Bug #21823: $! is nil inside rescue block when $! is accessed before raise in Ruby::Box

• Status: Open
• Assignee: tagomoris (Satoshi Tagomori)
• Target version: -
• ruby -v: ruby 4.0.0 (2025-12-25 revision 553f1675f3) +PRISM [arm64-darwin25]
• Backport: 3.2: DONTNEED, 3.3: DONTNEED, 3.4: DONTNEED, 4.0: REQUIRED
• Tags: box

[ahogappa (sho hirano)]

Description

When Ruby::Box is enabled (RUBY_BOX=1), accessing $! before a raise statement causes $! to be nil inside the subsequent rescue block, instead of containing the raised exception.

Reproduction

# test.rb
begin
  $!  # Accessing $! before raise triggers the bug
  raise "test error"
rescue
  puts "$! = #{$!.inspect}"
end

Without Ruby::Box:

$ ruby test.rb
$! = #<RuntimeError: test error>

With Ruby::Box:

$ RUBY_BOX=1 ruby test.rb
$! = nil

Expected behavior

$! should contain the raised exception (#<RuntimeError: test error>) inside the rescue block, regardless of whether $! was accessed before the raise.

Actual behavior

When RUBY_BOX=1 is set and $! is accessed before raise, $! becomes nil inside the rescue block.

[nobu (Nobuyoshi Nakada)]

• Tags set to box
• Assignee set to tagomoris (Satoshi Tagomori)
• Backport changed from 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN, 4.0: UNKNOWN to 3.2: DONTNEED, 3.3: DONTNEED, 3.4: DONTNEED, 4.0: REQUIRED