fixes #4220

lib/system.nim

@@ -2005,7 +2005,13 @@ iterator countdown*[T](a, b: T, step = 1): T {.inline.} =
   ## step count. `T` may be any ordinal type, `step` may only
   ## be positive. **Note**: This fails to count to ``low(int)`` if T = int for
   ## efficiency reasons.
-  when T is IntLikeForCount:
+  when T is (uint|uint64):
+    var res = a
+    while res >= b:
+      yield res
+      if res == b: break
+      dec(res, step)
+  elif T is IntLikeForCount:
     var res = int(a)
     while res >= int(b):
       yield T(res)

tests/misc/tunsignedcmp.nim

@@ -1,7 +1,15 @@
 discard """
   output: '''true
 true
-true'''
+true
+5
+4
+3
+2
+1
+0
+it should stop now
+'''
 """
 
 # bug 1420
@@ -11,3 +19,15 @@ echo x > y # works
 
 echo((40'i32) > (30'i32))
 echo((40'u32) > (30'u32)) # Error: ordinal type expected
+
+# bug #4220
+
+const count: uint = 5
+var stop_me = false
+
+for i in countdown(count, 0):
+  echo i
+  if stop_me: break
+  if i == 0:
+    echo "it should stop now"
+    stop_me = true