improve error message when trying to change email address to one used by a staged user

config/locales/server.en.yml

@@ -522,6 +522,7 @@ en:
     confirmed: "Your email has been updated."
     please_continue: "Continue to %{site_name}"
     error: "There was an error changing your email address. Perhaps the address is already in use?"
+    error_staged: "There was an error changing your email address. The address is already in use by a staged user."
     already_done: "Sorry, this confirmation link is no longer valid. Perhaps your email was already changed?"
     authorizing_old:
       title: "Thanks for confirming your current email address"

lib/email_updater.rb

@@ -26,7 +26,11 @@ class EmailUpdater
     email = Email.downcase(email_input.strip)
     EmailValidator.new(attributes: :email).validate_each(self, :email, email)
 
-    errors.add(:base, I18n.t('change_email.error')) if User.find_by_email(email)
+    if existing_user = User.find_by_email(email)
+      error_message = 'change_email.error'
+      error_message << '_staged' if existing_user.staged?
+      errors.add(:base, I18n.t(error_message))
+    end
 
     if errors.blank?
       args = {

spec/components/email_updater_spec.rb

@@ -5,6 +5,17 @@ describe EmailUpdater do
   let(:old_email) { 'old.email@example.com' }
   let(:new_email) { 'new.email@example.com' }
 
+  it "provides better error message when a staged user has the same email" do
+    Fabricate(:user, staged: true, email: new_email)
+
+    user = Fabricate(:user, email: old_email)
+    updater = EmailUpdater.new(user.guardian, user)
+    updater.change_to(new_email)
+
+    expect(updater.errors).to be_present
+    expect(updater.errors.messages[:base].first).to be I18n.t("change_email.error_staged")
+  end
+
   context 'as a regular user' do
     let(:user) { Fabricate(:user, email: old_email) }
     let(:updater) { EmailUpdater.new(user.guardian, user) }