=== What steps will reproduce the problem? ===

  var t = collection.immutable.TreeMap.empty[Int, Int]
  for (i <- 1 to 100000)
    t = (t + (i -> i) - i)   

=== What is the expected behavior? ===

The loop should require constant memory, regardless of the number of iterations.

=== What do you see instead? ===

The memory footprint is increasing while the loop is running.
When it finishes, there are exactly 100001 TreeMap and RedBlack$$Empty$$ instances that cannot be garbage collected until the reference t goes out of scope.

If you run this loop for a larger number of iterations, you will get an OutOfMemoryError (out of JVM heap space).

=== Additional information ===

The problem goes away, when you always keep at least one element in the TreeMap. This code works correctly:

var t = collection.immutable.TreeMap(0 -> 0)  // never removed
for (i <- 1 to 100000)
  t = (t + (i -> i) - i) 

JVisualVM suggests also that the problem might be related to the $$outer reference preventing 0-sized TreeMap objects from being GCed. (https://lampsvn.epfl.ch/trac/scala/ticket/1419)

=== What versions of the following are you using? ===

• Scala: 2.8.1.final
• Java: 1.6.0 update 23
• Operating system: Windows Vista Home Premium, 32-bit