No generic signature emitted for trait getter

[smarter]

Given:

trait Foo {
  val bla: List[String] = List("foo")
}
class Bar extends Foo

Decompiling with cfr shows:

public interface Foo {
    default public void $init$() {
    }

    public List<String> bla();

    default public List initial$bla() {
        return package$.MODULE$.List().apply((Seq)Predef$.MODULE$.wrapRefArray((Object[])new String[]{"foo"}));
    }
}

public class Bar
implements Foo {
    private final List bla;

    public Bar() {
        this.bla = Foo.super.initial$bla();
        Foo.super.$init$();
    }

    public List bla() {
        return this.bla;
    }
}

Notice the lack of generic signature on bla() in Bar, since trait getters are emitted in Mixin after erasure, getting a generic signature is not trivial. For mixin forwarders we work around this by emitting them as bridges (cf 6d0f9ca), but we should probably not do that for trait setters since they are not legitimately bridges.

We could also try to move trait getter (and setter, and maybe more stuff) before erasure since that's what scalac appear to be doing.

[smarter]

The lack of generic signature for trait getters leads to the following warning when compiling dotty-sbt-bridge in the Dotty build:

> dotty-sbt-bridge/compile
[info] Updating dotty-sbt-bridge...
[info] Done updating.
[info] Compiling 9 Java sources to /home/smarter/opt/dotty/sbt-bridge/src/target/classes ...
[warn] /home/smarter/opt/dotty/sbt-bridge/src/xsbt/DelegatingReporter.java:24:1: dotty$tools$dotc$reporting$UniqueMessagePositions$$positions() in dotty.tools.dotc.reporting.AbstractReporter implements dotty$tools$dotc$reporting$UniqueMessagePositions$$positions() in dotty.tools.dotc.reporting.UniqueMessagePositions
[warn]   return type requires unchecked conversion from scala.collection.mutable.HashMap to scala.collection.mutable.HashMap<scala.Tuple2<dotty.tools.dotc.util.SourceFile,java.lang.Object>,java.lang.Object>
[warn] final public class DelegatingReporter extends AbstractReporter {
[info] Done compiling.

[odersky]

We could also try to move trait getter (and setter, and maybe more stuff) before erasure since that's what scalac appear to be doing.

Note that the mixin in scalac is somewhat of a mess, precisely because there is no good time to do it. So we should step very carefully before trying to copy things.

[lrytz]

This also shows up in Akka. Looking at the bytecode, the trait getter, setter and also the field are lacking signatures.

trait L[+T]
trait T { private var transitionEvent: L[String] = ??? }
class C extends T

public class Test extends C { }

Test.java:1: warning: [unchecked] T$$transitionEvent() in C implements T$$transitionEvent() in T
public class Test extends C {
       ^
  return type requires unchecked conversion from L to L<String>

Note that the mixin in scalac is somewhat of a mess

Mixin got a good overhaul for 2.12 in scala/scala#5141 (and, I guess, some follw-ups).

[michelou]

Here is a minimization of the warning mentioned by @smarter (and taken from sbt-bridge); it consists of 4 files which are compiled in 2 steps (see below) :

Source file src\main\scala\Reporter.scala :

// minimized from compiler/src/dotty/tools/dotc/reporting/Reporter.scala

abstract class Reporter // extends interfaces.ReporterResult

Source file src\main\scala\UniqueMessagePositions.scala :

// minimized from compiler/src/dotty/tools/dotc/reporting/UniqueMessagePositions.scala

import scala.collection.mutable

trait UniqueMessagePositions extends Reporter {
  private val positions = new mutable.HashMap[(/*SourceFile*/String, Int), Int]
}

Source file src\main\scala\AbstractReporter.scala :

// minimized from compiler/src/dotty/tools/dotc/reporting/AbstractReporter.scala

abstract class AbstractReporter extends Reporter with UniqueMessagePositions

Java source file src\test\java\DelegatingReporter.java

// minimized from sbt-bridge/src/dotty/tools/xsbt/DelegatingReporter.java

final public class DelegatingReporter extends AbstractReporter {
}

2-steps compilation (Java 1.8.0u302 and Scala 3.0.2-RC2 on Windows):

call %SCALA3_HOME%\bin\scalac -d target\classes src\main\scala\AbstractReporter.scala src\main\scala\Reporter.scala src\main\scala\UniqueMessagePositions.scala

%JAVA_HOME%\bin\javac.exe -Xlint:unchecked -cp %SCALA3_HOME%\lib\scala-asm-9.1.0-scala-1.jar;%SCALA3_HOME%\lib\scala-library-2.13.6.jar;%SCALA3_HOME%\lib\scala3-compiler_3-3.0.2-RC2.jar;%SCALA3_HOME%\lib\scala3-interfaces-3.0.2-RC2.jar;%SCALA3_HOME%\lib\scala3-library_3-3.0.2-RC2.jar;target\classes -d target\classes src\test\java\DelegatingReporter.java

Output :

src\test\java\DelegatingReporter.java:3: warning: [unchecked] UniqueMessagePositions$$positions() in AbstractReporter implements UniqueMessagePositions$$positions() in UniqueMessagePositions
final public class DelegatingReporter extends AbstractReporter {
             ^
  return type requires unchecked conversion from HashMap to HashMap<Tuple2<String,Object>,Object>
1 warning

[odersky]

Is this still an issue after we changed the mixin scheme?

[smarter]

Yes, this issue is still present.

[smarter]

It's possible we could fix this just by emitting the getter/setter in the subclasses with ACC_SYNTHETIC (in my initial comment I mentioned emitting methods as bridges but I was confused: the only flag javac cares about is ACC_SYNTHETIC which we can emit on any method). However, using ACC_SYNTHETIC when implementing trait methods can lead to other issues as tracked in #13104