I though an example clarifying Enumerable#{to_a,entries} arity fulfills following conditions.

* `each` accepts arguments.
* `to_a` is defined on Enumerable.

Prime is enough for this condition. But it is stdlib yeah.
As far as I know, IO is the one.

```ruby
IO.instance_method(:each).owner #=> IO
IO.instance_method(:each).arity #=> -1 
IO.instance_method(:to_a).owner #=> Enumerable
```

But I couldn't find simple example with IO#to?a :(
It is an idea.

```shell
ruby -e 'p $stdin.to_a(",")' <<'EOD'
a,b
x,y
EOD

# => ["a,", "b\nx,", "y\n"]
```