Issue #17737 has been updated by jeremyevans0 (Jeremy Evans).


If this is a bug, it doesn't just affect `Array#permutation`, but many `Enumerable` methods:

```ruby
[].each_entry(2)
# => #<Enumerator: []:each_entry(2)>
[].each_entry(2).to_a
# ArgumentError (wrong number of arguments (given 1, expected 0))

[].each_with_index(1)
# => #<Enumerator: []:each_with_index(1)>
[].each_with_index(1).to_a
# ArgumentError (wrong number of arguments (given 1, expected 0))

{}.reverse_each(1)
# => #<Enumerator: {}:reverse_each(1)>
{}.reverse_each(1).to_a
# ArgumentError (wrong number of arguments (given 1, expected 0))
```

This basically happens for all Enumerable methods that accept a variable number of arguments and use `RETURN_SIZED_ENUMERATOR` before checking arity.  If we consider this a bug, we should audit the code and fix all cases, not just `Array#permutation`.



----------------------------------------
Bug #17737: `Array#permutation` does not immediately check the arity when no block is given
https://bugs.ruby-lang.org/issues/17737#change-91148

* Author: kachick (Kenichi Kamiya)
* Status: Open
* Priority: Normal
* ruby -v: ruby 3.0.0p0 (2020-12-25 revision 95aff21468) [x86_64-darwin20]
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN
----------------------------------------
```ruby
p [].permutation(1) #=> #<Enumerator: []:permutation(1)>
p [].permutation(1, 3) #=> #<Enumerator: []:permutation(1, 3)>
p [].permutation(1, 3).to_a #=> `permutation': wrong number of arguments (given 2, expected 0..1) (ArgumentError)
```

```ruby
p [].cycle(1) #=> #<Enumerator: []:cycle(1)>
p [].cycle(1, 3) #=> `cycle': wrong number of arguments (given 2, expected 0..1) (ArgumentError)
```

Is this an intentional behavior?
I would expect `To raise ArgumentError immediately if the arity is wrong` like Array#cycle.

PR: https://github.com/ruby/ruby/pull/4267



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