fixes #25591; error with capture in closure iterator

compiler/lambdalifting.nim

@@ -520,6 +520,8 @@ proc detectCapturedVars(n: PNode; owner: PSym; c: var DetectionPass) =
   of nkLambdaKinds, nkIteratorDef:
     if n.typ != nil:
       detectCapturedVars(n[namePos], owner, c)
+  of nkClosure:
+    detectCapturedVars(n[1], owner, c)
   of nkReturnStmt:
     detectCapturedVars(n[0], owner, c)
   of nkIdentDefs:

tests/iter/titer_issues.nim

@@ -239,6 +239,17 @@ block t2023_objiter:
   var o = init()
   echo(o.iter())
 
+  block: # bug #25591
+    iterator h(): int =
+      let n = 0
+      (proc() = discard n)()
+      yield 0
+
+    proc a() =
+      iterator m(): int {.closure.} = (for _ in h(): discard)
+      let _ = m
+
+    a()
 
 block:
   # bug #13739