It’s a bit tricky, but it’s possible to do this. However, before I get to that I wanted to note that according to SemVer spec 2.0 0.x.y versions do not come with any compatibility guarantees.

ExDoc version requirement based on Elixir version

One option would be to determine the ExDoc version requirement based on the current Elixir version:

  defp deps do
    [
      {:ex_doc, ex_doc_version(), only: :dev, runtime: false}
    ]
  end

  defp ex_doc_version do
    if System.version() >= "1.7", do: "~> 0.19", else: "~> 0.18.0"
  end

The drawback of this approach is that you now have an inconsistent mix.lock file between different Elixir versions. This might not be a problem if you primarily work with 1.7 and just want to include 1.6 on CI.

Use ExDoc 0.18 from escript

Another approach would be to stick to "~> 0.19" in your deps and use the older version from an escript install. On your CI if it’s running with 1.6 you would need something like this to generate the docs:

$ mix escript.install hex ex_doc '~> 0.18.0'
$ mix compile # there will be some warnings relating to ex_doc 0.19, but those can be ignored
$ path/to/escripts/ex_doc YourProject 0.1.0 _build/dev/lib/your_project/ebin -m YourProject

I prefer this approach because it avoids the mix.lock inconsistency and restricts the necessary changes to your CI configuration.