Regression for java compatibility in multiple `akka` / `pekko` projects

[WojciechMazur]

Based on the OpenCB failures found in following projects:

• apache/pekko-http - build logs
• akka/akka-http - build logs
• apache/pekko-connectors - build logs
• akka/akka
• apache/pekko

Compiler version

Last good release: 3.8.0-RC1-bin-20250921-834e8e9-NIGHTLY
First bad release: 3.8.0-RC1-bin-20250922-9bd7774-NIGHTLY
Bisect points to: f4330f0 (most likely related to the new stdlib)

Minimized code

final class Flow[In, Out]:
  def collect[T](pf: PartialFunction[Out, T]): Flow[In, T] = ???

object Flow:
  def create[T](): Flow[T, T] = ???
  
  
abstract class Message:
  def asTextMessage: TextMessage
  
abstract class TextMessage extends Message

abstract class JavaPartialFunction[A, B] extends PartialFunction[A, B]:
  @throws(classOf[Exception]) // required, compiles without annotation
  def apply(x: A, isCheck: Boolean): B
  
  final def isDefinedAt(x: A): Boolean = ???
  final override def apply(x: A): B = ???
  final override def applyOrElse[A1 <: A, B1 >: B](x: A1, default: A1 => B1): B1 = ???

public class JavaTestServer {
  public static Flow<Message, Message> greeter() {
    return Flow.<Message>create()
        .collect(
            new JavaPartialFunction<Message, Message>() {
              @Override
              public Message apply(Message msg, boolean isCheck) throws Exception {
                if (isCheck)  throw new RuntimeException(); 
                else return handleTextMessage(msg.asTextMessage());
              }
            });
  }

  public static TextMessage handleTextMessage(TextMessage msg) {
    return null;
  }
}

Output

Compiling project (Scala 3.8.0-RC1-bin-20251004-9f70200-NIGHTLY, JVM (21))
[error] ./JavaTestServer.java:5:57
[error]  error: name clash: compose(scala.Function1<A#1,Message>) in Function1 and compose(scala.Function1<A#2,A#2>) in JavaPartialFunction have the same erasure, yet neither overrides the other
[error]             new JavaPartialFunction<Message, Message>() {
[error]                                                         ^
Warning: javac exited with exit code 1

scala-cli trimmed the output, here's the full, original output:

[error] /build/repo/http-core/src/test/java/org/apache/pekko/http/JavaTestServer.java:69:57: <R>compose(scala.PartialFunction<R,T1>) in scala.runtime.AbstractPartialFunction cannot implement <R>compose(scala.PartialFunction<R,A>) in scala.PartialFunction
[error]   return type scala.PartialFunction<R,R> is not compatible with scala.PartialFunction<R,org.apache.pekko.http.javadsl.model.ws.Message>
[error] {
[error]               @Override
[error]               public Message apply(Message msg, boolean isCheck) throws Exception {
[error]                 if (isCheck)
[error]                   if (msg.isText()) return null;
[error]                   else throw noMatch();
[error]                 else return handleTextMessage(msg.asTextMessage());
[error]               }
[error]             }
[error]             

Notes

Original code refered to scala.runtime.AbstractPartialFunction (which is unsafe) but the same error can be reproduced when refering to public scala.PartialFunction

Expectation

Should compile