Fix #3675: don't unnecessarily wrap arrays when eliminating java varargs

When eliminating varargs in a java method call, we need to convert Seqs
into Arrays. However, if the object we're passing is already a wrapped
array, we just need to unwrap it.

Testing
=======

Eliminating varargs in

import java.nio.file._
class Test {
  def test(xs: Array[String]) = {
     val p4 = Paths.get("Hello", xs: _*)
  }
}

now gives

val p4: java.nio.file.Path = java.nio.file.Paths.get("Hello", xs)

Author: Abel Nieto

compiler/src/dotty/tools/dotc/transform/ElimRepeated.scala

@@ -94,6 +94,8 @@ class ElimRepeated extends MiniPhase with InfoTransformer { thisPhase =>
   private def seqToArray(tree: Tree, pt: Type)(implicit ctx: Context): Tree = tree match {
     case SeqLiteral(elems, elemtpt) =>
       JavaSeqLiteral(elems, elemtpt)
+    case Apply(fun, args) if TreeGen.isRefOrPrimWrappedArray(tree, tree.tpe.elemType) =>
+      args.head // There's only one argument: the wrapped array
     case _ =>
       val elemType = tree.tpe.elemType
       var elemClass = elemType.classSymbol

compiler/src/dotty/tools/dotc/transform/TreeGen.scala

@@ -23,4 +23,15 @@ object TreeGen {
       .select(wrapArrayMethodName(elemtp))
       .appliedToTypes(if (elemtp.isPrimitiveValueType) Nil else elemtp :: Nil)
       .appliedTo(tree)
+
+  /** Determines whether the given `tree` an application of `wrapRefArray` or `wrapXArray` */
+  def isRefOrPrimWrappedArray(tree: Tree, elemTpe: Type)(implicit ctx: Context): Boolean = {
+    tree match {
+      case Apply(fun, _) =>
+        val sname = fun.symbol.name
+        sname == nme.wrapRefArray || sname == nme.wrapXArray(elemTpe.classSymbol.name)
+      case _ =>
+        false
+    }
+  }
 }