java.lang.ClassFormatError: Method foo in class CLibrary has illegal modifiers: 0x101

[michelou]

Minimized code

trait CLibrary {
  @native
  def foo(x: Int): Unit
}

object I9432 {
  def main(args: Array[String]): Unit = {
    new CLibrary {
      override def foo(x: Int): Unit = println(x)
    }.foo(1)
  }
}

Output

$ java -version
openjdk version "11.0.8" 2020-07-14

$ C:\opt\dotty-0.26.0-RC1\bin\dotc.bat -deprecation -feature -d target\classes src\main\scala\I9432.scala

$ C:\opt\dotty-0.26.0-RC1\bin\dotr.bat -classpath target\classes I9432
Exception in thread "main" java.lang.ClassFormatError: Method foo in class CLibrary has illegal modifiers: 0x101
        at java.base/java.lang.ClassLoader.defineClass1(Native Method)
        at java.base/java.lang.ClassLoader.defineClass(ClassLoader.java:1017)
        at java.base/java.security.SecureClassLoader.defineClass(SecureClassLoader.java:174)
        at java.base/jdk.internal.loader.BuiltinClassLoader.defineClass(BuiltinClassLoader.java:800)
        at java.base/jdk.internal.loader.BuiltinClassLoader.findClassOnClassPathOrNull(BuiltinClassLoader.java:698)
        at java.base/jdk.internal.loader.BuiltinClassLoader.loadClassOrNull(BuiltinClassLoader.java:621)
        at java.base/jdk.internal.loader.BuiltinClassLoader.loadClass(BuiltinClassLoader.java:579)
        at java.base/jdk.internal.loader.ClassLoaders$AppClassLoader.loadClass(ClassLoaders.java:178)
        at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:522)
        at I9432.main(I9432.scala)

$ javap -cp target\classes CLibrary
Compiled from "I9432.scala"
public interface CLibrary {
  public static void foo$(CLibrary, int);
  public native default void foo(int);
}

Expectation

Modifiers native and default (since Java 8) are mutual exclusive (see above javap output).

Source file src\main\scala\I9432.scala should not compile.

[smarter]

FYI, in scalac this fails with:

A trait cannot define a native method.

So we're just missing a check that leads to an error (which could be done in RefChecks, like it is in scala 2)

[michelou]

Quick code revue gives:

• Scala 2: RefChecks.scala:1662
• Scala 3: Checking.scala:467

@smarter In method checkWellFormed(sym: Symbol) most fail(..) invocations take a Message as argument; what is the rule for adding new error messages like this one (file messages.scala is a minefield..) ?!

NB. The above minimized code does not provide the whole picture of my original code example which makes use of the JNA library. In my case the Java version works fine and I still have issues with the native modifier and varargs when using Dotty.

[smarter]

We try to make new message case class for new error messages, but you can instead just pass a String (there's an implicit conversion from String to Message).

[tudorv91]

Hi @smarter @michelou . I have created a PR for this issue here: #9799 . Can you have a look?