Did Ruby 2.5 introduce a new bug, or fix an old bug? The implicit `flatten` -> `to_ary` -> `respond_to_missing` call has changed.

flatten.rb

class A
  def respond_to?(method, include_all = false)
    ::Kernel.puts "respond_to?(#{method.inspect}, #{include_all.inspect})"
  end
end

class B
  def respond_to_missing?(method, include_all = false)
    ::Kernel.puts "respond_to_missing?(#{method.inspect}, #{include_all.inspect})"
  end
end

puts RUBY_VERSION
[A.new, B.new].flatten

# Output:
#
# 2.4.2
# respond_to?(:to_ary, true)
# respond_to_missing?(:to_ary, false)
#
# 2.5.0
# respond_to?(:to_ary, true)
# respond_to_missing?(:to_ary, true)