Fix #6152: Dotty tries to override Java bridge methods

Dotty overrides Java bridge methods when the user intends to override
the method for which the bridge was generated. This commit forbids
Dotty to override bridge methods altogether.

Author: anatoliykmetyuk

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

@@ -211,7 +211,7 @@ class ClassfileParser(
       if (method) Flags.Method | methodTranslation.flags(jflags)
       else fieldTranslation.flags(jflags)
     val name = pool.getName(in.nextChar)
-    if (!(sflags is Flags.Private) || name == nme.CONSTRUCTOR) {
+    if (!(sflags.is(Flags.Private) || sflags.is(Flags.Bridge)) || name == nme.CONSTRUCTOR) {
       val member = ctx.newSymbol(
         getOwner(jflags), name, sflags, memberCompleter, coord = start)
       getScope(jflags).enter(member)

tests/pos/i6152/A_1.java

@@ -0,0 +1,8 @@
+abstract class A_1 {
+  public abstract Object f();
+
+  public static abstract class B extends A_1 {
+    @Override
+    public abstract String f();
+  }
+}

tests/pos/i6152/B_2.scala

@@ -0,0 +1,3 @@
+class C extends A_1.B {
+  def f() = "hello"
+}