Allow error summary to contain links to other pages

This allows for a less-common pattern where the error summary is used on a page, but to summarise errors in form fields on a separate page.

lib/summarise_errors_matcher.rb

@@ -60,12 +60,21 @@ def all_error_messages_links_are_valid
           link = error_message_item.all(:link).first
 
           if link
-            link_fragment = link[:href].split('#').last
+            uri = URI(link[:href])
 
-            link_target= html.all(id: link_fragment).first || html.all(:field, name: link_fragment).first
+            # If the link is a fragment (href=#name) link, check that
+            # it actually links to a field on the page.
+            if uri.fragment && uri.path == ""
+              link_target = html.all(:field, id: uri.fragment).first || html.all(:field, name: uri.fragment).first
+
+              link_target
+            else
+              # Skip - error summary links to another page
+              true
+            end
 
-            link_target
           else
+            # No links found
             false
           end
         end

spec/summarise_errors_matcher_spec.rb

@@ -223,4 +223,38 @@
       }.to fail_with("Error message ‘Enter your full name’ links to #full-name but no input field has this ID or name")
     end
   end
+
+  # This allows for a less-common pattern where the error summary
+  # is used on a page, but to summarise errors in form fields on a
+  # separate page.
+  context "where the errors are linked to a another page" do
+
+    let(:html) { '
+    <title>Error: Your details</title>
+    <div class="govuk-error-summary" data-module="govuk-error-summary">
+      <div role="alert">
+        <h2 class="govuk-error-summary__title">
+          There is a problem
+        </h2>
+        <div class="govuk-error-summary__body">
+          <ul class="govuk-list govuk-error-summary__list">
+            <li>
+              <a href="/name">Enter your full name</a>
+            </li>
+            <li>
+              <a href="/nationality">Select if you are British, Irish or a citizen of a different country</a>
+            </li>
+          </ul>
+        </div>
+      </div>
+    </div>'}
+
+    it "should pass if all errors listed" do
+      expect(html).to summarise_errors([
+        "Enter your full name",
+        "Select if you are British, Irish or a citizen of a different country"
+      ])
+    end
+  end
+
 end