Bug #21757: Splatted args array is mutated when passing unexpected kwargs

• Status: Closed
• Assignee: jeremyevans0 (Jeremy Evans)
• Target version: -
• ruby -v: ruby 3.4.7 (2025-10-08 revision 7a5688e2a2) +PRISM [x86_64-linux]
• Backport: 3.2: DONTNEED, 3.3: DONTNEED, 3.4: REQUIRED

[apotonick (Nick Sutterer)]

The bug is very simple to provoke.

proc = ->(*) { } # we do NOT expect any keyword arguments.

args = [1, 2]

proc.(*args, bug: true)

pp args #=> [1, 2, {bug: true}]

The args array now has a third element which is exactly what got passed as keyword arguments. This is only in Ruby 3.4, 3.3 and below discard the "unexpected" keyword arguments as expected.

[jeremyevans0 (Jeremy Evans)]

• Assignee set to jeremyevans0 (Jeremy Evans)
• Backport changed from 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN to 3.2: DONTNEED, 3.3: DONTNEED, 3.4: REQUIRED

Thank you for the report. This is a bug in the allocationless anonymous splat optimization in Ruby 3.4, still present in the master branch. I'll work on a fix.

[jeremyevans0 (Jeremy Evans)]

I submitted a pull request to fix this: https://github.com/ruby/ruby/pull/15383

• Status changed from Open to Closed

Applied in changeset git|6409715212d22699bd2751a363b050a5d8b94b83.

---

Fix allocationless anonymous splat keyword argument check

Previously, if an argument splat and keywords are provided by
the caller, it did not check whether the method/proc accepted
keywords before avoiding the allocation. This is incorrect,
because if the method/proc does not accept keywords, the
keywords passed by the caller are added as a positional
argument, so there must be an allocation to avoid mutating
the positional splat argument.

Add a check that if the caller passes keywords, the
method/proc must accept keywords in order to optimize.
If the caller passes a keyword splat, either the
method/proc must accept keywords, or the keyword splat must
be empty in order to optimize.

If keywords are explicitly disallowed via **nil, the
optimization should be skipped, because the array is mutated
before the ArgumentError exception is raised.

In addition to a test for the correct behavior, add an
allocation test for a method that accepts an anonymous splat
without keywords.

Fixes [Bug #21757]