Bug #22381
Updated by jhawthorn (John Hawthorn) 3 days ago
```ruby
m = /a/.match("a").dup # dup a MatchData (has no "busy" flag set)
$~ = m # assign to $~
/b/ =~ "b" # perform a match that sets $~
an
p m
# "b" on Ruby 4.0 (and 1.8-2.7, incorrect)
# "a" on Ruby 3.0-3.4 (correct)
```
This is because the MATCH_BUSY flag isn't set correct and so the implicit match reuse (removed in #17507, re-added in #20652) mutates this escaped value