Reenable and fix -Ycheck of FirstTransform group

It was accidentally disabled because elimJavaPackages was renamed.

This required a change to `isWildcardStarArg` to handle retyping repeated
arguments that have been lifted in Typer, for example:

    foo(1, 2, 3)

might be lifted to:

    val args: Int* = [1, 2, 3]
    foo(args)

Author: smarter

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

@@ -170,14 +170,15 @@ trait TreeInfo[T >: Untyped <: Type] { self: Trees.Instance[T] =>
     case _ => false
   }
 
-  /** Is this argument node of the form <expr> : _* ?
+  /** Is this argument node of the form <expr> : _*, or is it a reference to
+   *  such an argument ? The latter case can happen when an argument is lifted.
    */
   def isWildcardStarArg(tree: Tree)(implicit ctx: Context): Boolean = unbind(tree) match {
     case Typed(Ident(nme.WILDCARD_STAR), _) => true
     case Typed(_, Ident(tpnme.WILDCARD_STAR)) => true
-    case Typed(_, tpt: TypeTree) => tpt.hasType && tpt.tpe.isRepeatedParam
+    case Typed(_, tpt: TypeTree) => tpt.typeOpt.isRepeatedParam
     case NamedArg(_, arg) => isWildcardStarArg(arg)
-    case _ => false
+    case arg => arg.typeOpt.widen.isRepeatedParam
   }
 
   /** If this tree has type parameters, those.  Otherwise Nil.

compiler/test/dotty/tools/vulpix/TestConfiguration.scala

@@ -45,7 +45,7 @@ object TestConfiguration {
   }
 
   // Ideally should be Ycheck:all
-  val yCheckOptions = Array("-Ycheck:elimJavaPackages,refchecks,splitter,arrayConstructors,erasure,capturedVars,getClass,elimStaticThis,labelDef")
+  val yCheckOptions = Array("-Ycheck:firstTransform,refchecks,splitter,arrayConstructors,erasure,capturedVars,getClass,elimStaticThis,labelDef")
 
   val basicDefaultOptions = checkOptions ++ noCheckOptions ++ yCheckOptions
   val defaultUnoptimised = TestFlags(classPath, runClassPath, basicDefaultOptions)