SI-6611 Tighten up an unsafe array optimization

The net was cast too wide and was unsafely optimizing away array
copies.

2 files changed, 22 insertions, 5 deletions

diff --git a/src/compiler/scala/tools/nsc/transform/CleanUp.scala b/src/compiler/scala/tools/nsc/transform/CleanUp.scala
index 1f353bb31c..bdcebf47b8 100644
--- a/src/compiler/scala/tools/nsc/transform/CleanUp.scala
+++ b/src/compiler/scala/tools/nsc/transform/CleanUp.scala
@@ -15,6 +15,7 @@ abstract class CleanUp extends Transform with ast.TreeDSL {
   import global._
   import definitions._
   import CODE._
+  import treeInfo.StripCast
 
   /** the following two members override abstract members in Transform */
   val phaseName: String = "cleanup"
@@ -618,14 +619,16 @@ abstract class CleanUp extends Transform with ast.TreeDSL {
         }
         transformApply
 
-      // This transform replaces Array(Predef.wrapArray(Array(...)), <tag>)
-      // with just Array(...)
-      case Apply(appMeth, List(Apply(wrapRefArrayMeth, List(array)), _))
+      // Replaces `Array(Predef.wrapArray(ArrayValue(...).$asInstanceOf[...]), <tag>)`
+      // with just `ArrayValue(...).$asInstanceOf[...]`
+      //
+      // See SI-6611; we must *only* do this for literal vararg arrays.
+      case Apply(appMeth, List(Apply(wrapRefArrayMeth, List(arg @ StripCast(ArrayValue(_, _)))), _))
       if (wrapRefArrayMeth.symbol == Predef_wrapRefArray &&
           appMeth.symbol == ArrayModule_overloadedApply.suchThat {
-            _.tpe.resultType.dealias.typeSymbol == ObjectClass
+            _.tpe.resultType.dealias.typeSymbol == ObjectClass  // [T: ClassTag](xs: T*): Array[T] post erasure
           }) =>
-        super.transform(array)
+        super.transform(arg)
 
       case _ =>
         super.transform(tree)
diff --git a/src/reflect/scala/reflect/internal/TreeInfo.scala b/src/reflect/scala/reflect/internal/TreeInfo.scala
index 66326c90e9..bee92b446b 100644
--- a/src/reflect/scala/reflect/internal/TreeInfo.scala
+++ b/src/reflect/scala/reflect/internal/TreeInfo.scala
@@ -265,6 +265,20 @@ abstract class TreeInfo {
       tree
   }
 
+  /** Strips layers of `.asInstanceOf[T]` / `_.$asInstanceOf[T]()` from an expression */
+  def stripCast(tree: Tree): Tree = tree match {
+    case TypeApply(sel @ Select(inner, _), _) if isCastSymbol(sel.symbol) =>
+      stripCast(inner)
+    case Apply(TypeApply(sel @ Select(inner, _), _), Nil) if isCastSymbol(sel.symbol) =>
+      stripCast(inner)
+    case t =>
+      t
+  }
+
+  object StripCast {
+    def unapply(tree: Tree): Some[Tree] = Some(stripCast(tree))
+  }
+
   /** Is tree a self or super constructor call? */
   def isSelfOrSuperConstrCall(tree: Tree) = {
     // stripNamedApply for SI-3584: adaptToImplicitMethod in Typers creates a special context