Issue #13257 has been updated by Satoshi TAGOMORI.


My motivation is to know whether there is an class/module in ancestors or not.

I needed it at this patch: https://github.com/msgpack/msgpack-ruby/pull/132/files
The feature implemented in this patch serialize/deserialize objects in a configured format if the object is an kind of registered classes or modules.
It should consider about the order of inheritance, so it can't use `#is_a?` method, and it should consider about modules added by `#extend`, so it should check ancestors of singleton classes.

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Bug #13257: Symbol#singleton_class should be undef
https://bugs.ruby-lang.org/issues/13257#change-63304

* Author: Satoshi TAGOMORI
* Status: Feedback
* Priority: Normal
* Assignee: 
* Target version: 
* ruby -v: ruby 2.4.0p0 (2016-12-24 revision 57164) [x86_64-darwin15]
* Backport: 2.2: UNKNOWN, 2.3: UNKNOWN, 2.4: UNKNOWN
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Objects of some classes doesn't have singleton classes (e.g., Symbol, Integer, Float...).
Symbol#singleton_class raises TypeError.

But Symbol#respond_to?(:singleton_class) returns true, and we cannot know when #singleton_class returns a valid class or raise errors in any way (except for calling it).

I think that such #singleton_class methods should be undef.



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