From: "Eregon (Benoit Daloze) via ruby-core" <ruby-core@...>
Date: 2026-10-19T19:25:23+00:00
Subject: [ruby-core:127177] [Ruby Bug#22452] `rb_equal` returns true for `NaN == NaN`

Issue #22452 has been updated by Eregon (Benoit Daloze).


There is also `equal?`:
```ruby
irb(main):003> RUBY_DESCRIPTION
=> "ruby 4.0.7 (2026-09-15 revision 229531a6cf) +PRISM [x86_64-linux]"
irb(main):001> Float::NAN == Float::NAN
=> false
irb(main):002> Float::NAN.equal?(Float::NAN)
=> true
irb(main):008> Float::NAN.equal?(-(0.0/0.0))
=> false
irb(main):009> Float::NAN.equal?(0.0/0.0)
=> false
irb(main):001> [Float::NAN, 0.0/0.0, -(0.0/0.0)].pack('d3').bytes.each_slice(8).to_a
=> [[0, 0, 0, 0, 0, 0, 248, 127], [0, 0, 0, 0, 0, 0, 248, 127], [0, 0, 0, 0, 0, 0, 248, 255]]
```
And on TruffleRuby:
```ruby
irb(main):008> RUBY_DESCRIPTION
=> "truffleruby 40.0.0-dev-da7373fe (2026-09-03), like ruby 4.0.2, Oracle GraalVM Native [x86_64-linux]"
irb(main):003> Float::NAN == Float::NAN
=> false
irb(main):005> Float::NAN.equal?(Float::NAN)
=> true
irb(main):006> Float::NAN.equal?(-(0.0/0.0))
=> true
irb(main):007> Float::NAN.equal?(0.0/0.0)
=> false
irb(main):002> [Float::NAN, 0.0/0.0, -(0.0/0.0)].pack('d3').bytes.each_slice(8).to_a
=> [[0, 0, 0, 0, 0, 0, 248, 127], [0, 0, 0, 0, 0, 0, 248, 255], [0, 0, 0, 0, 0, 0, 248, 127]]
```

(for I'm not sure what reason the NaN bit pattern is inverted for `0.0/0.0` and `-(0.0/0.0)`, on CRuby and TruffleRuby, maybe on C vs JVM)

----------------------------------------
Bug #22452: `rb_equal` returns true for `NaN == NaN`
https://bugs.ruby-lang.org/issues/22452#change-119590

* Author: byroot (Jean Boussier)
* Status: Open
* Backport: 3.3: UNKNOWN, 3.4: UNKNOWN, 4.0: UNKNOWN
----------------------------------------
I noticed this while trying to cleanup and simplify `rb_equal`.

```ruby
>> Float::NAN == Float::NAN
=> false
>> [Float::NAN].index(Float::NAN)
=> 0
```

I would expect it to return `nil`.

`Array#index` is just an example, any core method, and even C extension relying on `rb_equal` is similarly impacted.

This bug is present on all Ruby versions, except for versions between 2.1 and 2.4. 

The bug seem to have been re-introduced in 2.5 by [6e0e067de31511cebec01d176f628442371db879](https://github.com/ruby/ruby/commit/6e0e067de31511cebec01d176f628442371db879)

JRuby seem to have the same behavior, but TruffleRuby correctly returns`nil`.



-- 
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