[ruby-core:92563] [Ruby trunk Bug#15830] SortedSet's lazy setup causes unpredictable behaviour in subclass's initialize

Issue #15830 has been reported by viko (Viko Viko).

----------------------------------------
Bug #15830: SortedSet's lazy setup causes unpredictable behaviour in subclass's initialize
https://bugs.ruby-lang.org/issues/15830

* Author: viko (Viko Viko)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
* ruby -v: ruby 2.6.3p62 (2019-04-16 revision 67580) [x86_64-darwin18]
* Backport: 2.4: UNKNOWN, 2.5: UNKNOWN, 2.6: UNKNOWN
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``` ruby
require 'set'

class SortedSetWithoutLowest < SortedSet
  def initialize contents=[]
    puts "initialize, #{contents}"
    super
    lowest = to_a[0]
    puts "lowest from #{contents} is #{lowest}, removing"
    delete lowest
    puts "done initializing #{contents} to #{self}"
  end
end

3.times { SortedSetWithoutLowest[1,2,3,4,5] }
```

```
initialize, [1, 2, 3, 4, 5]
initialize, [1, 2, 3, 4, 5]
lowest from [1, 2, 3, 4, 5] is 1, removing
done initializing [1, 2, 3, 4, 5] to #<SortedSetWithoutLowest: {2, 3, 4, 5}>
lowest from [1, 2, 3, 4, 5] is 2, removing
done initializing [1, 2, 3, 4, 5] to #<SortedSetWithoutLowest: {3, 4, 5}>
initialize, [1, 2, 3, 4, 5]
lowest from [1, 2, 3, 4, 5] is 1, removing
done initializing [1, 2, 3, 4, 5] to #<SortedSetWithoutLowest: {2, 3, 4, 5}>
initialize, [1, 2, 3, 4, 5]
lowest from [1, 2, 3, 4, 5] is 1, removing
done initializing [1, 2, 3, 4, 5] to #<SortedSetWithoutLowest: {2, 3, 4, 5}>
```

The first such set created ends up with only [3, 4, 5], because SortedSet does some setup and then re-calls initialize (`set.rb` line 807). All further such sets are fine. All the sets are fine if any SortedSet has been made before, so calling `SortedSet.new` in the subclass's `initialize` and discarding the result is a viable workaround.



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