RFR (15): JDK-8242214: NullPointerException in JDK 14 javac compiling a method reference

[Jan Lahoda]

The patch that provoked this was JDK-8234729:
https://bugs.openjdk.java.net/browse/JDK-8234729

Before that, a method reference like this would not be unrolled into a 
lambda, and all would work. But now more method references are unrolled 
into lambdas, which is triggering the bug.

Jan

On 17. 06. 20 18:15, Vicente Romero wrote:
> Hi Jan,
> 
> Out of curiosity, as this is a regression, do we know what was the 
> change that introduced the issue and what was the related code like 
> before the change that provoked the regression?
> 
> Thanks,
> Vicente
> 
> On 6/17/20 9:15 AM, Jan Lahoda wrote:
>> Hi,
>>
>> There is a tricky issue related to unbound instance method references 
>> to a protected method in a superclass.
>>
>> Consider:
>> ---base/Base.java:
>> package base;
>>
>> public class Base {
>>     protected void test() {}
>> }
>> ---impl/Impl.java:
>> package impl;
>>
>> import base.Base;
>> import java.util.function.Consumer;
>>
>> public class Impl extends Base {
>>     Consumer<Impl> consumer() {
>>         return Impl::test;
>>     }
>> }
>> ---
>>
>> Since JDK-8234729, javac tries to expand the method reference to a 
>> lambda:
>> (/*synthetic*/ Object rec$)->((Base)rec$).test()
>>
>> But this is not quite right (and crashes javac) - Impl cannot invoke 
>> test on "Base":
>> ((Base) rec$).test();
>>
>> it needs to invoke (if I understand JLS 6.6.2.1. correctly) test on 
>> "Impl":
>> ((Impl) rec$).test();
>>
>> So, the expanded lambda for the method reference should be:
>> (/*synthetic*/ Object rec$)->((Impl)rec$).test()
>>
>> Which is what the proposed patch is trying to do.
>>
>> Webrev:
>> http://cr.openjdk.java.net/~jlahoda/8242214/webrev.00/
>>
>> JBS:
>> https://bugs.openjdk.java.net/browse/JDK-8242214
>>
>> How does this look?
>>
>> Thanks,
>>     Jan
>