Never generate function parents for companions of enum cases

The previous condition when we can and cannot generate a parent is quite obscure,
so we should drop it altogether.

Author: odersky

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

@@ -599,10 +599,10 @@ object desugar {
               constrVparamss.length > 1 ||
               mods.is(Abstract) ||
               restrictedAccess ||
-              isEnumCase && applyResultTpt.isEmpty) anyRef
+              isEnumCase) anyRef
           else
             // todo: also use anyRef if constructor has a dependent method type (or rule that out)!
-            (constrVparamss :\ (if (isEnumCase) applyResultTpt else classTypeRef)) (
+            (constrVparamss :\ classTypeRef) (
               (vparams, restpe) => Function(vparams map (_.tpt), restpe))
         def widenedCreatorExpr =
           (creatorExpr /: widenDefs)((rhs, meth) => Apply(Ident(meth.name), rhs :: Nil))

tests/neg/enums.scala

@@ -19,6 +19,12 @@ enum E3[-T <: Ordered[T]] {
 
 enum E4 {
   case C
+  case C4(x: Int)
+  val x3: Int => E4 = C4(_)
+}
+object E4 {
+  val x1: Int => E4 = C4    // error: found: C4, required: Int => E4
+  val x2: Int => E4 = C4(_) // ok
 }
 
 case class C4() extends E4 // error: cannot extend enum

tests/run/enums.scala

@@ -105,6 +105,7 @@ object Test6 {
     case Green  extends Color(3)
     case Red    extends Color(2)
     case Violet extends Color(Green.x + Red.x)
+    case RGB(xx: Int) extends Color(xx)
   }
 }