Mysterious fix for #8550 (#8561)

Replacing the `for` body with a nkEmpty node is not the right thing to
do.

compiler/transf.nim

@@ -539,11 +539,8 @@ proc transformFor(c: PTransf, n: PNode): PTransNode =
   if call.kind notin nkCallKinds or call.sons[0].kind != nkSym or
       call.sons[0].typ.callConv == ccClosure:
     n.sons[length-1] = transformLoopBody(c, n.sons[length-1]).PNode
-    if not c.tooEarly:
-      n.sons[length-2] = transform(c, n.sons[length-2]).PNode
-      result[1] = lambdalifting.liftForLoop(c.graph, n, getCurrOwner(c)).PTransNode
-    else:
-      result[1] = newNode(nkEmpty).PTransNode
+    n.sons[length-2] = transform(c, n.sons[length-2]).PNode
+    result[1] = lambdalifting.liftForLoop(c.graph, n, getCurrOwner(c)).PTransNode
     discard c.breakSyms.pop
     return result
 

tests/closure/t8550.nim

@@ -0,0 +1,12 @@
+discard """
+  output: "@[\"42\"]"
+"""
+
+proc chk_fail(): seq[string] =
+  iterator x(): int {.closure.} = yield 42
+  proc f(cl: iterator(): int {.closure.}): seq[string] =
+    result = @[]
+    for i in cl(): result.add($i)
+  result = f(x)
+
+echo(chk_fail())