From: "ozydingo (Andrew Schwartz) via ruby-core" <ruby-core@...>
Date: 2024-01-12T14:34:27+00:00
Subject: [ruby-core:116182] [Ruby master Bug#20180] Inconsistent evaluation of `**{}` depending on position in array

Issue #20180 has been reported by ozydingo (Andrew Schwartz).

----------------------------------------
Bug #20180: Inconsistent evaluation of `**{}` depending on position in array
https://bugs.ruby-lang.org/issues/20180

* Author: ozydingo (Andrew Schwartz)
* Status: Open
* Priority: Normal
* ruby -v: 3.3
* Backport: 3.0: UNKNOWN, 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN
----------------------------------------
Reproduced on ruby:3.3 docker container

The evaluation of `**{}` differs if it appears alone (evaluates as empty / no content) in an array vs after another element (evaluates as an empty Hash).

```rb
args = []; kwargs = {}
[*args]
# => []
[**kwargs]
# => []
[*args, **kwargs]
# => [{}]
[*args] + [**kwargs] == [*args, **kwargs]
=> false
```

I claim this violates the Principle of Least Surprise. I will admit that beyond a thin example I will give below, I am struggling to come up with a more convincing pragmatic reason that this should be addressed, but due to how surprising it is and the bugs it cause our team I wanted to submit it for tracking. This may be related to https://bugs.ruby-lang.org/issues/20064?tab=notes though the issues are distinct.

Specifically, in my use case, I am writing a class responsible for adapting arguments in one form to another (json object to method args and vice versa). My tests broken when adding support for keyword args due to expectations of no args:

```
# RSpec
expect(foo).to have_received(:bar).with([]) # This now need `with([], {})`
```

Again, this is a bit thin as by itself this isn't problematic as I can add the `{}`. However, this does require that my test knows more about the implementation that it should. That is, my implementation might be

```
if kwargs.present?
  call(*args, **kwargs)
else
  call(*args)
end
```

This change does not change the behavior of the class, but will break the test. I therefore think this behavior of `**kwargs` is problematic.






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