Add all trait ancestors to the list of parents in bytecode

... to allow us to call arbitrary super methods with invokespecial

Author: retronym

src/compiler/scala/tools/nsc/backend/jvm/BTypesFromSymbols.scala

@@ -251,7 +251,11 @@ class BTypesFromSymbols[G <: Global](val global: G) extends BTypes {
         //  class C extends Object with A  // minimizeParents removes Object
         ifs
     }
-    interfaces.map(_.typeSymbol)
+    // Add all mixin traits as direct parents of classes so we can emit arbitrary super calls to default methods with invokespecial
+    // TODO also reflect this in the generic signatures (?)
+    val mixinTraitsOfClass = if (!classSym.isJavaDefined && !classSym.isTrait) classSym.ancestors.filter(_.isTrait) else Nil
+    (interfaces.map(_.typeSymbol) ::: mixinTraitsOfClass).distinct
+
   }
 
   /**