better assertion message, getSuperInterfaces()

Author: magarciaEPFL

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

@@ -1210,12 +1210,28 @@ abstract class BCodeTypes extends SubComponent with BytecodeWriters {
       sups.distinct
     };
 
-    assert(!superInterfaces.contains(NoSymbol), "found NoSymbol among: " + superInterfaces.mkString)
-    assert(superInterfaces.forall(s => s.isInterface || s.isTrait), "found non-interface among: " + superInterfaces.mkString)
+        def checkSuperIfaces() {
+
+          assert(!superInterfaces.contains(NoSymbol), s"found NoSymbol among: ${prettyPrintFullnames(superInterfaces)}")
+
+          val nonIfaces = superInterfaces.filter(s => !s.isInterface && !s.isTrait)
+          assert(
+            nonIfaces.isEmpty,
+            s"found non-interfaces ${prettyPrintFullnames(nonIfaces)} among: ${prettyPrintFullnames(superInterfaces)}"
+          )
+
+        }
+
+    checkSuperIfaces()
 
     minimizeInterfaces(superInterfaces)
   }
 
+  def prettyPrintFullnames(syms: List[Symbol]): String = {
+    val lst = mapWithIndex(syms)({ case (sym, idx) => s"( $idx : ${sym.fullName})" })
+    lst.mkString
+  }
+
   final def exemplarIfExisting(iname: String): Tracked = {
     val bt = lookupRefBTypeIfExisting(iname)
     if(bt != null) exemplars.get(bt)