Issue #17116 has been reported by S_H_ (Shun Hiraoka).

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Feature #17116: raise ArgumentError in Enumerator#new in no given blocks
https://bugs.ruby-lang.org/issues/17116

* Author: S_H_ (Shun Hiraoka)
* Status: Open
* Priority: Normal
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Now, `Enumerator#new` is allow this code(no given blocks).

```ruby
obj = Object.new
Enumerator.new(obj)
```

But, warning is displayed(since `ruby-2.0.0-preview2`).


```bash
-e:1: warning: Enumerator.new without a block is deprecated; use Object#to_enum instead
```

I think it's okay to remove this behaviour.
And I think that it seems natural to raise ArgumentError if no block given.




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