Compilation errors with latest nightly build

[ptziegler]

Consider the following snippet:

import java.util.Collections;
import java.util.List;
import java.util.function.BiConsumer;
import java.util.function.Function;

public class Foo {
    private List<? extends Bar> value;
    
    public void set(List<? extends Bar> value) {
        this.value = value;
    }
    
    public List<Bar> get() {
        return Collections.unmodifiableList(value);
    }
    
    public static void main(String[] args) {
        test(Foo::get, Foo::set);
    }
    
    public static <U> void test(Function<Foo, List<U>> getter, BiConsumer<Foo, List<? extends U>> setter) {
        // noop
    }
    
    public static interface Bar{}
}

This snippet compiles with the M1, but not with the I20250419-0820:

[iloveeclipse]

@ptziegler : which exact javac version is happy with this code? My java 21 javac says:

/usr/lib/jvm/java-21/bin/javac Foo.java
/data/runtime-Java_21/A/src/Foo.java:18: error: incompatible types: invalid method reference
        test(Foo::get, Foo::set);
                       ^
    incompatible types: List<CAP#1> cannot be converted to List<? extends Bar>
  where U is a type-variable:
    U extends Object declared in method <U>test(Function<Foo,List<U>>,BiConsumer<Foo,List<? extends U>>)
  where CAP#1 is a fresh type-variable:
    CAP#1 extends U from capture of ? extends U
Note: Some messages have been simplified; recompile with -Xdiags:verbose to get full output
1 error

@stephan-herrmann : please check whether the compilation error is expected (I guess yes).
Related PR introducing this error is #3915.

[ptziegler]

@ptziegler : which exact javac version is happy with this code? My java 21 javac says:

/usr/lib/jvm/java-21/bin/javac Foo.java
/data/runtime-Java_21/A/src/Foo.java:18: error: incompatible types: invalid method reference
        test(Foo::get, Foo::set);
                       ^
    incompatible types: List<CAP#1> cannot be converted to List<? extends Bar>
  where U is a type-variable:
    U extends Object declared in method <U>test(Function<Foo,List<U>>,BiConsumer<Foo,List<? extends U>>)
  where CAP#1 is a fresh type-variable:
    CAP#1 extends U from capture of ? extends U
Note: Some messages have been simplified; recompile with -Xdiags:verbose to get full output
1 error

@stephan-herrmann : please check whether the compilation error is expected (I guess yes). Related PR introducing this error is #3915.

I haven't compiled this with javac. This error appeared in my workspace after updating the IDE, meaning something that happened from one ECJ version to another.

However, I get the same error with javac, so I assume this regression is intentional/desirable.

[iloveeclipse]

However, I get the same error with javac, so I assume this regression is intentional/desirable.

This is my assumption. The original problem fixed was #3457.
But I let the generics guru give the final answer here :-)

[srikanth-sankaran]

But I let the generics guru give the final answer here

And if the generics guru confirms the behavior is desired/intended, it will actually progression and not regression :)

[stephan-herrmann]

The behavior change is caused by capturing type List<? extends U#1> yielding List<capture#4-of ? extends U#1>, during findSuperTypeOriginatingFrom(). This is in line with JLS §4.10.2 as pointed out in #3457 (comment)

Note to self: this test case also invokes the new InferenceContext18.maybeUncapture() from within TypeBinding.isTypeArgumentContainedBy(). There's a smell about this (invocation is marked "not backed by JLS"): in #3915 I gave the following excuse "uncapture in a situation that previously didn't capture in the 1.place". Unfortunately, the excuse doesn't apply here, the capture is not due to recent changes. OTOH, that uncapturing has no impact on the current test.

For comparison, here's the full error report from javac:

Foo.java:18: error: method test in class Foo cannot be applied to given types;
                                test(Foo::get, Foo::set);
                                ^
  required: Function<Foo,List<U>>,BiConsumer<Foo,List<? extends U>>
  found:    Foo::get,Foo::set
  reason: cannot infer type-variable(s) U
    (argument mismatch; invalid method reference
      method set in class Foo cannot be applied to given types
        required: List<? extends Bar>
        found:    Foo,List<? extends U>
        reason: argument mismatch; List<CAP#1> cannot be converted to List<? extends Bar>)
  where U is a type-variable:
    U extends Object declared in method <U>test(Function<Foo,List<U>>,BiConsumer<Foo,List<? extends U>>)
  where CAP#1 is a fresh type-variable:
    CAP#1 extends U from capture of ? extends U
1 error

While this has some "similarity" to ecj's report, I was puzzled by "reason: cannot infer type-variable(s) U". In fact, I am able to infer the type variable and when asking compilers to type check this modified invocation, both agree to my solution:

			        Foo.<Bar>test(Foo::get, Foo::set);

Sorry, type checking java generics between JLS and javac is not a hard science when wildcards / captures are involved.

So the best answer I can give is threefold:

• the relevant change from Method call compiles with javac but not ecj #3915 is backed by JLS
• the conclusions drawn by javac and ecj look "similar"
• but, as shown by the extended invocation, there is no hard reason why inference must fail.

Anyway, I'll push the new test and then call this issue done.

[srikanth-sankaran]

Should this be closed @stephan-herrmann - I see a PR with a junit merged in,