[ruby-core:127158] [Ruby Bug#22452] `rb_equal` returns true for `NaN == NaN`

17 Oct 2026

      Issue #22452 has been updated by byroot (Jean Boussier).

Possibly a duplicate of [Bug #1720] where it was established that `Float::NaN == Float::NaN` is "undefined" therefore OK.

It seems that the motivation was to not prevent optimization of `rb_equal`.

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Bug #22452: `rb_equal` returns true for `NaN == NaN`
https://bugs.ruby-lang.org/issues/22452#change-119564

* Author: byroot (Jean Boussier)
* Status: Open
* Backport: 3.3: UNKNOWN, 3.4: UNKNOWN, 4.0: UNKNOWN
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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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