Show backwars routing error for final page

app/services/routes/build_service.rb

@@ -28,7 +28,18 @@ def build_routes
   def options_for_goto_page(page, selected = nil)
     next_page = form.next_page_after(page)
 
-    return [] unless next_page
+    # If there's no next page then it's the last page of the form.
+    # We only need options for this page if there's an error that needs correcting.
+    if next_page.nil?
+      selected_page = if selected && selected != Forms::RouteInput::DEFAULT_VALUE
+                        option_for_select(form.pages.find { |page| page.id == selected })
+                      end
+
+      return [
+        selected_page,
+        [END_OF_FORM_OPTION.first, Forms::RouteInput::DEFAULT_VALUE],
+      ].compact
+    end
 
     # Don't include the current page or pages before in the options,
     # unless the goto page for the existing condition is before the current page,

app/views/routes/show.html.erb

@@ -25,7 +25,7 @@
               row.with_value do
                 capture do %>
                 <p> <%= page.question_text %> </p>
-                <% if @routes_input.form.next_page_after(page).present? %>
+                <% if @routes_input.form.next_page_after(page).present? || routes.any?(&:invalid?) %>
                   <ul class="govuk-list">
                     <% routes.each do |route| %>
                       <li>

spec/services/routes/build_service_spec.rb

@@ -205,8 +205,23 @@
       end
     end
 
-    it "returns an empty array if the page has no next page" do
-      expect(service.options_for_goto_page(pages.third)).to be_empty
+    context "when the page has no next page" do
+      it "returns the end of the form option" do
+        expected_options = [["End of the form", "default"]]
+
+        expect(service.options_for_goto_page(pages.third)).to match_array(expected_options)
+      end
+
+      context "when there is a selected page" do
+        it "returns the selected page and end of the form options" do
+          expected_options = [
+            ["1. question 1", pages.first.id],
+            ["End of the form", "default"],
+          ]
+
+          expect(service.options_for_goto_page(pages.third, pages.first.id)).to match_array(expected_options)
+        end
+      end
     end
 
     it "returns a list of all possible goto options" do

spec/views/routes/show.html.erb_spec.rb

@@ -171,6 +171,43 @@ def select_options(select_field)
           end
         end
       end
+
+      context "when the routing page is the final page" do
+        let(:pages) do
+          [
+            build_stubbed(:page, id: 101),
+            build_stubbed(:page, id: 102),
+            build_stubbed(
+              :page,
+              id: 103,
+              routing_conditions: [
+                build_stubbed(
+                  :condition,
+                  routing_page_id: 103,
+                  goto_page_id: 101,
+                  answer_value: nil,
+                ),
+              ],
+            ),
+          ]
+        end
+
+        let(:routes_input) do
+          build(:routes_input, form:).assign_form_values.tap do |routes_input|
+            routes_input.routes.each { |route| allow(route).to receive(:invalid?).and_return(true) }
+          end
+        end
+
+        it "shows the selected goto page for the route" do
+          render_page
+
+          expect(rendered).to have_selector('.govuk-select[name="forms_routes_input[routes_attributes][2][goto]"]') do |field|
+            expect(field).to have_selector("option[selected]") do |option|
+              expect(option["value"]).to eq "101"
+            end
+          end
+        end
+      end
     end
 
     context "when a page is a select from a list question" do
@@ -247,7 +284,7 @@ def select_options(select_field)
     end
   end
 
-  context "when there are not enoough pages" do
+  context "when there are not enough pages" do
     let(:pages) do
       [
         build_stubbed(:page, id: 101),