Bug #22378: `Ractor::IsolationError` in code not using Ractors

• Status: Open
• Assignee: ractor
• Target version: -
• ruby -v: ruby 4.1.0dev (2026-09-23T07:42:54Z master 8cdc7b5957) +PRISM [x86_64-linux]
• Backport: 3.3: UNKNOWN, 3.4: UNKNOWN, 4.0: UNKNOWN

[Earlopain (Earlopain _)]

# Reduced from the `httpx` gem
@var = Class.new.freeze.dup
@var.instance_variable_set(:@test, 123)

I expect this code to succeed. dup returns an unfrozen copy, so I should be able to set instance variables on it. However, I get the following error even though the code is ractor-free:

test.rb:2:in 'Kernel#instance_variable_set': can't modify instance variables of a shareable Class (Ractor::IsolationError)

It works on 4.0 but fails on current master. Caused by #22291

[Earlopain (Earlopain _)]

• Related to Bug #22291: Instance variables should be forbidden on Ractor-shareable objects added

[Earlopain (Earlopain _)]

• Description updated (diff)

[luke-gru (Luke Gruber)]

• Assignee set to ractor