[ruby-core:107559] [Ruby master Bug#18582] Hash.group_by not grouping correctly with SortedSets

Issue #18582 has been updated by nobu (Nobuyoshi Nakada).

Status changed from Open to Third Party's Issue
Description updated

It is not because of ruby versions, by whether `SortedSet` uses RBTree gem or not.
```shell-session
$ ruby2.7 -rset -e 's1 = SortedSet["fubar"]; s2 = SortedSet["fubar"]; p s1.eql?(s2)'
true

$ gem2.7 i --user sorted_set
Fetching set-1.0.2.gem
Fetching rbtree-0.4.5.gem
Fetching sorted_set-1.0.3.gem
WARNING:  You don't have /Users/nobu/.gem/ruby/2.7.0/bin in your PATH,
	  gem executables will not run.
Building native extensions. This could take a while...
Successfully installed rbtree-0.4.5
Successfully installed set-1.0.2
Successfully installed sorted_set-1.0.3
Parsing documentation for rbtree-0.4.5
Installing ri documentation for rbtree-0.4.5
Parsing documentation for set-1.0.2
Installing ri documentation for set-1.0.2
Parsing documentation for sorted_set-1.0.3
Installing ri documentation for sorted_set-1.0.3
Done installing documentation for rbtree, set, sorted_set after 0 seconds
3 gems installed

$ ruby2.7 -rsorted_set -e 's1 = SortedSet["fubar"]; s2 = SortedSet["fubar"]; p s1.eql?(s2)'
false
```

RBTree gem needs to implement `#hash` and `#eql?` methods, to be hash keys.

----------------------------------------
Bug #18582: Hash.group_by not grouping correctly with SortedSets
https://bugs.ruby-lang.org/issues/18582#change-96472

* Author: [e-mail] (Mike Carlton)
* Status: Third Party's Issue
* Priority: Normal
* ruby -v: ruby 3.0.3p157 (2021-11-24 revision 3fb7d2cadc) [x86_64-darwin20]
* Backport: 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN, 3.1: UNKNOWN
----------------------------------------
With Ruby 3.0.3, when using SortedSets as group_by value for Hash, equal SortedSets are not grouped as they should be.
This works correctly in Ruby 2.7.1 (when rbtree gem is not present, not tested with rbtree gem)

This works correctly using Sets as the group_by value in both 2.7.1 and 3.0.3

This test code:
```ruby
require 'set'
require 'sorted_set' if RUBY_VERSION > '3'

puts RUBY_VERSION

# works when keys are Sets
s1 = Set['fubar']
s2 = Set['fubar']
warn "expected #{s1} to equal #{s2}" unless s1 == s2

grouped = { 'a' => s1, 'b' => s2 }.group_by { |_, v| v }
puts "grouped by Sets: #{grouped}"
warn "expected 1 key in hash grouped by Sets, got #{grouped.keys.size}" unless grouped.keys.size == 1

# 3.0.3 fails when keys are SortdSets
ss1 = SortedSet['fubar']
ss2 = SortedSet['fubar']
warn "expected #{ss1} to equal #{ss2}" unless ss1 == ss2

grouped = { 'a' => ss1, 'b' => ss2 }.group_by { |_, v| v }
puts "grouped by SortedSets: #{grouped}"
warn "expected 1 key in hash grouped by SortedSets, got #{grouped.keys.size}" unless grouped.keys.size == 1
```
prints this under 2.7.1:
```
2.7.1
grouped by Sets: {#<Set: {"fubar"}>=>[["a", #<Set: {"fubar"}>], ["b", #<Set: {"fubar"}>]]}
grouped by SortedSets: {#<SortedSet: {"fubar"}>=>[["a", #<SortedSet: {"fubar"}>], ["b", #<SortedSet: {"fubar"}>]]}
```

but prints this under 3.0.3:
```
3.0.3
grouped by Sets: {#<Set: {"fubar"}>=>[["a", #<Set: {"fubar"}>], ["b", #<Set: {"fubar"}>]]}
grouped by SortedSets: {#<SortedSet: {"fubar"}>=>[["a", #<SortedSet: {"fubar"}>]], #<SortedSet: {"fubar"}>=>[["b", #<SortedSet: {"fubar"}>]]}
expected 1 key in hash grouped by SortedSets, got 2
```



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