Invalid Type with ChannelTest Sockets #5509

Environment

• Elixir version (elixir -v): 1.15.2
• Phoenix version (mix deps): 1.7.6
• Operating system: macOS 13.4.1

Actual behavior

Hello 👋🏼

This relates to the intersection of production code and test code, so it may not be an actual issue.

The type Phoenix.Socket.t() specifies the types of most fields, however Phoenix.ChannelTest initializes sockets with several fields that are either missing (such as channel_pid or have a different type (like transport). As a result, the following code (from the ChannelTest module documentation) is caught by Dialyzer as having no return:

{:ok, _, socket} =
  socket(UserSocket, "user:id", %{some_assigns: 1})
  |> subscribe_and_join(RoomChannel, "room:lobby", %{"id" => 3})

# or...

{:ok, socket} = connect(UserSocket, %{"some" => "params"}, %{})
{:ok, _, socket} = subscribe_and_join(socket, "room:lobby", %{"id" => 3})

At the application level, Dialyzer generally doesn't give a descriptive error beyond "has no local return", however if you copy the library code in place of the calls, it warns about the call to Phoenix.Channel.Server.join/4:

The function call will not succeed.

Phoenix.Channel.Server.join(
  _socket :: %Phoenix.Socket{
    :assigns => _,
    :channel => nil,
    :channel_pid => nil,
    :endpoint => atom(),
    :handler => _,
    :id => _,
    :join_ref => nil,
    :joined => false,
    :private => %{},
    :pubsub_server => _,
    :ref => nil,
    :serializer => Phoenix.ChannelTest.NoopSerializer,
    :topic => nil,
    :transport => {Phoenix.ChannelTest, pid()},
    :transport_pid => pid()
  },
  MyAppWeb.RoomChannel,
  _message :: %Phoenix.Socket.Message{
    :event => <<_::64>>,
    :join_ref => nil,
    :payload => %{},
    :ref => integer(),
    :topic => <<_::56>>
  },
  [
    {:starter,
     (_, _, _ -> {:error, _} | {:ok, :undefined | pid()} | {:ok, :undefined | pid(), _})},
    ...
  ]
)

breaks the contract
(Phoenix.Socket.t(), module(), Phoenix.Socket.Message.t(), :elixir.keyword()) ::
  {:ok, term(), pid()} | {:error, term()}

Notably, I discovered this mismatch only because I chose to call ChannelTest functions in the setup block contained within my application's ChannelCase module. Normally, Dialyzer would not have the chance to catch this code in test files.

Expected behavior

It seems unlikely that the Phoenix.Socket.t() type should be expanded to include the socket created in ChannelTest (except possibly for fields like transport), but on the other hand I wonder if it is even feasible for the fake Socket to be "complete" in this context. Since these functions are generally called in test files anyway, maybe it's not worth exploring further.

❤️