[ruby-core:97453] [Ruby master Bug#16690] [BUG] Set of sets: containing set no longer believes member set is a member after mutating its member set

Issue #16690 has been reported by keithyjohnson (Keith Johnson).

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Bug #16690: [BUG] Set of sets: containing set no longer believes member set is a member after mutating its member set
https://bugs.ruby-lang.org/issues/16690

* Author: keithyjohnson (Keith Johnson)
* Status: Open
* Priority: Normal
* ruby -v: 2.3.0
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN
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Please correct me if I'm wrong but I expected that when mutating a member set of a containing set, that that mutated member set, would still be considered a member of the containing set.  But after merging another set into the member set, the containing set no longer believes it to be a member.  I was surprised by this, but if that is expected behavior, please let me know!  Also, please let me know if you have any more questions.

``` ruby
[1] pry(#<Clustering>)> clusters.class
=> Set
[2] pry(#<Clustering>)> smaller_cluster.class
=> Set
[3] pry(#<Clustering>)> larger_cluster.class
=> Set
[4] pry(#<Clustering>)> x = larger_cluster.object_id
=> 70237123254460
[5] pry(#<Clustering>)> clusters.member?(larger_cluster)
=> true
[6] pry(#<Clustering>)> larger_cluster.merge(smaller_cluster)
=> #<Set: {387, 92}>
[7] pry(#<Clustering>)> larger_cluster.object_id == x
=> true
[8] pry(#<Clustering>)> clusters.member?(larger_cluster)
=> false
[9] pry(#<Clustering>)> clusters.map(&:object_id).include?(larger_cluster.object_id)
=> true
[10] pry(#<Clustering>)> RUBY_VERSION
=> "2.3.0"
```





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