Issue #12370 has been reported by Daniel Ferreira.

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Bug #12370: require 'mkmf' changes Object#respond_to? behaviour 
https://bugs.ruby-lang.org/issues/12370

* Author: Daniel Ferreira
* Status: Open
* Priority: Normal
* Assignee: 
* ruby -v: 2.3.0
* Backport: 2.1: UNKNOWN, 2.2: UNKNOWN, 2.3: UNKNOWN
----------------------------------------
Replication steps:

```
$ irb
> RUBY_VERSION
=> '2.3.0'
> require 'yaml'
> YAML.public_method_defined?(:configuration)
=> false
> YAML.respond_to?(:configuration)
=> false
> require 'mkmf'
> YAML.public_method_defined?(:configuration)
=> false
> YAML.respond_to?(:configuration)
=> true
```

```
Object.constants do |constant|
  constant.respond_to?(:configuration) ==> true
end
```

Also there is a different behaviour for core classes over library or gem classes and/or modules under Object namespace.





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