kwargs_method_missing.rb

class A
  def f(x: nil)
    p x
  end
end

class Wrapper
  def initialize
    @a = A.new
  end

  def method_missing(meth, *args)
    @a.send(meth, *args)
  end
end

Wrapper.new.f(x: 1)

result.txt

$ ruby kwargs_method_missing.rb
kwargs_method_missing.rb:13: warning: The last argument is used as the keyword parameter
kwargs_method_missing.rb:2: warning: for `f' defined here
1