Add test for #1836

tests/iter/tconcat.nim

@@ -0,0 +1,17 @@
+discard """
+  output: '''DabcD'''
+"""
+
+proc toIter*[T](s: Slice[T]): iterator: T =
+  iterator it: T {.closure.} =
+    for x in s.a..s.b:
+      yield x
+  return it
+
+iterator concat*[T](its: varargs[T, toIter]): auto =
+  for i in its:
+    for x in i():
+      yield x
+
+for i in concat(1..4, 20..23):
+  echo i