For meta-programming/debugging purposes, I would like to request the inverse of
Object#singleton_class. Namely, a method that is called on a class that is a singleton class, and returns the object it is a singleton of. Since the
Singleton module in the standard library http://ruby-doc.org/stdlib-2.3.0/libdoc/singleton/rdoc/Singleton.html assigns the method name
instance to such classes, I think
Class#instance should be the name for such feature.
Array.singleton_class.instance # => Array "foo".singleton_class.instance # => "foo"
When the receiver is a class but is not a singleton class, then it should raise an error.
Array.instance # => error