RFR: JDK-8207224: Javac compiles source code despite illegal use of unchecked conversions

[Liam Miller-Cushon]

Hi Vicente,

I did some testing with your fix to get a sense of the source compatibility
impact, and it looks minor: I only found a handful of examples that are
rejected after the change.

One of them is in icu4j. Is this change an expected result of the fix?

icu/icu4j/main/classes/core/src/com/ibm/icu/impl/locale/XLikelySubtags.java:47:
error: make() in <anonymous com.ibm.icu.impl.locale.XLikelySubtags$Maker$1>
cannot override <V>make() in Maker
             public Map<Object,Object> make() {
                                       ^
   return type Map<Object,Object> is not compatible with V
   where V is a type-variable:
     V extends Object declared in method <V>make()
...
https://github.com/unicode-org/icu/blob/master/icu4j/main/classes/core/src/com/ibm/icu/impl/locale/XLikelySubtags.java#L47

Thanks,

On Sat, Dec 1, 2018 at 11:47 AM Vicente Romero <vicente.romero at oracle.com>
wrote:

> Please review fix for [1] at [2]. Javac is accepting this program as valid:
>
> class ReturnTypeSubstitutableTest {
>     abstract class AbstractDemo<Request extends AbstractResult, Response
> extends AbstractResult> {
>         protected abstract Response test(Request request);
>     }
>
>     abstract interface AbstractResult {}
>
>     abstract interface SimpleResult extends AbstractResult {}
>
>     class Result1 implements SimpleResult {}
>
>     class OtherResult implements AbstractResult {}
>
>     public class SimpleDemo<Request extends AbstractResult, Response
> extends AbstractResult> extends AbstractDemo<Request, Response> {
>         @Override
>         protected SimpleResult test(AbstractResult request) {
> <----------- this method is accepted as a good override for
> AbstractDemo::test
>             return null;
>         }
>     }
> }
>
> From Dan's evaluation at the bug entry [1]:
>
> Per JLS 8.4.8.3, the first method must be return-type-substitutable for
> the second. This can be satisfied in 3 ways (JLS 8.4.5):
> - SimpleResult is a subtype of Response (no)
> - SimpleResult can be converted to a subtype of Response by unchecked
> conversion (no)
> - The methods have different signatures (yep) and SimpleResult =
> erasure(Response) (no)
>
> No case applies, so an error should occur. This fix syncs javac with the
> spec.
>
> Thanks,
> Vicente
>
> [1] https://bugs.openjdk.java.net/browse/JDK-8207224
> [2] http://cr.openjdk.java.net/~vromero/8207224/webrev.00/
>
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