This proposal has been accepted and moved to the issues tracker under issues #5734 and #5735.
josevalim
Creator of Elixir
4
Qqwy
TypeCheck Core Team
Let me re-ask my earlier question because it got overlooked because of the discussion that sparked here directly afterwards 
:
How can we properly migrate to this new way of doing things, while for the time being keeping our code backwards-compatible with older Elixir versions?Use @optional_callbacks but also write defoverridable for backwards compatibility? Or is there a way to switch between these statements at compile-time based on the Elixir version, to make it very explicit that part of it is legacy-support that might be removed in the future?
2
josevalim
Creator of Elixir
The suggestions here should be backwards compatible. For example, imagine we make handle_call/3 an optional callback and we change use GenServer to no longer implement handle_call/3 by default. As long as we change the GenServer implementation to also cope with that, then for the user it makes no difference. If the user code defines its own handle_call/3, then it has nothing to override now and it should just work.
@impl true is not a concern because it applies only to those consuming the behaviour (and not the ones implementing it).
Does it clarify your concerns?
3
Qqwy
TypeCheck Core Team
samphilipd
I implemented this behaviour as specified in Jose’s original proposal.
Defoverridable now takes module name as argument (merged) - https://github.com/elixir-lang/elixir/pull/6022
@impl warnings (needs feedback) - https://github.com/elixir-lang/elixir/pull/6031
Comments are welcome
2