FEATURE: the hide_email_address_taken setting works with the change email address form in user preferences

config/locales/server.en.yml

@@ -2660,11 +2660,11 @@ en:
       title: "Account already exists"
       subject_template: "[%{email_prefix}] Account already exists"
       text_body_template: |
-        You just tried to create an account at %{site_name}. However, an account already exists for %{email}.
+        You just tried to create an account at %{site_name}, or tried to change the email of an account to %{email}. However, an account already exists for %{email}.
 
         If you forgot your password, [reset it now](%{base_url}/password-reset).
 
-        If you didn’t try to create an account for %{email}, don’t worry – you can safely ignore this message.
+        If you didn’t try to create an account for %{email} or change your email address, don’t worry – you can safely ignore this message.
 
         If you have any questions, [contact our friendly staff](%{base_url}/about).
 

lib/email_updater.rb

@@ -27,12 +27,16 @@ class EmailUpdater
     EmailValidator.new(attributes: :email).validate_each(self, :email, email)
 
     if existing_user = User.find_by_email(email)
+      if SiteSetting.hide_email_address_taken
+        Jobs.enqueue(:critical_user_email, type: :account_exists, user_id: existing_user.id)
+      else
         error_message = 'change_email.error'
         error_message << '_staged' if existing_user.staged?
         errors.add(:base, I18n.t(error_message))
       end
+    end
 
-    if errors.blank?
+    if errors.blank? && existing_user.nil?
       args = {
         old_email: @user.email,
         new_email: email,

spec/components/email_updater_spec.rb

@@ -131,4 +131,25 @@ describe EmailUpdater do
       end
     end
   end
+
+  context 'hide_email_address_taken is enabled' do
+    before do
+      SiteSetting.hide_email_address_taken = true
+    end
+
+    let(:user) { Fabricate(:user, email: old_email) }
+    let(:existing) { Fabricate(:user, email: new_email) }
+    let(:updater) { EmailUpdater.new(user.guardian, user) }
+
+    it "doesn't error if user exists with new email" do
+      updater.change_to(existing.email)
+      expect(updater.errors).to be_blank
+      expect(user.email_change_requests).to be_empty
+    end
+
+    it 'sends an email to the owner of the account with the new email' do
+      Jobs.expects(:enqueue).once.with(:critical_user_email, has_entries(type: :account_exists, user_id: existing.id))
+      updater.change_to(existing.email)
+    end
+  end
 end

spec/requests/users_email_controller_spec.rb

@@ -96,6 +96,11 @@ describe UsersEmailController do
       context 'when the new email address is taken' do
         let!(:other_user) { Fabricate(:coding_horror) }
 
+        context 'hide_email_address_taken is disabled' do
+          before do
+            SiteSetting.hide_email_address_taken = false
+          end
+
           it 'raises an error' do
             put "/u/#{user.username}/preferences/email.json", params: {
               email: other_user.email
@@ -113,6 +118,21 @@ describe UsersEmailController do
           end
         end
 
+        context 'hide_email_address_taken is enabled' do
+          before do
+            SiteSetting.hide_email_address_taken = true
+          end
+
+          it 'responds with success' do
+            put "/u/#{user.username}/preferences/email.json", params: {
+              email: other_user.email
+            }
+
+            expect(response).to be_success
+          end
+        end
+      end
+
       context 'when new email is different case of existing email' do
         let!(:other_user) { Fabricate(:user, email: 'case.insensitive@gmail.com') }