From 97286db546a9a33da95d12b35e288c76625d4d07 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Derek=20=E5=91=86?=
 <116649+derekdai@users.noreply.github.com>
Date: Sat, 30 Oct 2021 01:55:48 +0800
Subject: [PATCH] fix #18971 (#19070) [backport:1.6]

since the example code return value from global variable, instead
of first argument, the `n.len` is 1 which causes compiler crashes.

(cherry picked from commit f755e452d2a2c7656f07793b7f01dc654a7c5253)
---
 compiler/varpartitions.nim |  2 +-
 tests/arc/t18971.nim       | 10 ++++++++++
 2 files changed, 11 insertions(+), 1 deletion(-)
 create mode 100644 tests/arc/t18971.nim

diff --git a/compiler/varpartitions.nim b/compiler/varpartitions.nim
index 8f422face8e1..709d00fa0ea8 100644
--- a/compiler/varpartitions.nim
+++ b/compiler/varpartitions.nim
@@ -479,7 +479,7 @@ proc destMightOwn(c: var Partitions; dest: var VarIndex; n: PNode) =
         # calls do construct, what we construct must be destroyed,
         # so dest cannot be a cursor:
         dest.flags.incl ownsData
-      elif n.typ.kind in {tyLent, tyVar}:
+      elif n.typ.kind in {tyLent, tyVar} and n.len > 1:
         # we know the result is derived from the first argument:
         var roots: seq[(PSym, int)]
         allRoots(n[1], roots, RootEscapes)
diff --git a/tests/arc/t18971.nim b/tests/arc/t18971.nim
new file mode 100644
index 000000000000..9b587d956249
--- /dev/null
+++ b/tests/arc/t18971.nim
@@ -0,0 +1,10 @@
+discard """
+  cmd: "nim c --gc:arc $file"
+"""
+
+type MyObj = ref object
+
+var o = MyObj()
+proc x: var MyObj = o
+
+var o2 = x()