Issue #16990 has been updated by Eregon (Benoit Daloze).


Because Array and Set are fundamentally different, I think ensuring both operands have the same type, explicitly or internally, is completely reasonable.
I don't expect Array "set" operations to magically know about the Set representation.

Having something that is both fast for `include?` and `+` means it needs to keep both a set-like representation and an array-like representation, which is a memory trade-off that `FastArray` in your PRs makes.

> my_list_of_stuff + OtherClass::STUFF

Which is what type?
`set + array` already does `set + array.to_set` and returns a Set (dedup'd elements)

`array + set` is not so well defined.
Does it do `array + set.to_a` with duplicated elements? And then that makes the return type inconsistent with `set + array`.
Doing `array.to_set + set` implicitly doesn't seem nice either as @knu said.

I think a few practical examples from RuboCop would help to figure what makes most sense.
Maybe having a specialized abstraction like FastArray is what makes most sense for RuboCop if non-set operations are used frequently.

----------------------------------------
Feature #16990: Sets: operators compatibility with Array
https://bugs.ruby-lang.org/issues/16990#change-87401

* 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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