fixes #14139 (#15107)

* fix #14139
* Update lib/pure/collections/heapqueue.nim

Co-authored-by: Andreas Rumpf <rumpf_a@web.de>
Co-authored-by: Clyybber <darkmine956@gmail.com>

lib/pure/collections/heapqueue.nim

@@ -156,10 +156,10 @@ proc replace*[T](heap: var HeapQueue[T], item: T): T =
 
 proc pushpop*[T](heap: var HeapQueue[T], item: T): T =
   ## Fast version of a push followed by a pop.
-  if heap.len > 0 and heapCmp(heap[0], item):
-    swap(item, heap[0])
+  result = item
+  if heap.len > 0 and heapCmp(heap.data[0], item):
+    swap(result, heap.data[0])
     siftup(heap, 0)
-  return item
 
 proc clear*[T](heap: var HeapQueue[T]) =
   ## Remove all elements from `heap`, making it empty.

tests/stdlib/t14139.nim

@@ -0,0 +1,9 @@
+import heapqueue
+
+var test_queue : HeapQueue[int]
+
+test_queue.push(7)
+test_queue.push(3)
+test_queue.push(9)
+let i = test_queue.pushpop(10)
+assert i == 3