Which Ruby classes support .clone?

Question

Ruby defines #clone in Object. To my suprise, some classes raise Exceptions when calling it. I found NilClass, TrueCla

Answer 1

I did a git grep "can't clone" of YARV's source code, and got

lib/singleton.rb:    raise TypeError, "can't clone instance of singleton #{self.class}"
object.c:        rb_raise(rb_eTypeError, "can't clone %s", rb_obj_classname(obj));
test/test_singleton.rb:    expected = "can't clone instance of singleton TestSingleton::SingletonTest"

The first and third lines indicate you can't clone a singleton.

The second line refers to rb_special_const_p(obj). But this is going beyond my ken.