Issue #16990 has been updated by marcandre (Marc-Andre Lafortune).


Eregon (Benoit Daloze) wrote in #note-6:
> I don't expect Array "set" operations to magically know about the Set representation.

I would like to expect it 

> `array + set` is not so well defined.
> Does it do `array + set.to_a` 

Yes

> And then that makes the return type inconsistent with `set + array`.

Yes. Note that it is already inconsistent (`Set` vs raising an error).

----------------------------------------
Feature #16990: Sets: operators compatibility with Array
https://bugs.ruby-lang.org/issues/16990#change-87437

* Author: marcandre (Marc-Andre Lafortune)
* Status: Open
* Priority: Normal
----------------------------------------
We currently have `set <operator> array` work fine:

```ruby
Set[1] + [2] # => Set[1, 2]
```

Nothing works in the reverse order:

```ruby
[1] + Set[2] # => no implicit conversion of Set into Array
# should be:
[1] + Set[2] # => [1, 2]
```

#### set-like operators

Note that the situation is particularly frustrating for `&`, `|` and `-`.
If someone wants to do `ary - set`, one **has** to do `ary - set.to_a` which will, internally, do a `to_set`, so what is happening is `set.to_a.to_set`!! (assuming `ary` is over `SMALL_ARRAY_LEN == 16` size, otherwise it's still doing in `O(ary * set)` instead of `O(ary)`).

The same holds with `&` and `|`; see order issue as to why this can *not* (officially) be done any other way.

Reminder:
```ruby
ary & ary.reverse # => ary
Set[*ary] & Set[*ary.reverse]  # => Set[*ary.reverse], officially order is indeterminate
```




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