error msg for #13864 (#13962)

Co-authored-by: cooldome <ariabushenko@bk.ru>

tests/arc/tref_cast_error.nim

@@ -0,0 +1,15 @@
+discard """
+  cmd: "nim c --gc:arc $file"
+  errormsg: "expression cannot be cast to ref RootObj"
+  joinable: false
+"""
+
+type Variant* = object
+    refval: ref RootObj
+
+proc newVariant*[T](val: T): Variant =
+    let pt = T.new()
+    pt[] = val
+    result = Variant(refval: cast[ref RootObj](pt))
+
+var v = newVariant(@[1, 2, 3])