Allow an absolute controller path inside a module scope

Fixes #12777

actionpack/CHANGELOG.md

+*   Allow an absolute controller path inside a module scope
+
+    Fixes #12777
+
 *   Unique the segment keys array for non-optimized url helpers
 
     In Rails 3.2 you only needed pass an argument for dynamic segment once so

actionpack/lib/action_dispatch/routing/mapper.rb

@@ -218,8 +218,12 @@ def default_controller_and_action
               controller ||= default_controller
               action     ||= default_action
 
-              unless controller.is_a?(Regexp)
-                controller = [@scope[:module], controller].compact.join("/").presence
+              if @scope[:module] && !controller.is_a?(Regexp)
+                if controller =~ %r{\A/}
+                  controller = controller[1..-1]
+                else
+                  controller = [@scope[:module], controller].compact.join("/").presence
+                end
               end
 
               if controller.is_a?(String) && controller =~ %r{\A/}

actionpack/test/dispatch/routing_test.rb

@@ -2882,6 +2882,18 @@ def test_multiple_positional_args_with_the_same_name
     assert_equal '/downloads/1/1.tar', download_path('1', '1')
   end
 
+  def test_absolute_controller_namespace
+    draw do
+      namespace :foo do
+        get '/', to: '/bar#index', as: 'root'
+      end
+    end
+
+    get '/foo'
+    assert_equal 'bar#index', @response.body
+    assert_equal '/foo', foo_root_path
+  end
+
 private
 
   def draw(&block)

guides/source/routing.md

@@ -239,6 +239,8 @@ In each of these cases, the named routes remain the same as if you did not use `
 | PATCH/PUT | /admin/posts/:id      | posts#update      | post_path(:id)      |
 | DELETE    | /admin/posts/:id      | posts#destroy     | post_path(:id)      |
 
+TIP: _If you need to use a different controller namespace inside a `namespace` block you can specify an absolute controller path, e.g: `get '/foo' => '/foo#index'`._
+
 ### Nested Resources
 
 It's common to have resources that are logically children of other resources. For example, suppose your application includes these models: