2 files changed, 21 insertions, 5 deletions
diff --git a/src/compiler/scala/tools/nsc/backend/icode/ICodes.scala b/src/compiler/scala/tools/nsc/backend/icode/ICodes.scala
index 9553d5cf9b..06c3ee2a9e 100644
--- a/src/compiler/scala/tools/nsc/backend/icode/ICodes.scala
+++ b/src/compiler/scala/tools/nsc/backend/icode/ICodes.scala
@@ -35,7 +35,7 @@ abstract class ICodes extends AnyRef
                                  with Repository
 {
   val global: Global
-  import global.{ definitions, settings, perRunCaches }
+  import global.{ log, definitions, settings, perRunCaches }
 
   /** The ICode representation of classes */
   val classes = perRunCaches.newMap[global.Symbol, IClass]()
@@ -72,11 +72,22 @@ abstract class ICodes extends AnyRef
   }
 
   def checkValid(m: IMethod) {
-    for (b <- m.code.blocks)
-      if (!b.closed) {
+    // always dicey to iterate over mutable structures
+    val bs = m.code.blocks.toList
+
+    for (b <- bs ; if !b.closed) {
+      // Something is leaving open/empty blocks around (see SI-4840) so
+      // let's not kill the deal unless it's nonempty.
+      if (b.isEmpty) {
+        log("!!! Found open but empty block while inlining " + m + ": removing from block list.")
+        m.code removeBlock b
+      }
+      else {
+        Console.println("Fatal bug in inliner: found open block when inlining " + m)
         m.dump
-        global.abort("Open block: " + b + " " + b.flagsString)
+        global.abort("Open block was: " + b + " " + b.flagsString)
       }
+    }
   }
 
   object liveness extends Liveness {
diff --git a/src/compiler/scala/tools/nsc/backend/icode/Linearizers.scala b/src/compiler/scala/tools/nsc/backend/icode/Linearizers.scala
index 9f22a6de90..8130c99978 100644
--- a/src/compiler/scala/tools/nsc/backend/icode/Linearizers.scala
+++ b/src/compiler/scala/tools/nsc/backend/icode/Linearizers.scala
@@ -14,6 +14,8 @@ import mutable.ListBuffer
 
 trait Linearizers {
   self: ICodes =>
+
+  import global.debuglog
   import opcodes._
 
   abstract class Linearizer {
@@ -178,11 +180,14 @@ trait Linearizers {
      * Prepend b to the list, if not already scheduled.
      * @return Returns true if the block was added.
      */
-    def add(b: BasicBlock) =
+    def add(b: BasicBlock) = {
+      debuglog("Linearizer adding block " + b.label)
+
       if (!added(b.label)) {
         added += b.label
         blocks = b :: blocks;
       }
+    }
   }
 
   /** A 'dump' of the blocks in this method, which does not