Improve User username error for namespace conflict

app/models/user.rb

@@ -1377,7 +1377,14 @@ class User < ApplicationRecord
 
   def set_username_errors
     namespace_path_errors = self.errors.delete(:"namespace.path")
-    self.errors[:username].concat(namespace_path_errors) if namespace_path_errors
+
+    return unless namespace_path_errors&.any?
+
+    if namespace_path_errors.include?('has already been taken') && !User.exists?(username: username)
+      self.errors.add(:base, :username_exists_as_a_different_namespace)
+    else
+      self.errors[:username].concat(namespace_path_errors)
+    end
   end
 
   def username_changed_hook

changelogs/unreleased/215309-improve-the-error-message-username-has-already-been-taken-on-user-.yml

+---
+title: Improve error message when username and namespace conflict
+merge_request: 47537
+author:
+type: changed

config/locales/en.yml

@@ -169,6 +169,7 @@ en:
     format: "%{attribute} %{message}"
     messages:
       label_already_exists_at_group_level: "already exists at group level for %{group}. Please choose another one."
+      username_exists_as_a_different_namespace: A user, alias, or group already exists with that username.
       wrong_size: "is the wrong size (should be %{file_size})"
       size_too_small: "is too small (should be at least %{file_size})"
       size_too_big: "is too big (should be at most %{file_size})"

spec/models/user_spec.rb

@@ -4280,7 +4280,7 @@ RSpec.describe User do
             it 'adds the namespace errors to the user' do
               user.update(username: new_username)
 
-              expect(user.errors.full_messages.first).to eq('Username has already been taken')
+              expect(user.errors.full_messages.first).to eq('A user, alias, or group already exists with that username.')
             end
           end
         end

spec/services/users/update_service_spec.rb

@@ -31,7 +31,7 @@ RSpec.describe Users::UpdateService do
         result = update_user(user, { username: 'taken' })
       end.not_to change { user.reload.username }
       expect(result[:status]).to eq(:error)
-      expect(result[:message]).to eq('Username has already been taken')
+      expect(result[:message]).to eq('A user, alias, or group already exists with that username.')
     end
 
     it 'updates the status if status params were given' do