2 files changed, 60 insertions, 1 deletions
diff --git a/src/library/scala/collection/mutable/OpenHashMap.scala b/src/library/scala/collection/mutable/OpenHashMap.scala
index 24f5761cf5..5f8f5b9a0a 100644
--- a/src/library/scala/collection/mutable/OpenHashMap.scala
+++ b/src/library/scala/collection/mutable/OpenHashMap.scala
@@ -81,6 +81,9 @@ extends AbstractMap[Key, Value]
     h ^ (h >>> 7) ^ (h >>> 4)
   }
 
+  /** Increase the size of the table.
+    * Copy only the occupied slots, effectively eliminating the deleted slots.
+    */
   private[this] def growTable() = {
     val oldSize = mask + 1
     val newSize = 4 * oldSize
@@ -92,8 +95,18 @@ extends AbstractMap[Key, Value]
     deleted = 0
   }
 
+  /** Return the index of the first slot in the hash table (in probe order)
+    * that either is empty, or is or was last occupied by the given key.
+    */
   private[this] def findIndex(key: Key) : Int = findIndex(key, hashOf(key))
 
+  /** Return the index of the first slot in the hash table (in probe order)
+    * that either is empty, or is or was last occupied by the given key.
+    * 
+    * This method is an optimization for when the hash value is in hand.
+    * 
+    * @param hash hash value for `key`
+    */
   private[this] def findIndex(key: Key, hash: Int): Int = {
     var j = hash
 
@@ -136,7 +149,11 @@ extends AbstractMap[Key, Value]
       None
     } else {
       val res = entry.value
-      if (entry.value == None) { size += 1; modCount += 1 }
+      if (entry.value == None) {
+        size += 1
+        deleted -= 1
+        modCount += 1
+      }
       entry.value = Some(value)
       res
     }
diff --git a/test/junit/scala/collection/mutable/OpenHashMapTest.scala b/test/junit/scala/collection/mutable/OpenHashMapTest.scala
new file mode 100644
index 0000000000..1459c14d78
--- /dev/null
+++ b/test/junit/scala/collection/mutable/OpenHashMapTest.scala
@@ -0,0 +1,42 @@
+package scala.collection.mutable
+
+import org.junit.Test
+import org.junit.Assert._
+import org.junit.runner.RunWith
+import org.junit.runners.JUnit4
+
+/** Tests for [[OpenHashMap]]. */
+@RunWith(classOf[JUnit4])
+class OpenHashMapTest {
+  /** Test that an [[OpenHashMap]] correctly maintains its internal `deleted` count. */
+  @Test
+  def maintainsDeletedCount {
+    val m = OpenHashMap.empty[Int, Int]
+
+    // Reflect to get the private `deleted` field's value, which should be zero.
+
+    /* TODO Doesn't work, due to SI-9306.
+    import scala.reflect.runtime.{universe => ru}
+
+    val mirror = ru.runtimeMirror(m.getClass.getClassLoader)
+    val mapType = ru.typeOf[OpenHashMap[Int, Int]]
+    val termSym = mapType.decls
+      .filterNot { s => s.isMethod }
+      .filter { s => s.fullName.endsWith("deleted") }
+      .head.asTerm
+
+    val fieldMirror = mirror.reflect(m).reflectField(termSym)
+		*/
+    // Use Java reflection instead for now.
+    val field = m.getClass.getDeclaredField("scala$collection$mutable$OpenHashMap$$deleted")
+    field.setAccessible(true)
+
+    m.put(0, 0)
+    m.remove(0)
+    assertEquals(1, field.getInt(m))
+
+    m.put(0, 0)  // Add an entry with the same key
+    // TODO assertEquals(0, fieldMirror.get.asInstanceOf[Int])
+    assertEquals(0, field.getInt(m))
+  }
+}