widen-unbox-widen vs JEP 455

[Stephan Herrmann]

Has anyone found the time to check which outcome is intended according to JEP 455?

Do I understand correctly, that the pattern 'int i' should always match at 
runtime in all cases of the below code?

Alternatively, should we propose to drop the rules for widen-then-unbox 
conversions, because any real benefit of such conversion would require that 
'final' is dropped from any of the boxing classes?

thanks,
Stephan

Am 01.09.24 um 10:44 schrieb Stephan Herrmann:
> I just learned that reference widening followed by unboxing (followed by 
> primitive widening) is deemed a relevant conversion [1]. So I made sure that ecj 
> can handle this in various situation, only to find out that this will make ecj 
> produce code that is behaviorally different from what javac emits.
> 
> Here's my test code challenging this aspect of JEP 455:
> //
> <T extends Short> void typeVariableSingle(T single) {
>      int i1 = single;
>      System.out.print(i1);
>      if (single instanceof int i)
>          System.out.print(i);
>      else
>          System.out.print('-');
>      switch (single) {
>          case int i -> System.out.print(i);
>          default -> System.out.print('-');
>      }
>      System.out.println();
> }
> <T extends Short> void typeVariableList(List<T> list) {
>      int i1 = list.get(0);
>      System.out.print(i1);
>      if (list.get(0) instanceof int i)
>          System.out.print(i);
>      else
>          System.out.print('-');
>      switch (list.get(0)) {
>          case int i -> System.out.print(i);
>          default -> System.out.print('-');
>      }
>      System.out.println();
> }
> void wildcard(List<? extends Short> list) {
>      int i1 = list.get(0);
>      System.out.print(i1);
>      if (list.get(0) instanceof int i)
>          System.out.print(i);
>      else
>          System.out.print('-');
>      switch (list.get(0)) {
>          case int i -> System.out.print(i);
>          default -> System.out.print('-');
>      }
>      System.out.println();
> }
> void main() {
>      Short s = 1;
>      typeVariableSingle(s);
>      typeVariableList(Collections.singletonList(s));
>      wildcard(Collections.singletonList(s));
> }
> //
> 
> compiled with ecj this gives
> 111
> 111
> 111
> 
> compiled with javac the program prints
> 111
> 1-1
> 1-1
> 
> Is this a simple bug in javac, or is there more to it?
> thanks
> Stephan
> 
> [1] see answers to 
> https://mail.openjdk.org/pipermail/amber-spec-experts/2024-August/004204.html