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

[Maurizio Cimadamore]

Looks good - btw, if the compatibility cost proves to be too much we 
could consider putting the new behavior behind the source switch (as we 
did with override/hide string clash checking for some time).

Cheers
Maurizio

On 03/12/2018 21:01, Vicente Romero wrote:
> Hi,
>
> Please review the related CSR [1]
>
> Thanks,
> Vicente
>
> [1] https://bugs.openjdk.java.net/browse/JDK-8214729
>
> On 12/1/18 2:46 PM, Vicente Romero 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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