Fix shorthand routes where controller and action are in the scope

Merge `:action` from routing scope and assign endpoint if both `:controller`
and `:action` are present. The endpoint assignment only occurs if there is
no `:to` present in the options hash so should only affect routes using the
shorthand syntax (i.e. endpoint is inferred from the the path).

Fixes #9856

actionpack/CHANGELOG.md

+*   Merge `:action` from routing scope and assign endpoint if both `:controller`
+    and `:action` are present. The endpoint assignment only occurs if there is
+    no `:to` present in the options hash so should only affect routes using the
+    shorthand syntax (i.e. endpoint is inferred from the the path).
+
+    Fixes #9856
+
+    *Yves Senn*, *Andrew White*
+
 *   ActionView extracted from ActionPack
 
     *Piotr Sarnacki*, *Łukasz Strzałkowski*

actionpack/lib/action_dispatch/routing/mapper.rb

@@ -11,8 +11,8 @@ module Routing
     class Mapper
       URL_OPTIONS = [:protocol, :subdomain, :domain, :host, :port]
       SCOPE_OPTIONS = [:path, :shallow_path, :as, :shallow_prefix, :module,
-                       :controller, :path_names, :constraints, :defaults,
-                       :shallow, :blocks, :options]
+                       :controller, :action, :path_names, :constraints,
+                       :shallow, :blocks, :defaults, :options]
 
       class Constraints #:nodoc:
         def self.new(app, constraints, request = Rack::Request)
@@ -874,6 +874,10 @@ def merge_controller_scope(parent, child) #:nodoc:
             child
           end
 
+          def merge_action_scope(parent, child) #:nodoc:
+            child
+          end
+
           def merge_path_names_scope(parent, child) #:nodoc:
             merge_options_scope(parent, child)
           end
@@ -1383,6 +1387,10 @@ def match(path, *rest)
             raise ArgumentError, "Unknown scope #{on.inspect} given to :on"
           end
 
+          if @scope[:controller] && @scope[:action]
+            options[:to] ||= "#{@scope[:controller]}##{@scope[:action]}"
+          end
+
           paths.each do |_path|
             route_options = options.dup
             route_options[:path] ||= _path if _path.is_a?(String)