`Protocol.UndefinedError` when printing out a struct that is from a stubbed module

[sevenseacat]

This was a super-bizarro one that I just came across! I've tried to minimize the example as much as I can.

Given a module that defines a struct, eg.

defmodule Log do
  defstruct [:name]
  def hello, do: nil
end

I can use that struct quite happily in tests -

# in test
dbg(%Log{})

# prints out
%Log{} #=> %Log{name: nil}

but if I want to stub out a function on that module, it throws up a really confusing error

# in test 
stub(Log, :hello, fn -> "hello" end)
dbg(%Log{})

# prints out 
%Log{} #=> #Inspect.Error<
  got Protocol.UndefinedError with message:

      """
      protocol Enumerable not implemented for nil of type Atom. This protocol is implemented for
      the following type(s): DBConnection.PrepareStream, DBConnection.Stream, Date.Range, 
      Ecto.Adapters.SQL.Stream, File.Stream, Floki.HTMLTree, Function, GenEvent.Stream, HashDict, 
      HashSet, IO.Stream, IP.Prefix, Jason.OrderedObject, List, Map, MapSet, MerkleMap, 
      Phoenix.LiveView.LiveStream, Postgrex.Stream, Range, Stream, StreamData
      """

  while inspecting:

      %{__struct__: Log, name: nil}

  Stacktrace:

    (elixir 1.14.3) lib/enum.ex:1: Enumerable.impl_for!/1
    (elixir 1.14.3) lib/enum.ex:166: Enumerable.reduce/3
    (elixir 1.14.3) lib/enum.ex:4307: Enum.reduce/3
    (elixir 1.14.3) lib/inspect.ex:595: Inspect.Any.inspect/2
    (elixir 1.14.3) lib/inspect/algebra.ex:341: Inspect.Algebra.to_doc/2
    (elixir 1.14.3) lib/kernel.ex:2254: Kernel.inspect/2
    (elixir 1.14.3) lib/macro.ex:2576: Macro.dbg_format_ast_to_debug/2
    (elixir 1.14.3) lib/macro.ex:2546: Macro.__dbg__/3
    test/my_app/my_test.exs:44: MyApp.MyTest."test stuff"/1
    (ex_unit 1.14.3) lib/ex_unit/runner.ex:512: ExUnit.Runner.exec_test/1
    (stdlib 4.3.1) timer.erl:235: :timer.tc/1
    (ex_unit 1.14.3) lib/ex_unit/runner.ex:463: anonymous fn/4 in ExUnit.Runner.spawn_test_monitor/4
>

I can't work out if this is an error in displaying the struct, or actually using the struct for things. As I want to use the struct in assertions for tests, trying to print out test failures is itself raising protocol errors like this, and it's rather confusing!

[bvobart]

Did you ever end up fixing this issue? I'm running into the exact same problem in my Elixir project, though I am using IO.inspect instead of dbg. As you said, it makes printing out test failures fail with these protocol errors, making it incredibly difficult to debug why my tests are failing.

[sevenseacat]

I didn't, unfortunately - I ended up working around it to not do it :(

[edgurgel]

Hey team Mimic 1.8.0 1.8.1 should fix this issue. Please reopen if that's not the case!