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.
--
Mark Raynsford | https://www.io7m.com
