Fix #6152: dotc requires override to implement abstract java method

The reason is that bridges are allowed to participate in overriding
relationships. This commit does the same scalac does, that is,
prevent bridges from participation in such relationships.

Author: anatoliykmetyuk

compiler/src/dotty/tools/dotc/core/SymDenotations.scala

@@ -1141,7 +1141,7 @@ object SymDenotations {
      *  either as overrider or overridee.
      */
     final def memberCanMatchInheritedSymbols(implicit ctx: Context): Boolean =
-      !isConstructor && !is(Private)
+      !isConstructor && !is(Private | Artifact)
 
     /** The symbol, in class `inClass`, that is overridden by this denotation in class `siteClass`.*/
     final def overriddenSymbol(inClass: ClassSymbol, siteClass: ClassSymbol = owner.asClass)(implicit ctx: Context): Symbol =

compiler/src/dotty/tools/dotc/core/classfile/ClassfileConstants.scala

@@ -341,7 +341,7 @@ object ClassfileConstants {
       case JAVA_ACC_PRIVATE    => Private
       case JAVA_ACC_PROTECTED  => Protected
       case JAVA_ACC_FINAL      => Final
-      case JAVA_ACC_SYNTHETIC  => Synthetic
+      case JAVA_ACC_SYNTHETIC  => Synthetic | Artifact
       case JAVA_ACC_STATIC     => JavaStatic
       case JAVA_ACC_ABSTRACT   => if (isClass) Abstract else Deferred
       case JAVA_ACC_INTERFACE  => PureInterfaceCreationFlags | JavaDefined

tests/pos/i6152/A_1.java

@@ -0,0 +1,8 @@
+abstract class A {
+  public abstract Object f();
+
+  public static abstract class B extends A {
+    @Override
+    public abstract String f();
+  }
+}

tests/pos/i6152/Test_2.scala

@@ -0,0 +1,8 @@
+class C extends A.B {
+  def f() = "hello"
+}
+
+object Main extends App {
+  val c: A = new C
+  println(c.f())
+}