7 files changed, 68 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
diff --git a/test/files/pos/bug4840.flags b/test/files/pos/bug4840.flags
new file mode 100644
index 0000000000..eb4d19bcb9
--- /dev/null
+++ b/test/files/pos/bug4840.flags
@@ -0,0 +1 @@
+-optimise
+\ No newline at end of file
diff --git a/test/files/pos/bug4840.scala b/test/files/pos/bug4840.scala
new file mode 100644
index 0000000000..bf44f71d7a
--- /dev/null
+++ b/test/files/pos/bug4840.scala
@@ -0,0 +1,13 @@
+class Crashy {
+  def g(): Option[Any] = None
+
+  def crashy() = {
+    for (_ <- g()) {
+      (null: Any) match {
+        case Some(_) => 5
+        case None    => sys.error("")
+      }
+    }
+  }
+}
+
diff --git a/test/files/run/run-bug4840.check b/test/files/run/run-bug4840.check
new file mode 100644
index 0000000000..cdfc1f911f
--- /dev/null
+++ b/test/files/run/run-bug4840.check
@@ -0,0 +1,2 @@
+Some(5)
+Some(Some(Some(5)))
diff --git a/test/files/run/run-bug4840.flags b/test/files/run/run-bug4840.flags
new file mode 100644
index 0000000000..eb4d19bcb9
--- /dev/null
+++ b/test/files/run/run-bug4840.flags
@@ -0,0 +1 @@
+-optimise
+\ No newline at end of file
diff --git a/test/files/run/run-bug4840.scala b/test/files/run/run-bug4840.scala
new file mode 100644
index 0000000000..dda280fd17
--- /dev/null
+++ b/test/files/run/run-bug4840.scala
@@ -0,0 +1,30 @@
+object Test {
+  def g(x: Boolean): Option[String] = if (x) Some("booya") else None
+
+  def f1() = {
+    for (x <- g(true)) yield {
+      g(false) match {
+        case Some(_) => sys.error("")
+        case None    => 5
+      }
+    }
+  }
+
+  def f2() = {
+    for (x <- g(true) ; y <- g(true) ; z <- g(true)) yield {
+      for (x <- g(true) ; y <- g(true) ; z <- g(true)) yield {
+        g(true) map { _ =>
+          (null: Any) match {
+            case Some(x: Int) => x
+            case _            => 5
+          }
+        }
+      }
+    }
+  }
+
+  def main(args: Array[String]): Unit = {
+    println(f1())
+    println(f2())
+  }
+}