I think the functional part of this PR is that if clazz is a singleton it will return public which does seems to match vm_define_method in MRI. This PR with only the singleton check of the if statement may be what you want (although we should be able to see some test/spec pass too as a result).

I am not sure what NO_SUPER_CLASS and NOT_IMPLEMENTED checks are for? They are not doing what you expect but if you explain what you think they are doing I can maybe help you do the right thing. NO_SUPER_CLASS.toString() -> "NO_SUPER_CLASS" which would only have an effect if your Ruby method was named that.

You can ping me or @headius on matrix to talk about this. Deciphering our codebase and MRI's codebase can be tricky at times :)

Looks ok to me. Concur with @enebo that the main change here is to use public visibility for singleton classes.