Bug #21707: Destructuring assignment of SimpleDelegator wrapped array bug with YJIT

• Status: Open
• Assignee: -
• Target version: -
• ruby -v: ruby 4.0.0dev (2025-11-08T15:08:09Z master 75d25a42e6) +PRISM [x86_64-linux]
• Backport: 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN

[tompng (tomoya ishida)]

Destructuring assignment does not work after a few iterations if YJIT is enabled.
Happens in ruby 3.4.1 and 4.0.0dev.

root@05305c0005f4:/# ruby -v
ruby 4.0.0dev (2025-11-23T19:10:29Z master 8d73a18187) +PRISM [x86_64-linux]
root@05305c0005f4:/# ruby --enable-yjit -rdelegate -e "p 40.times.map { x, = SimpleDelegator.new([1,2,3]); x }"
[1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, [1, 2, 3], [1, 2, 3], [1, 2, 3], [1, 2, 3], [1, 2, 3], [1, 2, 3], [1, 2, 3], [1, 2, 3], [1, 2, 3], [1, 2, 3], [1, 2, 3]]

Works perfectly if yjit is disabled or zjit is enabled.

root@05305c0005f4:/# ruby --disable-yjit -rdelegate -e "p 40.times.map { x, = SimpleDelegator.new([1,2,3]); x }"
[1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1]
root@05305c0005f4:/# ruby --enable-zjit -rdelegate -e "p 40.times.map { x, = SimpleDelegator.new([1,2,3]); x }"
[1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1]

Found in a code like this:

code = '1;' * 100
lex_result = Prism.lex_compat(code) # Prism::LexCompat::Token is a subclass of SimpleDelegator
lex_result.value.map do |(pos, kind, text, state)|
  p pos
end