Improve robustness of computing type parameters in completers

We need to take DerivedTypeTrees into account when computing
the type parameters of a type completer.

Author: odersky

compiler/src/dotty/tools/dotc/typer/Namer.scala

@@ -795,10 +795,16 @@ class Namer { typer: Typer =>
         nestedCtx = localContext(sym).setNewScope
         myTypeParams = {
           implicit val ctx = nestedCtx
-          val tparams = original.rhs match {
-            case LambdaTypeTree(tparams, _) => tparams
+          def typeParamTrees(tdef: Tree): List[TypeDef] = tdef match {
+            case TypeDef(_, original) =>
+              original match {
+                case LambdaTypeTree(tparams, _) => tparams
+                case original: DerivedFromParamTree => typeParamTrees(original.watched)
+                case _ => Nil
+              }
             case _ => Nil
           }
+          val tparams = typeParamTrees(original)
           completeParams(tparams)
           tparams.map(symbolOfTree(_).asType)
         }