[ruby-core:117067] [Ruby master Feature#20326] Add an `undefined` for use as a default argument.

Issue #20326 has been reported by shan (Shannon Skipper).

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Feature #20326: Add an `undefined` for use as a default argument.
https://bugs.ruby-lang.org/issues/20326

* Author: shan (Shannon Skipper)
* Status: Open
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Variations around `UNDEFINED = Object.new` are a fairly common pattern to see used as default arguments to distinguish between `nil` and no argument provided. For example, a Ruby implementation of Array#count might look something like:

``` ruby
class Array
  UNDEFINED = Object.new
  def UNDEFINED.inspect = 'UNDEFINED'
  UNDEFINED.freeze

  def count(item = UNDEFINED)
    if item == UNDEFINED
      # ...
    end
  end
end
```

I'd like to propose adding an `undefined` module function method on Kernel to remove the boilerplate for this fairly common use case. An `__undefined__` method or `__UNDEFINED__` keyword would be alternatives to `undefined`. An `undefined?` helper would also be an optional nicety:

``` ruby
class Array
  def count(item = undefined)
    if item.undefined?
      # ...
    end
  end
end
```

A Ruby implementation might look like:

``` ruby
module Kernel
  UNDEFINED = Object.new
  def UNDEFINED.inspect = -'undefined'
  UNDEFINED.freeze
  private_constant :UNDEFINED

  def undefined? = self == UNDEFINED

  module_function

  def undefined = UNDEFINED
end
```




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