Bug #20180: Inconsistent evaluation of `**{}` depending on position in array

• Status: Closed
• Assignee: -
• Target version: -
• ruby -v: 3.3
• Backport: 3.0: REQUIRED, 3.1: REQUIRED, 3.2: REQUIRED, 3.3: REQUIRED

[ozydingo (Andrew Schwartz)]

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).

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.

[jeremyevans0 (Jeremy Evans)]

I submitted a pull request to fix this: https://github.com/ruby/ruby/pull/9624

[byroot (Jean Boussier)]

• Backport changed from 3.0: UNKNOWN, 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN to 3.0: REQUIRED, 3.1: REQUIRED, 3.2: REQUIRED, 3.3: REQUIRED

Seems like this behavior changed in 2.7.

[jeremyevans0 (Jeremy Evans)]

• Related to Bug #20229: Empty keyword splat in array not removed in ARGSPUSH case added

[jeremyevans0 (Jeremy Evans)]

• Status changed from Open to Closed

Fixed by 77c1233f79a0f96a081b70da533fbbde4f3037fa. However, that adds an instruction, so the implementation is not backportable. To backport, use the change in https://github.com/ruby/ruby/pull/9624