Add unit test and address review comments

Author: Abel Nieto

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

@@ -94,8 +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 app@Apply(fun, args) if isWrappedArray(app) =>
-      args.head // There's only one argument: the wrapped array
+    case app@Apply(fun, args) if isWrappedArray(app) => // rewrite a call to `wrapXArray(arr)` to `arr`
+      args.head
     case _ =>
       val elemType = tree.tpe.elemType
       var elemClass = elemType.classSymbol
@@ -108,7 +108,7 @@ class ElimRepeated extends MiniPhase with InfoTransformer { thisPhase =>
           // Because of phantomclasses, the Java array's type might not conform to the return type
   }
 
-  /** Determines whether the given `tree` represents a wrapped array */
+  /** Determines whether `tree` is a method call to Predef.wrapXArray */
   private def isWrappedArray(tree: Apply)(implicit ctx: Context): Boolean = {
     val elemTpe = tree.tpe.elemType
     val wrapMethodName = TreeGen.wrapArrayMethodName(elemTpe)

compiler/test/dotty/tools/backend/jvm/DottyBytecodeTests.scala

@@ -187,4 +187,32 @@ class TestBCode extends DottyBytecodeTest {
         diffInstructions(instructions1, instructions2))
     }
   }
+
+  /** Verifies that arrays are not unnecessarily wrapped when passed to Java varargs methods */
+  @Test def dontWrapArraysInJavaVarargs = {
+    val source =
+      """
+        |import java.nio.file._
+        |class Test {
+        |  def test(xs: Array[String]) = {
+        |     val p4 = Paths.get("Hello", xs: _*)
+        |  }
+        |}
+      """.stripMargin
+
+    checkBCode(source) { dir =>
+      val moduleIn = dir.lookupName("Test.class", directory = false)
+      val moduleNode = loadClassNode(moduleIn.input)
+      val method = getMethod(moduleNode, "test")
+
+      val arrayWrapped = instructionsFromMethod(method).exists { instr: Instruction =>
+        instr match {
+          case inv: Invoke => inv.name.contains("wrapRefArray")
+          case _ => false
+        }
+      }
+
+      assert(!arrayWrapped, "Arrays should not be wrapped when passed to a Java varargs method\n")
+    }
+  }
 }