Issue #13817 has been updated by usa (Usaku NAKAMURA).

Backport changed from 2.2: REQUIRED, 2.3: REQUIRED, 2.4: DONE to 2.2: REQUIRED, 2.3: DONE, 2.4: DONE

ruby_2_3 r59880 merged revision(s) 59613.

----------------------------------------
Bug #13817: test/unit breaks Hash
https://bugs.ruby-lang.org/issues/13817#change-66639

* Author: rovf (Ronald Fischer)
* Status: Closed
* Priority: Normal
* Assignee: ktsj (Kazuki Tsujimoto)
* Target version: 
* ruby -v: ruby 2.4.1p111 (2017-03-22 revision 58053) [x86_64-linux]
* Backport: 2.2: REQUIRED, 2.3: DONE, 2.4: DONE
----------------------------------------
Consider the following program:

~~~ruby
#!/usr/bin/env ruby
BEGIN {$VERBOSE = true}
require 'test/unit'
class Hash
  STDERR.puts method_defined?(:<<)
  alias << merge!
  STDERR.puts method_defined?(:<<)
end
~~~

In older Ruby versions (1.9.3, 2.3.3), the output of this program was:

```
false
true
```

In Ruby 2.4.1, the output is

```
false
~/work/stash/vp5/src/test/test_xx.rb:6: warning: method redefined; discarding old <<
true
```

Please note:

1. The warning occurs, even though `method_defined?` tells us that no `<<` method is in `Hash`.
2. The warning disappears, when we do **not** require test/unit, which suggests that the problem is caused by test/unit
3. Changing the offending line to `alias :<< :merge!` does not make any difference




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