I am using unity 4.6.6 to create a Xcode build. When I create a build I am getting following error
Cannot initialize return object of type 'Singleton_1_t6738 *' with an rvalue of type 'Singleton_1_t6737 *'
https://msdn.microsoft.com/en-us/library/ff650316.aspx
http://www.dotnetperls.com/singleton
Hope these links help.
Have you consulted the Unity website?