Issue #17336 has been reported by k0kubun (Takashi Kokubun).

----------------------------------------
Feature #17336: using refined: do ... end
https://bugs.ruby-lang.org/issues/17336

* Author: k0kubun (Takashi Kokubun)
* Status: Open
* Priority: Normal
----------------------------------------
## Problem
When we need a monkey patch which is used only in a single file (), we want to define a refinement and use it in a single place. The problem is that it needs deep indentation and `Module.new { ... }` which feels redundant.

```rb
class Foo
  using Module.new {
    refine Array do
      def flat_map!(&block)
        replace(flat_map(&block))
      end
    end
  }
  
  # ...
end
```

@tagomoris proposed an idea to reduce indentation and remove `Module.new { ... }`. This looks pretty convenient, but I want to write `do ... end`, which would make it a block of `using` here, because we almost always use `do ... end` for defining methods or modules.

```rb
module Kernel
  def refined(mod, &block)
    Module.new do
      refine(mod, &block)
    end
  end
end

class Foo
  using refined(Array) {
    def flat_map!(&block)
      replace(flat_map(&block))
    end
  }

  # ...
end
```

## Proposal
What about supporting this? Because `using` currently doesn't take a block, it doesn't conflict with the existing syntax.

```rb
class Foo
  using refined: Array do
    def flat_map!(&block)
      replace(flat_map(&block))
    end
  end

  # ...
end
```

This syntax is based on ideas of @tagomoris and @znz .



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