Bug #17595: [Pattern Matching] deconstruct_keys with zero patterns supplies nil for keys argument

• Status: Rejected
• Assignee: -
• Target version: -
• ruby -v: 3.0.0
• Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN

[baweaver (Brandon Weaver)]

Example:

Test = Struct.new(:a, :b) do
  def deconstruct_keys(ks)
    p ks: ks
    to_h.slice(*ks)
  end
end
# => Test

Test.new(1,2) in {}
# {:ks=>nil}
# => true

As users may call Array methods against the keys argument I consider this a potential bug that it returns nil for no supplied arguments.

Proposed Patch: Pass an empty array instead to maintain interface consistency.

[baweaver (Brandon Weaver)]

An alternative patch would to be raising an exception as an empty hash for a match feels like an invalid case, as mentioned by k_tsj.

[ktsj (Kazuki Tsujimoto)]

• Status changed from Open to Rejected

It is intentional behavior, so I close this ticket.

• When **rest is used in the pattern, nil is passed as a keys value.
• {} is equivalent to {**rest} if rest.empty?