From: "osyoyu (Daisuke Aritomo) via ruby-core" <ruby-core@...>
Date: 2025-12-01T15:16:02+00:00
Subject: [ruby-core:123966] [Ruby Bug#21723] `binding.irb` raises a LoadError under `bundle exec`

Issue #21723 has been reported by osyoyu (Daisuke Aritomo).

----------------------------------------
Bug #21723: `binding.irb` raises a LoadError under `bundle exec`
https://bugs.ruby-lang.org/issues/21723

* Author: osyoyu (Daisuke Aritomo)
* Status: Open
* ruby -v: ruby 4.0.0preview2 (2025-11-17 master 4fa6e9938c) +PRISM [arm64-darwin24]
* Backport: 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN
----------------------------------------
In Ruby 4.0, `binding.irb` raises a LoadError when some unknown condition is met.

```ruby
# main.rb
binding.irb
```

```
main.rb:1: warning: irb used to be loaded from the standard library, but is not part of the default gems since Ruby 4.0.0.
You can add irb to your Gemfile or gemspec to fix this error.
Failed to activate irb, please install it with 'gem install irb'
/Users/osyoyu/.rbenv/versions/master/lib/ruby/4.0.0+0/bundled_gems.rb:60:in 'Kernel.require': cannot load such file -- irb (LoadError)
Did you mean?  erb
        from /Users/osyoyu/.rbenv/versions/master/lib/ruby/4.0.0+0/bundled_gems.rb:60:in 'block (2 levels) in Kernel#replace_require'
        from main.rb:1:in 'Binding#irb'
        from main.rb:1:in '<main>'
/Users/osyoyu/.rbenv/versions/master/lib/ruby/4.0.0+0/bundled_gems.rb:60:in 'Kernel.require': cannot load such file -- irb (LoadError)
Did you mean?  erb
        from /Users/osyoyu/.rbenv/versions/master/lib/ruby/4.0.0+0/bundled_gems.rb:60:in 'block (2 levels) in Kernel#replace_require'
        from main.rb:1:in 'Binding#irb'
        from main.rb:1:in '<main>'
```

While irb has been changed to a bundle gem in #20309, it is agreed that `binding.irb` shall work under `bundle exec` without adding `irb` in Gemfile.
Thus, I've considered this behavior as an bug and created a ticket here.

One condition I have discovered is when the Gemfile contains a `gem` reference with a `path:` option.
I have created a repro repository in https://github.com/osyoyu/irbfail for that case.



-- 
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