[Feature #19538]

```
1)
Warning.[]= :experimental emits and suppresses warnings for :experimental FAILED
Expected "" =~ /is experimental/
to be truthy but was nil
```

https://github.com/ruby/dev-meeting-log/blob/master/DevelopersMeeting20201210Japan.md#feature-17371-reintroduce-expr-in-pat-ktsj