How to define a Ruby method in runtime so that Rubocop doesn't complain?

I'm doing this:

class F
  def foo
    @x = Object.new
    def @x.bar(_t)
      true
    end
  end
end

Rubocop complains:

a.rb:4:5: W: Lint/NestedMethodDefinition: Method definitions must not be nested. Use lambda instead.
    def @x.bar(_t) ...
    ^^^^^^^^^^^^^^

I can't understand how to refactor this code to make Rubocop happy.

Rubocop sees this as a nested method definition although it isn't. (I think this is because of the @, you might want to report this as an issue).

To make Rubocop "happy", you could just ignore that specific cop:

def foo
  @x = Object.new
  def @x.bar(_t) # rubocop:disable Lint/NestedMethodDefinition
    true
  end
end

Alternatively, you could define the method in other "good" ways:

class << @x
  def bar(_t)
    true
  end
end

@x.instance_eval do
  def bar(_t)
    true
  end
end

@x.define_singleton_method(:bar) do |_t|
  true
end