closes #5321

tests/iter/tclosureiters.nim

@@ -79,3 +79,27 @@ proc foo(f: (iterator(): int)) =
 
 let fIt = iterator(): int = yield 70
 foo fIt
+
+# bug #5321
+
+proc lineIter*(filename: string): iterator(): string =
+  result = iterator(): string {.closure.} =
+    for line in lines(filename):
+      yield line
+
+proc unused =
+  var count = 0
+  let iter = lineIter("temp10.nim")
+  for line in iter():
+    count += 1
+
+iterator lineIter2*(filename: string): string {.closure.} =
+  var f = open(filename, bufSize=8000)
+  defer: close(f)   # <-- commenting defer "solves" the problem
+  var res = TaintedString(newStringOfCap(80))
+  while f.readLine(res): yield res
+
+proc unusedB =
+  var count = 0
+  for line in lineIter2("temp10.nim"):
+    count += 1