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

• Status: Open
• 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.