[ruby-core:91003] [Ruby trunk Bug#15479] Array#reject! modifies literal Array

Issue #15479 has been updated by nagachika (Tomoyuki Chikanaga).

Backport changed from 2.4: REQUIRED, 2.5: REQUIRED, 2.6: REQUIRED to 2.4: REQUIRED, 2.5: DONE, 2.6: REQUIRED

ruby_2_5 r66784 merged revision(s) 66756.

----------------------------------------
Bug #15479: Array#reject! modifies literal Array
https://bugs.ruby-lang.org/issues/15479#change-76224

* Author: Eregon (Benoit Daloze)
* Status: Closed
* Priority: Normal
* Assignee: tenderlovemaking (Aaron Patterson)
* Target version: 
* ruby -v: ruby 2.6.0p0 (2018-12-25 revision 66547) [x86_64-linux]
* Backport: 2.4: REQUIRED, 2.5: DONE, 2.6: REQUIRED
----------------------------------------
This was found by running ruby/spec with -R2 (such as mspec -R2 -fs core/array/reject_spec.rb).
TravisCI log on https://travis-ci.org/ruby/spec/jobs/473175799#L539

Here is a simple reproducer.
MRI seems to modify the Array literal permanently:

```ruby
3.times do
  a = [1, 2, 3, 4]
  puts "initial: #{a}"
  begin
    a.reject! do |x|
      case x
      when 2 then true
      when 3 then raise StandardError, 'Oops'
      else false
      end
    end
  rescue StandardError
  end

  puts "after:   #{a}"
end
```

prints

```
initial: [1, 2, 3, 4]
after:   [1, 3, 4]
initial: [1, 3, 4, 4]
after:   [1, 3, 4, 4]
initial: [1, 3, 4, 4]
after:   [1, 3, 4, 4]
```

2.5.3 behaves fine, but trunk is also affected.



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