[ruby-core:113480] [Ruby master Bug#19632] Disable external iterator for frozen enumerator

Issue #19632 has been updated by Eregon (Benoit Daloze).


+1. And yes peek can have side effect so should be considered mutating method.

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Bug #19632: Disable external iterator for frozen enumerator
https://bugs.ruby-lang.org/issues/19632#change-103063

* Author: make_now_just (Hiroya Fujinami)
* Status: Open
* Priority: Normal
* Backport: 3.0: UNKNOWN, 3.1: UNKNOWN, 3.2: UNKNOWN
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Currently, methods to manipulate an external iterator like `#next` and `#feed` can be called even if a receiver of an enumerator is frozen. However, these methods change the state of an external iterator in an enumerator. Therefore, it seems a BUG to me, and these methods should raise FrozenError if the receiver is frozen.

```ruby
e = 3.times.freeze

# Current

e.next # => 1
e.next # => 2

# Expected

e.next # raise FrozenError
```

Two years ago, this issue was mentioned in [a comment](https://bugs.ruby-lang.org/issues/17195#note-5).

I suggest fixing the following methods to raise FrozenError against a frozen enumerator.

- `Enumerator#next`
- `Enumerator#next_values`
- `Enumerator#peek`
- `Enumerator#peek_values`
- `Enumerator#feed`
- `Enumerator#rewind`

Also, even if an enumerator is frozen, it does not affect other methods for internal iterators.



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