thanks to the heads up from Franck, I can work around KT-1646 - refactored previous Filter code to reuse the AbstractIterator base class and implemented filter/filterNot/filterIsInstance lazily

libraries/stdlib/src/Filter.kt

@@ -1,49 +0,0 @@
-package kotlin
-
-import java.util.*
-import java.lang.Iterable
-
-/** Filters given iterator */
-inline fun <T> java.util.Iterator<T>.filter(f: (T)-> Boolean) : java.util.Iterator<T> = FilterIterator<T>(this, f)
-
-/** Create iterator filtering given java.lang.Iterable */
-/*
-inline fun <T> java.lang.Iterable<T>.filter(f: (T)->Boolean) : java.util.Iterator<T> = (iterator() as java.util.Iterator<T>).filter(f)
-*/
-
-private class FilterIterator<T>(val original: java.util.Iterator<T>, val filter: (T)-> Boolean) : java.util.Iterator<T> {
-    var state = 0
-    var nextElement: T? = null
-
-    override fun hasNext(): Boolean =
-        when(state) {
-                1 -> true  // checked and next present
-                2 -> false // checked and next not present
-                else -> {
-                    while(original.hasNext()) {
-                        val candidate = original.next()
-                        if((filter)(candidate)) {
-                            nextElement = candidate
-                            state = 1
-                            true
-                        }
-                    }
-                    state = 2
-                    false
-                }
-        }
-
-    override fun next(): T =
-        if(state != 1)
-            throw java.util.NoSuchElementException()
-        else {
-            val res = nextElement as T
-            nextElement = null
-            state = 0
-            res
-        }
-
-    override fun remove() {
-        throw java.lang.UnsupportedOperationException()
-    }
-}

libraries/stdlib/src/Iterators.kt

@@ -21,33 +21,19 @@ private class FilterIsIterator<T, R :T>(val iter: java.util.Iterator<T>, val kla
     }
 }
 
-
-// TODO when we're past the compiler error
-// http://youtrack.jetbrains.com/issue/KT-1646
-//
-// we can use this code to replace the other
-// filter / FilterIterator class as it reuses the
-// core logic from AbstractIterator
-
 /** Returns a new lazy iterator containing all elements in this iteration which match the given predicate */
-inline fun <T> java.util.Iterator<T>.filter2(predicate: (T)-> Boolean) : java.util.Iterator<T> {
-    return FilterIterator2(this, predicate)
+inline fun <T> java.util.Iterator<T>.filter(predicate: (T)-> Boolean) : java.util.Iterator<T> {
+    return FilterIterator(this, predicate)
 }
 
-private class FilterIterator2<T>(val iter: java.util.Iterator<T>, val fn: (T)-> Boolean) : AbstractIterator<T>() {
+private class FilterIterator<T>(val iter: java.util.Iterator<T>, val predicate: (T)-> Boolean) : AbstractIterator<T>() {
     override protected fun computeNext(): T? {
-        /*
-
-        TODO this block generates a compiler error:
-        http://youtrack.jetbrains.com/issue/KT-1646
-
         while (iter.hasNext()) {
             val element = iter.next()
-            if (fn(element)) {
+            if ((predicate)(element)) {
                 return element
             }
         }
-        */
         done()
         return null
     }