SI-6089 better tail position analysis for matches

we mistakenly went into apply nodes to look for matchEnd-labeldefs
in tail positions -- only apply nodes that jump to labels in tailpos
should be traversed (this arises for nested matches)

1 files changed, 11 insertions, 5 deletions

diff --git a/src/compiler/scala/tools/nsc/transform/TailCalls.scala b/src/compiler/scala/tools/nsc/transform/TailCalls.scala
index 4c9d855413..d5bbc578fc 100644
--- a/src/compiler/scala/tools/nsc/transform/TailCalls.scala
+++ b/src/compiler/scala/tools/nsc/transform/TailCalls.scala
@@ -373,7 +373,7 @@ abstract class TailCalls extends Transform {
   // the labels all look like: matchEnd(x) {x}
   // then, in a forward jump `matchEnd(expr)`, `expr` is considered in tail position (and the matchEnd jump is replaced by the jump generated by expr)
   class TailPosLabelsTraverser extends Traverser {
-    val tailLabels = new collection.mutable.ListBuffer[Symbol]()
+    val tailLabels = new collection.mutable.HashSet[Symbol]()
 
     private var maybeTail: Boolean = true // since we start in the rhs of a DefDef
 
@@ -388,9 +388,15 @@ abstract class TailCalls extends Transform {
     def traverseTreesNoTail(trees: List[Tree]) = trees foreach traverseNoTail
 
     override def traverse(tree: Tree) = tree match {
-      case LabelDef(_, List(arg), body@Ident(_)) if arg.symbol == body.symbol => // we're looking for label(x){x} in tail position, since that means `a` is in tail position in a call `label(a)`
+      // we're looking for label(x){x} in tail position, since that means `a` is in tail position in a call `label(a)`
+      case LabelDef(_, List(arg), body@Ident(_)) if arg.symbol == body.symbol =>
         if (maybeTail) tailLabels += tree.symbol
 
+      // jumps to matchEnd are transparent; need this case for nested matches
+      // (and the translated match case below does things in reverse for this case's sake)
+      case Apply(fun, arg :: Nil) if hasSynthCaseSymbol(fun) && tailLabels(fun.symbol) =>
+        traverse(arg)
+
       // a translated casedef
       case LabelDef(_, _, body) if hasSynthCaseSymbol(tree) =>
         traverse(body)
@@ -400,9 +406,9 @@ abstract class TailCalls extends Transform {
         // the assumption is once we encounter a case, the remainder of the block will consist of cases
         // the prologue may be empty, usually it is the valdef that stores the scrut
         val (prologue, cases) = stats span (s => !s.isInstanceOf[LabelDef])
-        traverseTreesNoTail(prologue) // selector (may be absent)
-        traverseTrees(cases)
         traverse(expr)
+        traverseTrees(cases.reverse)  // reverse so that we enter the matchEnd LabelDef before we see jumps to it
+        traverseTreesNoTail(prologue) // selector (may be absent)
 
       case CaseDef(pat, guard, body) =>
         traverse(body)
@@ -426,7 +432,7 @@ abstract class TailCalls extends Transform {
         traverseTreesNoTail(catches)
         traverseNoTail(finalizer)
 
-      case EmptyTree | Super(_, _) | This(_) | Select(_, _) | Ident(_) | Literal(_) | Function(_, _) | TypeTree() =>
+      case Apply(_, _) | EmptyTree | Super(_, _) | This(_) | Select(_, _) | Ident(_) | Literal(_) | Function(_, _) | TypeTree() =>
       case _ => super.traverse(tree)
     }
   }