3:
>
> ~~
> final class SealedExample
> {
> private SealedExample()
> {
>
> }
>
> // Compiles if you remove `sealed` or if `I` is not generic.
> sealed interface I<T> {
> final class C implements I<Object> { }
> }
>
> static void f(final I<Object> x) {
> if (x instanceof I.C) {
>
> }
> }
> }
> ~~
>
> The error is:
>
> ~~
> src/main/java/SealedRecord.java:14: error: incompatible types:
> I<Object> cannot be converted to C if (x instanceof I.C) {
> ^
> 1 error
> ~~
>
> The error goes away if you remove the word "sealed" from the
> interface. In her words: "It breaks simple things like Option.".
>
> I can see the reasoning required on behalf of the compiler: You've
> handed me an I<Object>, and by the definition of I, there's exactly one
> class that could yield an I<Object>: C.
>
> I'm unsure of whether the compiler should be rejecting these
> definitions or not.
Hi Mark,
this is an interesting snippet, while i struggle to understand why someone want
to write a code like that, I is parametrized by T but the only possible
subclass implements I<Object> so T is useless here,
anyway, the compiler should not reject that code so it's a bug.
I've also tested with latest Eclipse and IntelliJ and in both cases it compiles
fine.
>
> --
> Mark Raynsford | https://www.io7m.com
Rémi
