How do you know that?
See the stacktrace:
* elixir /home/build/elixir/lib/elixir/lib/enum.ex:1Enumerable.impl_for!/1
* elixir /home/build/elixir/lib/elixir/lib/enum.ex:141Enumerable.reduce/3
* elixir lib/enum.ex:3015Enum.reverse/1
* elixir lib/enum.ex:2647Enum.to_list/1
* elixir lib/map.ex:181Map.new_from_enum/1
* guardian lib/guardian.ex:573Guardian.encode_and_sign/4 # <-- this is where user=claims
* guardian lib/guardian/plug.ex:208Guardian.Plug.sign_in/5
* lib/dog_web/controllers/session_controller.ex:29DogWeb.SessionController.login_reply/2
Try passing all arguments to sign_in without relying on the defaults, something strange is going on:
conn
|> put_flash(:info, "Welcome back!")
|> Guardian.Plug.sign_in(Guardian, user, %{}, [])
