Fix bug in minimizeParents, keep all java interfaces

Author: lrytz

src/compiler/scala/tools/nsc/transform/Erasure.scala

@@ -195,17 +195,20 @@ abstract class Erasure extends InfoTransform
    * as an immediate parent to support an `invokespecial`.
    */
   def minimizeParents(parents: List[Type]): List[Type] = if (parents.isEmpty) parents else {
-    def isRedundantParent(sym: Symbol) = sym.isInterface || sym.isTrait
+    def isRedundantParent(parent: Symbol, candidate: Symbol) =
+      !parent.isJavaDefined &&
+        parent.isTraitOrInterface &&
+        candidate.isSubClass(parent)
 
     var rest   = parents.tail
     var leaves = collection.mutable.ListBuffer.empty[Type] += parents.head
-    while(rest.nonEmpty) {
+    while (rest.nonEmpty) {
       val candidate = rest.head
       if (candidate.typeSymbol.isJavaDefined && candidate.typeSymbol.isInterface) leaves += candidate
       else {
         val nonLeaf = leaves exists { t => t.typeSymbol isSubClass candidate.typeSymbol }
         if (!nonLeaf) {
-          leaves = leaves filterNot { t => isRedundantParent(t.typeSymbol) && (candidate.typeSymbol isSubClass t.typeSymbol) }
+          leaves = leaves filterNot { t => isRedundantParent(t.typeSymbol, candidate.typeSymbol) }
           leaves += candidate
         }
       }

test/junit/scala/lang/traits/BytecodeTest.scala

@@ -379,6 +379,19 @@ class BytecodeTest extends BytecodeTesting {
     assertEquals(cls, Nil)
   }
 
+  @Test
+  def noMinimizeJavaInterfaces(): Unit = {
+    val jCode = List("interface T { default int f() { return 1; } }" -> "T.java")
+    val code =
+      """trait U extends T { override def f() = 2 }
+        |class C extends T with U { def t = super[T].f }
+      """.stripMargin
+    val List(c, u) = compileClasses(code, jCode)
+    assertEquals(c.interfaces.asScala.toList.sorted, List("T", "U"))
+    val ins = getMethod(c, "t").instructions
+    assert(ins contains Invoke(INVOKESPECIAL, "T", "f", "()I", true), ins.stringLines)
+  }
+
   def ifs(c: ClassNode, expected: List[String]) = assertEquals(expected, c.interfaces.asScala.toList.sorted)
   def invSt(m: Method, receiver: String, method: String = "f$", itf: Boolean = true): Unit =
     assert(m.instructions contains Invoke(INVOKESTATIC, receiver, method, s"(L$receiver;)I", itf), m.instructions.stringLines)