Incompilable instanceof with super-bound

[Tagir Valeev]

Hello, Maurizio!

Thank you for the quick response.

I added the main() method just to check whether the resulting program is
actually compilable and runnable by Eclipse compiler (and it is). If
there's another problem there, then probably it requires separate
investigation, so let's put it aside for now. For this particular issue,
let's replace the main method with

System.out.println(isSub(null));

With best regards,
Tagir Valeev.

On Thu, Mar 16, 2023 at 11:21 AM Maurizio Cimadamore <
maurizio.cimadamore at oracle.com> wrote:

> It seems an issue with cast conversion? E.g. the following:
>
> Sub<?> ss = (Sub<?>)next;
>
> Also fails (with same error). Now, there are two questions here:
>
>    1. should the cast be statically rejected - this means asking whether
>    Sub<?> and Super<? super Box<?>> are provably distinct parameterized
>    types;
>    2. should the cast be allowed without warnings - this means proving
>    that there is only one possible way to go from a Super<? super Box<?>>
>    down to a Sub<?>;
>
> But, before I try to reply any of this questions - can you please confirm
> me that there is another error in this program - namely with isSub(new
> Sub<>()) (I seem to get an inference failure here for the diamond
> expression).
>
> Maurizio
>
> On 16/03/2023 10:01, Tagir Valeev wrote:
>
> Hello!
>
> We've stumbled with the following code sample:
>
> class Test {
>     public static boolean isSub(Super<? super Box<?>> next) {
>         return next instanceof Sub<?>;
>     }
>
>     static class Super<T> {}
>
>     static class Sub<T> extends Super<Box<T>> { }
>
>     static class Box<T> {}
>
>     public static void main(String[] args) {
> System.out.println(isSub(new Sub<>()));
> }
> }
>
> javac 21-ea+12-971 reports an error here:
>
> Test.java:3: error: incompatible types: Super<CAP#1> cannot be converted
> to Sub<?>
>         return next instanceof Sub<?>;
>                ^
>   where CAP#1 is a fresh type-variable:
>     CAP#1 extends Object super: Box<?> from capture of ? super Box<?>
> 1 error
>
> On the other hand, Eclipse compiler compiles this correctly. From my
> understanding of the specification, this code should be accepted. Could you
> please take a look?
>
> Thank you in advance,
> Tagir Valeev.
>
> ​
>
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