Error compiling with JDK11 when a case class does not have its companion object

[julianmendez]

problem

If a case class does not have a companion object, the following error message appears:

 error: illegal combination of modifiers: abstract and static
[ERROR]   static public abstract  R apply ()  ;
[ERROR]                             ^

In JDK 8 using abstract and static is considered a warning. In JDK 11 this is an error.

expectation

A case class can be used without needing a companion object.

reproduction

To reproduce this issue, it is enough to create a file containing:

case class MyCaseClass() {}

However, the following does not have any problems:

case class MyCaseClass() {}

object MyCaseClass {}

In the generated Java code, one can see the difference. In the first case we see:

public  class MyCaseClass implements scala.Product, java.io.Serializable {
  static public abstract  R apply ()  ;
  static public  java.lang.String toString ()  { throw new RuntimeException(); }
  public   MyCaseClass ()  { throw new RuntimeException(); }
}

In the second case we see:

public  class MyCaseClass implements scala.Product, java.io.Serializable {
  public   MyCaseClass ()  { throw new RuntimeException(); }
}

configuration

This was compiled with:

• genjavadoc 0.15
• org.scala-tools.maven-scala-plugin 2.15.2
• net.alchim31.maven.scala-maven-plugin 4.3.0
• org.apache.maven.plugins.maven-javadoc-plugin 3.1.1
• org.apache.maven.plugins.maven-compiler-plugin 3.8.1
• Apache Maven 3.6.0
• Java 11.0.5
• Scala 2.13.1

related issues

These issues could be related:
#181
#154