Protocol functions with default arguments produces unclear error

[kipcole9]

Elixir and Erlang/OTP versions

elixir 1.20.0-rc.1-otp-28
erlang 28.3.1

Operating system

MacOS

Current behavior

Up to Elixir 1.19, the following protocol definition does not error. In Elixir 1.20 it produces an error which is quite tricky to work out.

defprotocol TestProtocol do
  def to_string(name, options \\ [])
end

The error is:

    warning: protocols can only define functions without implementation via def/1, found: to_string/1
    │
  1 │ defprotocol TestProtocol do
    │ ~~~~~~~~~~~~~~~~~~~~~~~~~~~
    │
    └─ lib/prototest_protocol.ex:1: TestProtocol (module)

It took me longer than expected to realise this is because the handling of default parameters produces multiple function clauses with a delegate implementation.

Expected behavior

1. Restore the behaviour of Elixir 1.19 or
2. Improve the error message when protocols attempt to define callbacks with default arguments.

[josevalim]

Thank you. We will restore this behaviour.

[josevalim]

Hi @kipcole9, I have written two patches, one for each option, and I decided it is probably cleaner to not allow mixing implementation concerns (default arguments) with protocol definitions, so for now we will clarify the error message.

EDIT: fixed typo, thanks @kipcole9.

[kipcole9]

I know you meant "it is probably cleaner to not allow mixing" - add the comment just in case anyone searches for this in the future.