[ruby-core:109368] [Ruby master Feature#18948] Add `with_private_method` option to `private_constant`

Issue #18948 has been updated by okuramasafumi (Masafumi OKURA).


In a real world code I'm facing, `M1` contains lots of other methods so including it in `InstanceMethods` causes confusion.

I pushed my code on GitHub.
https://github.com/okuramasafumi/alba/blob/jsonapi/lib/alba/resource.rb#L299-L301
https://github.com/okuramasafumi/alba/blob/jsonapi/lib/alba/jsonapi.rb#L30-L43

----------------------------------------
Feature #18948: Add `with_private_method` option to `private_constant`
https://bugs.ruby-lang.org/issues/18948#change-98511

* Author: okuramasafumi (Masafumi OKURA)
* Status: Open
* Priority: Normal
----------------------------------------
## Problem

I have the following code:

```ruby
module M1
  CONST = 'CONST'.freeze
end

module M2
  def self.included(base)
    base.include M1
    base.include InstanceMethods
  end

  module InstanceMethods
    def const?(value)
      M1::CONST == value
    end
  end
end

class C
  include M2
end

C.new.const?('CONST') # => true
```

`M2` module "inherits" `M1` module using `included` hook. This code works, but I prefer making constants private. However, when I add `private_constant :CONST` to `M1` then the code doesn't work anymore since we cannot refer `M1::CONST`.

## Workaround

One way to solve this is to define method that just returns the private constant.

```ruby
# Refined version of M1 with `const` method
module M1
  CONST = 'CONST'.freeze
  private_constant :CONST

  private
  def const
    CONST
  end
end

# Refined version of M2, using `const` method inherited from M1
module M2
  def self.included(base)
    base.include M1
    base.include InstanceMethods
  end

  module InstanceMethods
    def const?(value)
      const == value
    end
  end
end
```

This works anyway, but defining private method for every private constant could be cumbersome.

## Solution

So my suggestion here is to add `with_private_method` option to `private_constant` method. The auto-generated private method has the same name as the given constant and just returns it. So we can do the following:

```ruby
module M1
  CONST = 'CONST'.freeze
  private_constant :CONST, with_private_method: true
end
```

Now we can access `CONST` with `const` method. This makes is possible to refer private constants through private methods more easily, that helps some code like my example.



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