Intentional. Consider it an early deprecation warning powered by Dialyzer. :)

Unless I'm missing something does this change means anyone that uses Dialyzer can no longer use Phoenix.View as you can no longer set layouts to be used? This feels like a massive breaking change which didn't even get mentioned in the release notes.

You can use Phoenix.View and you can still set layouts. That's not the problem here. The old style still works and, if you want to avoid the Dialyzer warning (which does not stop the code from executing), it is a matter of replacing this:

plug :put_layout, {ExampleWeb.LayoutView, "app.html"}

by this:

plug :put_layout, html: {ExampleWeb.LayoutView, "app.html"}

plug :put_layout, html: {ExampleWeb.LayoutView, "app.html"}

This was not a drop-in replacement for me, as it seems to require that you are also using the new-style layouts:

put_layout and put_root_layout expects an module and template per format, such as:

    html: {MyView, :app}

Got:

    {ExampleWeb.LayoutView, "custom.html"}

If I use the new atom syntax html: {ExampleWeb.LayoutView, :custom}, there are no errors, but the custom layout is not used, it uses the app layout instead.

However, the old syntax works fine.

What I did was write a hack to hide it from dialyzer, until we have time to migrate properly:

  @spec put_layout_workaround(Plug.Conn.t(), {atom(), String.t()}) :: Plug.Conn.t()
  def put_layout_workaround(conn, arg) do
    # credo:disable-for-next-line Credo.Check.Refactor.Apply
    apply(Phoenix.Controller, :put_layout, [conn, arg])
  end

Then

plug :put_layout_workaround, {ExampleWeb.LayoutView, "custom.html"}

@adamu yeah I got the same exception trying the code @josevalim is suggesting. Your hack to get around dialyzer looks like it will be necessary as the new style does seem to require you to migrate your app which we just don't have time for in one go and don't want to get left behind.

Hrm, this is not supposed to happen. Can you folks please provide a custom app that reproduces the error? Then I will take a look at it immediately. :)

PS: @adamu my kid was very happy to see a Minecraft something (your avatar I believe) at his dad work computer :D

Haha, I made that avatar way back in 2012 or so. It's been a while since I actually played :)

I've made a reproduction of the issue in a dummy project here:

https://github.com/adamu/put_layout_repro/blob/c331d0addc38dad9493c152826f6a07a8d3f1ce8/lib/put_layout_repro_web/router.ex#L9-L16

    # ---------------------------------------------------------------
    # Working
    plug :put_layout, {PutLayoutReproWeb.LayoutView, "custom.html"}
    # Fails to compile
    # plug :put_layout, html: {PutLayoutReproWeb.LayoutView, "custom.html"}
    # Uses App layout
    # plug :put_layout, html: {PutLayoutReproWeb.LayoutView, :custom}
    # ---------------------------------------------------------------

Thank you so much! ❤️ I pushed a warning that hopefully clarifies the situation. TL;DR: change your :controller to do:

use Phoenix.Controller, layouts: [html: {PutLayoutReproWeb.LayoutView, :app}]