Raise Gitlab::View::Presenter::CannotOverrideMethodError if presentee already respond to method

Signed-off-by: Rémy Coutable <remy@rymai.me>

lib/gitlab/view/presenter/base.rb

 module Gitlab
   module View
     module Presenter
+      CannotOverrideMethodError = Class.new(StandardError)
+
       module Base
         extend ActiveSupport::Concern
 

lib/gitlab/view/presenter/delegated.rb

@@ -8,9 +8,11 @@ module Gitlab
           @subject = subject
 
           attributes.each do |key, value|
-            unless subject.respond_to?(key)
-              define_singleton_method(key) { value }
+            if subject.respond_to?(key)
+              raise CannotOverrideMethodError.new("#{subject} already respond to #{key}!")
             end
+
+            define_singleton_method(key) { value }
           end
 
           super(subject)

spec/lib/gitlab/view/presenter/delegated_spec.rb

@@ -17,10 +17,9 @@ describe Gitlab::View::Presenter::Delegated do
       expect(presenter.current_user).to eq('Jane Doe')
     end
 
-    it 'does not override the presentee attributes' do
-      presenter = presenter_class.new(project, user: 'Jane Doe')
-
-      expect(presenter.user).to eq('John Doe')
+    it 'raise an error if the presentee already respond to method' do
+      expect { presenter_class.new(project, user: 'Jane Doe') }.
+        to raise_error Gitlab::View::Presenter::CannotOverrideMethodError
     end
   end