[ruby-core:104732] [Ruby master Bug#17841] The first instance of SortedSet doesn't sort when rbtree is present

Issue #17841 has been updated by usa (Usaku NAKAMURA).

Backport changed from 2.6: DONTNEED, 2.7: REQUIRED, 3.0: DONTNEED to 2.6: DONTNEED, 2.7: DONE, 3.0: DONTNEED

merged into ruby_2_7 at 2a7235421fcd59b449c84306d059f22b4c5f0865

----------------------------------------
Bug #17841: The first instance of SortedSet doesn't sort when rbtree is present
https://bugs.ruby-lang.org/issues/17841#change-93066

* Author: joshc (Josh C)
* Status: Closed
* Priority: Normal
* ruby -v: ruby 2.7.3p183 (2021-04-05 revision 6847ee089d) [x86_64-linux]
* Backport: 2.6: DONTNEED, 2.7: DONE, 3.0: DONTNEED
----------------------------------------
If the `rbtree` gem is present, then in ruby 2.7, the first instance of `SortedSet` will return entries in insertion order. Future instances of `SortedSet` correctly return sorted entries.

Given:

```
require 'rbtree'
require 'set'

descending = [2, 1]

s1 = SortedSet.new
descending.each { |i| s1.add(i) }

s2 = SortedSet.new
descending.each { |i| s2.add(i) }

puts s1.to_a == s1.sort.to_a
puts s2.to_a == s2.sort.to_a

```

It prints:

```
false
true
```

It should print:

```
true
true
```

The reason for this is because the `SortedSet#initialize` method monkey patches *itself*. So the first instance of `SortedSet` uses the default `@hash = {}` instead of `@hash = RBTree.new`, and it doesn't monkey patch the `to_a` method to sort the hash keys.

This is an issue for anyone adding the `sorted_set` gem  dependency to support both ruby 2 and 3, since the `sorted_set` gem depends on `rbtree`



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