fixes #6724

tests/ccgbugs/trefseqsort.nim

@@ -0,0 +1,33 @@
+discard """
+  output: '''@[0, 4, 9, 1, 3, 2]
+@[0, 1, 2, 3, 9]'''
+"""
+# bug #6724
+import algorithm
+
+type
+  Bar = object
+    bar: ref seq[int]
+  Foo = ref Bar
+
+proc test(x: ref Foo) =
+  x.bar[].del(1)
+  x.bar[].sort(cmp)
+
+proc main() =
+  var foo: ref Foo
+  new(foo)
+
+  var s = @[0, 4, 9, 1, 3, 2]
+
+  var sr: ref seq[int]
+  new(sr)
+  sr[] = s
+
+  foo[] = Foo(bar: sr)
+  echo($foo.bar[])
+
+  test(foo)
+  echo($foo.bar[])
+
+main()