attempt to fix #16374 (#17232)

* remove unnecessary when statement

* remove outdated codes

* attempt to fix #16374

* fix

compiler/cgen.nim

@@ -406,6 +406,11 @@ proc resetLoc(p: BProc, loc: var TLoc) =
   if isImportedCppType(typ): return
   if optSeqDestructors in p.config.globalOptions and typ.kind in {tyString, tySequence}:
     assert rdLoc(loc) != nil
+
+    let atyp = skipTypes(loc.t, abstractInst)
+    if atyp.kind in {tyVar, tyLent}:
+      linefmt(p, cpsStmts, "$1->len = 0; $1->p = NIM_NIL;$n", [rdLoc(loc)])
+    else:
       linefmt(p, cpsStmts, "$1.len = 0; $1.p = NIM_NIL;$n", [rdLoc(loc)])
   elif not isComplexValueType(typ):
     if containsGcRef:

tests/ccgbugs/t16374.nim

@@ -0,0 +1,38 @@
+discard """
+  matrix: "--gc:refc; --gc:orc"
+"""
+
+block:
+  iterator mvalues(t: var seq[seq[int]]): var seq[int] =
+    yield t[0]
+
+  var t: seq[seq[int]]
+
+  while false:
+    for v in t.mvalues:
+      discard
+
+  proc ok =
+    while false:
+      for v in t.mvalues:
+        discard
+
+  ok()
+
+block:
+  iterator mvalues(t: var seq[seq[int]]): lent seq[int] =
+    yield t[0]
+
+  var t: seq[seq[int]]
+
+  while false:
+    for v in t.mvalues:
+      discard
+
+  proc ok =
+    while false:
+      for v in t.mvalues:
+        discard
+
+  ok()
+