"Reference to X is ambiguous" compiling Java code

[ignasi35]

The Akka codebase includes some Java code resulting from code generation using protoc from .protobuf files. This Java code is kept under SCM (git) so it is not regenerated over and over.

A particular protobuf file uses protobuf inner messages that result into generating inner classes when producing Java classes for the protobuf messages. Each of the generated classes includes a Builder inner class.

Compiling code that uses those Builder classes produces the following compiler error:

[error] -- [E049] Reference Error: /Users/ignasi/git/github/akka/akka/akka-persistence-scala3/akka-persistence/src/main/java/akka/persistence/serialization/MessageFormats.java:4113:16 
[error] 4113 |      protected Builder newBuilderForType(
[error]      |                ^^^^^^^
[error]      |            Reference to Builder is ambiguous,
[error]      |            it is both defined in object AtLeastOnceDeliverySnapshot
[error]      |            and inherited subsequently in class UnconfirmedDelivery

when compiling this code.
NOTE: this error is a sample, there are a few more identical to that one.

I already tried compiling java sources first as suggested offline by some colleagues (@lrytz) but that workaround is not feasible in this particular codebase, unfortunately, since there are other parts of the build that require compiling Scala first. Using Mixed compile ordering doesn't work around the issue.

Compiler version

3.0.1-RC1

Minimized code

public final class MessageFormats {
    public  static final class AtLeastOnceDeliverySnapshot extends ... {
         public static final class Builder extends ... {
         }
         public  static final class UnconfirmedDelivery extends ... {
             public static final class Builder extends ... {
             }
             // code referring to `Builder` in this block will no longer 
             // compile in scala3
             @java.lang.Override
             protected Builder newBuilderForType(...) {
                  Builder builder = new Builder(parent);
                  return builder;
             }
             ...
         }
    }
}

Or probably smaller:

public final class A {
  public static final class C { }
  public static final class D {
    public static final class C { }
    public C foo() { return new C(); }
  }
}

Expectation

The compile can compile the Java code as it did in 2.13 and previous versions.

[odersky]

I can take a look at this

[lrytz]

See scala/scala#5332 and scala/scala#7671 for fixes in this area in scala/scala. At some point in the past, I think the strategy was to represent a Java class

class C {
  static class D
  D foo()
}

using an import

object C:
  class D
class C:
  import C._
  def foo(): D

but we changed this and now there is a java-specific lookup method javaFindMember. It seems dotty is using the "old" strategy.

[smarter]

It seems dotty is using the "old" strategy.

No we actually ported that change: #12884

[lrytz]

Ah, OK. The imports seem to be there after parsing though:

public final class A {
  public static final class C { }
  public static final class D {
    public static final class C { }
    public C foo() { return new C(); }
  }
}

object Test {
  def foo: A.D.C = (new A.D).foo()
}

$> sc3 A.java Test.scala -Xprint:parser
parsed:
package <empty> {
  object A() {
    <static> object C() {}
    final <static> class C private[this](x$1: _root_.scala.Unit) extends _root_.java.lang.Object {
      def <init>()
    }
    <static> object D() {
      <static> object C() {}
      final <static> class C private[this](x$1: _root_.scala.Unit) extends _root_.java.lang.Object {
        def <init>()
      }
    }
    final <static> class D private[this](x$1: _root_.scala.Unit) extends _root_.java.lang.Object {
      import D.*
      def <init>()
      def foo(): C = _root_.scala.Predef.???
    }
  }
  final class A private[this](x$1: _root_.scala.Unit) extends _root_.java.lang.Object {
    import A.*
    def <init>()
  }
}

[smarter]

I don't see them on master, so I assume you're using an older compiler.

[lrytz]

Allright, I tried 3.0.1

[smarter]

3.0.1 was branched more than six weeks ago so it's old news already ;)

[som-snytt]

sdkman is out of the loop.

➜  ~ sdk list scala
================================================================================
Available Scala Versions
================================================================================
 > * 3.0.0             * 2.12.10             2.11.6
   + 3.0.0-RC1           2.12.9              2.11.5
   + 3.0.0-M3          * 2.12.8              2.11.4
   + 3.0.0-M2          * 2.12.7              2.11.3
   + 3.0.0-M1          * 2.12.6            * 2.11.2