Bug #22099: Keyword-only method silently accepts a positional argument

• Status: Closed
• Assignee: -
• Target version: -
• ruby -v: ruby 3.4.9 (2026-03-11 revision 76cca827ab) +PRISM [arm64-darwin24]
• Backport: 3.3: DONTNEED, 3.4: REQUIRED, 4.0: DONE

[rumoto (Hiroki Motoyoshi)]

A method that declares only keyword parameters accepts a positional argument without raising, but only from the second call onward; a prior valid keyword call to the same method is required to trigger it. Once triggered, the positional value is bound to the first keyword and an explicitly passed keyword of the same name is dropped.

class Foo
  def bar(a: nil, b: nil)
    p [:body, a: a, b: b]
  end
end

foo = Foo.new

foo.bar(a: 1, b: 2)   #=> [:body, {a: 1, b: 2}]
foo.bar(99, a: 0)     #=> [:body, {a: 99, b: nil}]
foo.bar(99, b: 0)     #=> [:body, {a: nil, b: 99}]

Although one positional argument is passed, no exception is raised. The positional value (99) is bound to the first keyword a, and the explicitly passed keyword (a: 0 / b: 0) is not reflected in the result.

Since bar accepts no positional arguments, an ArgumentError (wrong number of arguments (given 1, expected 0)) should be raised.

[nobu (Nobuyoshi Nakada)]

• Backport changed from 3.3: UNKNOWN, 3.4: UNKNOWN, 4.0: UNKNOWN to 3.3: DONTNEED, 3.4: REQUIRED, 4.0: REQUIRED

It appears to be since 3.4.

[jeremyevans0 (Jeremy Evans)]

I bisected the failure to 147ca9585ede559fd68e162cbbbaba84f009c9a1

CC @tenderlovemaking (Aaron Patterson) @jhawthorn (John Hawthorn)

[luke-gru (Luke Gruber)]

I created a PR. It's currently being reviewed.

• Status changed from Open to Closed

Applied in changeset git|034fa35ff042933ddb722cadf438e120dbe05881.

---

CI shape comparison needs to take into account # of kwargs given

If number of keyword arguments are not taken into account when comparing CI
shapes, invalid bindings happen:

class Foo
  def bar(a: nil, b: nil)
    p [:body, a: a, b: b]
  end
end

foo = Foo.new

foo.bar(a: 1, b: 2)   #=> [:body, {a: 1, b: 2}] # prime CC
foo.bar(99, a: 0)     #=> [:body, {a: 99, b: nil}] # BAD!

Also, set max keywords to 2**16-1 so we don't grow the cc_entries_entry struct.

Fixes [Bug #22099]

[k0kubun (Takashi Kokubun)]

ruby_4_0 backport draft PR: https://github.com/ruby/ruby/pull/17763

[k0kubun (Takashi Kokubun)]

• Backport changed from 3.3: DONTNEED, 3.4: REQUIRED, 4.0: REQUIRED to 3.3: DONTNEED, 3.4: REQUIRED, 4.0: DONE

ruby_4_0 aad9ef5ce42d07004cebe4dab1d8d661a195d550 merged revision(s) 034fa35ff042933ddb722cadf438e120dbe05881.