Fix problem with parameterized enums

The logic for generating the result type of apply was
wrong if value parameters were passed to the enum class.
Test case in objXfun.scala.

Author: odersky

compiler/src/dotty/tools/dotc/ast/Desugar.scala

@@ -431,11 +431,18 @@ object desugar {
     // For all other classes, the parent is AnyRef.
     val companions =
       if (isCaseClass) {
+        def extractType(t: Tree): Tree = t match {
+          case Apply(t1, _) => extractType(t1)
+          case TypeApply(t1, ts) => AppliedTypeTree(extractType(t1), ts)
+          case Select(t1, nme.CONSTRUCTOR) => extractType(t1)
+          case New(t1) => t1
+          case t1 => t1
+        }
         // The return type of the `apply` method
         val applyResultTpt =
           if (isEnumCase)
             if (parents.isEmpty) enumClassTypeRef
-            else parents.reduceLeft(AndTypeTree)
+            else parents.map(extractType).reduceLeft(AndTypeTree)
           else TypeTree()
 
         val parent =

tests/pos/objXfun.scala

@@ -0,0 +1,8 @@
+object Foo extends (Int => Int) { // OK
+  def apply(x: Int) = x
+}
+
+enum class E(x: Int)  // used to generate Int => new E(x) as the parent of object E --> crash
+object E {
+  case C(x: Int) extends E(x)
+}