Issue #13817 has been updated by kou (Kouhei Sutou).

Status changed from Open to Third Party's Issue
Assignee set to ktsj (Kazuki Tsujimoto)

It's not caused by test-unit. It's caused by power_assert.

Please report it to https://github.com/k-tsj/power_assert/issues/new with the following script:

```ruby
#!/usr/bin/env ruby
$VERBOSE = true
require 'power_assert/enable_tracepoint_events'
class Hash
  STDERR.puts method_defined?(:<<)
  alias << merge!
  STDERR.puts method_defined?(:<<)
end
```

----------------------------------------
Bug #13817: test/unit breaks Hash
https://bugs.ruby-lang.org/issues/13817#change-66185

* Author: rovf (Ronald Fischer)
* Status: Third Party's Issue
* 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: UNKNOWN, 2.3: UNKNOWN, 2.4: UNKNOWN
----------------------------------------
Consider the following program:

~~~
#!/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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