closes #20808 (#21077)

tests/effects/tfuncs_cannot_mutate3.nim

@@ -0,0 +1,35 @@
+discard """
+  errormsg: "cannot mutate location kid.parent within a strict func"
+  line: 16
+"""
+
+{.experimental: "strictFuncs".}
+
+type
+  Node = ref object
+    name: string
+    kids: seq[Node]
+    parent: Node
+
+func initParents(tree: Node) =
+  for kid in tree.kids:
+    kid.parent = tree
+    initParents(kid)
+
+proc process(intro: Node): Node =
+  var tree = Node(name: "root", kids: @[
+    intro,
+    Node(name: "one", kids: @[
+      Node(name: "two"),
+      Node(name: "three"),
+    ]),
+    Node(name: "four"),
+  ])
+  initParents(tree)
+
+proc main() =
+  var intro = Node(name: "intro")
+  var tree = process(intro)
+  echo intro.parent.name
+
+main()