Issue #17038 has been updated by marcandre (Marc-Andre Lafortune).

Status changed from Open to Rejected

Note that this change is listed in the `NEWS`

I'm thus closing this.

----------------------------------------
Bug #17038: On master, ancestry edits can lead to duplicates in Module#ancestors
https://bugs.ruby-lang.org/issues/17038#change-86635

* Author: alanwu (Alan Wu)
* Status: Rejected
* Priority: Normal
* ruby -v: ruby 2.8.0dev (2020-07-20T06:39:31Z master 935d0b3d05) [x86_64-linux]
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN
----------------------------------------
Running the following [script](https://wandbox.org/permlink/0f48S2QhvEO1cOBF) on master(935d0b3d05dfc8b30bba505792129bf0e33ebe3b),
`A` appears three times in the lookup chain of `C` and `A#foo` is called multiple
times with `super`.

```ruby
module A
  def foo
    [:a] + super
  end
end

module B
  def foo
    [:b] + super
  end
end

class Object
  def foo
    [:object]
  end
end

module C
  def foo
    [:c] + super
  end
end

class D
  prepend A
  include C
  include B


  def foo
    [:d] + super
  end
end

B.include A
C.include A

p D.ancestors
p D.new.foo
p RUBY_REVISION

__END__

[A, D, B, A, C, A, Object, Kernel, BasicObject]
[:a, :d, :b, :a, :c, :a, :object]
"935d0b3d05dfc8b30bba505792129bf0e33ebe3b"
```

This change was introduced in #9573. Is this behavior intentional?
In my opinion it's a bit odd since it's not possible to have duplicates
in the lookup chain on released versions. Allowing duplicates can
surprise peopole that are used to not having duplicates and lead to bugs.
If `A#foo` had side effects, for example, calling it multiple times
could be undesirable.

Also, logically it doesn't make sense to have a module be the ancestor of itself.
Comparison operators make less sense with this new setup.



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