From: "k0kubun (Takashi Kokubun) via ruby-core" <ruby-core@...>
Date: 2026-07-09T22:02:59+00:00
Subject: [ruby-core:126012] [Ruby Bug#22099] Keyword-only method silently accepts a positional argument

Issue #22099 has been updated by 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 commit:aad9ef5ce42d07004cebe4dab1d8d661a195d550 merged revision(s) commit:034fa35ff042933ddb722cadf438e120dbe05881.

----------------------------------------
Bug #22099: Keyword-only method silently accepts a positional argument
https://bugs.ruby-lang.org/issues/22099#change-117991

* Author: rumoto (Hiroki Motoyoshi)
* Status: Closed
* ruby -v: ruby 3.4.9 (2026-03-11 revision 76cca827ab) +PRISM [arm64-darwin24]
* Backport: 3.3: DONTNEED, 3.4: REQUIRED, 4.0: DONE
----------------------------------------
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. 


```ruby
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.



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