fix #17385, len must be declared before items (#17386)

tests/sets/m17385.nim

@@ -0,0 +1,11 @@
+import std/sets
+
+type
+  Diff*[T] = object
+    data: T
+
+proc test*[T](diff: Diff[T]) =
+  var bPopular = initHashSet[T]()
+  for element in bPopular.items():
+    echo element
+

tests/sets/t17385.nim

@@ -0,0 +1,4 @@
+import m17385
+
+let a = Diff[int]()
+a.test()