Clause cannot match warning on line 1 after upgrading to elixir 1.6

[rhnonose]

Environment

• Elixir & Erlang versions (elixir --version): 1.6.0 and 20
• Operating system: macOS High Sierra 10.13.1

Current behavior

When compiling, the following module gives the warnings:

warning: this clause cannot match because a previous clause at line 1 always matches
  lib/re_web/router.ex:1

warning: this clause cannot match because a previous clause at line 1 always matches
  lib/re_web/router.ex:1

Here's the full module:

defmodule ReWeb.Router do
  use ReWeb, :router

  pipeline :browser do
    plug(:accepts, ["html"])
    plug(:fetch_session)
    plug(:fetch_flash)
    plug(:protect_from_forgery)
    plug(:put_secure_browser_headers)
  end

  pipeline :public_api do
    plug(:accepts, ["json"])
  end

  pipeline :private_api do
    plug(:accepts, ["json"])
    plug(ReWeb.GuardianPipeline)
  end

  scope "/", ReWeb do
    pipe_through(:public_api)

    resources("/neighborhoods", NeighborhoodController, only: [:index])
    resources("/listings", ListingController, only: [:index, :show])
    resources("/interests", InterestController, only: [:create])
    post("/users/login", AuthController, :login)
    post("/users/register", AuthController, :register)
  end

  scope "/", ReWeb do
    pipe_through(:private_api)

    resources "/listings", ListingController, except: [:new] do
      resources("/images", ImageController, only: [:index, :create, :delete])
    end

    put("/listings/:listing_id/image_order", ListingController, :order)
  end

  if Mix.env() == :dev do
    scope "/dev" do
      pipe_through(:browser)

      forward("/mailbox", Plug.Swoosh.MailboxPreview, base_path: "/dev/mailbox")
    end
  end
end

There's no warning in Elixir 1.5.2 and commenting out the line 25 (resources("/listings", ListingController, only: [:index, :show])) also removes the warning.

I'm also not sure if this belongs here or in Phoenix.

Expected behavior

Give warning on the proper line or no warning at all.

[OvermindDL1]

I think that is because a bug was fixed. There is overlap in the calls between both of the "/listings" resources so that error 'should' have been there, but because of a bug it was not being shown before? Proper line would be much better though...

[josevalim]

Good catch. I am on it. Let's see what we can do.

[josevalim]

Fixed on Phoenix and back-ported to v1.3 there.

[atomkirk]

How can I update my phoenix 1.3 dep to use this commit? I ran mix deps.update phoenix and it didn't change the hash in my mix.lock…

Also tried

mix deps.unlock phoenix
mix deps.get

[josevalim]

SInce it was not released yet, you need to move to a git dependency.

[atomkirk]

Is there a way to do that while making it so that my dependency that relies on phoenix 1.0 < x < 1.3 still work?

[josevalim]

Try that out and mix should guide you to use the override: true option.

[josevalim]

You will also need to pass the branch: "v1.3" option.

[atomkirk]

ok I'm using phoenixframework/phoenix@c7a6569 and I'm still getting:

warning: this clause cannot match because a previous clause at line 2 always matches
  lib/zipbooks_web/router.ex:2

warning: this clause cannot match because a previous clause at line 2 always matches
  lib/zipbooks_web/router.ex:2

warning: this clause cannot match because a previous clause at line 2 always matches
  lib/zipbooks_web/router.ex:2

warning: this clause cannot match because a previous clause at line 2 always matches
  lib/zipbooks_web/router.ex:2

warning: this clause cannot match because a previous clause at line 2 always matches
  lib/zipbooks_web/router.ex:2

warning: this clause cannot match because a previous clause at line 2 always matches
  lib/zipbooks_web/router.ex:2

defmodule ZB.Router do
  use Phoenix.Router <------ line 2
  use Plug.ErrorHandler
  use Sentry.Plug

[ijunaid8989]

I am trying to upgrade to Elixir 1.7 from 1.5.2 and still getting this warning (Phoenix 1.3)

warning: this clause cannot match because a previous clause at line 160 always matches
  lib/evercam_media_web/controllers/archive_controller.ex:190
warning: this clause cannot match because a previous clause at line 1 always matches
  lib/evercam_media_web/router.ex:1
warning: this clause cannot match because a previous clause at line 1 always matches
  lib/evercam_media_web/router.ex:1
warning: this clause cannot match because a previous clause at line 1 always matches
  lib/evercam_media_web/router.ex:1
warning: this clause cannot match because a previous clause at line 1 always matches
  lib/evercam_media_web/router.ex:1

[OvermindDL1]

@ijunaid8989 You should post support requests to the Elixir Forums, and for your issue you should definitely post your router.ex file in the post (with proper code fences). :-)

[nelsonic]

Getting this error on a fresh Phoenix 1.7 app: dwyl/phoenix-todo-list-tutorial#59 (comment)

warning: this clause cannot match because a previous clause at line 1 always matches
  lib/app_web/router.ex:1

router.ex file is: github.com/dwyl/phoenix-todo-list-tutorial/blob/main/lib/app_web/router.ex