Restore to: option with an implicit controller

The `:to` option for routes can once again be a Symbol or a String
without a controller if the controller is implicitly provided by a
nesting `controller` or `resources` call.

actionpack/lib/action_dispatch/routing/mapper.rb

@@ -231,12 +231,21 @@ def normalize_options!(options, path_params, modyoule)
               if to.nil?
                 controller = default_controller
                 action = default_action
-              elsif to.is_a?(String) && to.include?("#")
-                to_endpoint = to.split("#").map!(&:-@)
-                controller  = to_endpoint[0]
-                action      = to_endpoint[1]
               else
-                raise ArgumentError, ":to must respond to `action` or `call`, or it must be a String that includes '#'"
+                if to.is_a?(String)
+                  controller_or_action, action = to.split("#", 2).map!(&:-@)
+                  if !action || action.empty?
+                    action = controller_or_action
+                  else
+                    controller = controller_or_action
+                  end
+                elsif to.is_a?(Symbol)
+                  action = to
+                end
+                controller ||= default_controller
+                unless controller
+                  raise ArgumentError, ":to must respond to `action` or `call`, or it must be a String that includes '#', or the controller should be implicit"
+                end
               end
 
               controller = add_controller_module(controller, modyoule)

actionpack/test/dispatch/routing_test.rb

@@ -4037,6 +4037,7 @@ def draw(&block)
     routes = ActionDispatch::Routing::RouteSet.new
     routes.draw(&block)
     @app = self.class.build_app routes
+    @routes = routes
   end
 
   def test_missing_controller
@@ -4057,6 +4058,17 @@ def test_missing_controller_with_to
     assert_match(/:to must respond to/, ex.message)
   end
 
+  def test_implicit_controller_with_to
+    draw do
+      controller :foo do
+        get "/foo/bar", to: "bar"
+        get "/foo/baz", to: :baz
+      end
+    end
+    assert_routing "/foo/bar", controller: "foo", action: "bar"
+    assert_routing "/foo/baz", controller: "foo", action: "baz"
+  end
+
   def test_to_is_a_symbol
     ex = assert_raises(ArgumentError) {
       draw do
@@ -4066,13 +4078,13 @@ def test_to_is_a_symbol
     assert_match(/:to must respond to/, ex.message)
   end
 
-  def test_missing_action_on_hash
+  def test_missing_action_with_to
     ex = assert_raises(ArgumentError) {
       draw do
         get "/foo/bar", to: "foo#"
       end
     }
-    assert_match(/Missing :action/, ex.message)
+    assert_match(/:to must respond to/, ex.message)
   end
 
   def test_valid_controller_options_inside_namespace