As for your actual question:

defguard is_query(a) when is_map(a) and :erlang.map_get(:__struct__, a) == Ecto.Query

Ha, I like this a lot. Thanks for pointing it out.

It is impossible to do in a defguard. I think it is possible, with big caveats so maybe actually impossible too, with a defmacro. If the protocol is consolidated, you can grab the list of modules implementing it. So if your protocol is consolidated by the time you call your guard, which generally may not necessarily be the case, that should work. Here’s a sketch:

defmodule Macros do
  defmacro is_impl(term, protocol) do
    protocol = Macro.expand(protocol, __ENV__)

    impls =
      case protocol.__protocol__(:impls) do
        {:consolidated, impls} ->
          impls

        :not_consolidated ->
          raise "#{inspect(protocol)} is not consolidated"
      end

    quote do
      unquote(term).__struct__ in unquote(impls)
    end
  end
end

defmodule Foo do
  import Macros

  def f(enumerable) when is_impl(enumerable, Enumerable) do
    Enum.to_list(enumerable)
  end
end

besides the caveat around consolidation time, in the list of impls we have things like Atom, Integer, …, Any, and they all would have to be handled too.

So yeah, not worth it.