Conflict with ISO8601 serializer and parser on ActiveSupport::Duration

[crashtech]

Steps to reproduce

duration = ActiveSupport::Duration.build(1000000)
duration_iso = duration.iso8601
ActiveSupport::Duration.parse(duration_iso)

Expected behavior

{:weeks => 1, :days=>4, :hours=>13, :minutes=>46, :seconds=>40}

Actual behavior

ActiveSupport::Duration::ISO8601Parser::ParsingError: Invalid ISO 8601 duration: "P1W4DT13H46M40S" mixing weeks with other date parts not allowed

System configuration

Rails 5.2.1
Ruby 2.3.1

There's an Exception prohibiting mixing weeks with other date pieces. Although it seems a correct Exception, it makes the process confusing since I could measure a duration by "2 months and 1 week" in any other aspect, and it would also provide a correct behavior on ActiveSupport::Duration#parse

The solutions simply remove the exception and allow it to move forward.
https://github.com/rails/rails/blob/master/activesupport/lib/active_support/duration/iso8601_parser.rb#L106

[rails-bot]

This issue has been automatically marked as stale because it has not been commented on for at least three months.
The resources of the Rails team are limited, and so we are asking for your help.
If you can still reproduce this error on the 5-2-stable branch or on master, please reply with all of the information you have about it in order to keep the issue open.
Thank you for all your contributions.

[quedvi]

I can confirm this bug is still present in Rails 5.2.3 / Ruby 2.6.3

Checking the ISO8601, mixing of week with other date parts is not allowed. Therefore I would suggest to change the ActiveSupport::Duration::ISO8601Serializer to:

 def serialize
    parts, sign = normalize
    return "PT0S" if parts.empty?

    if parts.key?(:weeks) && parts.except(:weeks).present?
      parts[:days] = (parts[:days] || 0) + 7 * parts[:weeks]
      parts.delete(:weeks)
    end

    output = +"P"
    output << "#{parts[:years]}Y"   if parts.key?(:years)
    output << "#{parts[:months]}M"  if parts.key?(:months)
    output << "#{parts[:weeks]}W"   if parts.key?(:weeks)
    output << "#{parts[:days]}D"    if parts.key?(:days)
    time = +""
    time << "#{parts[:hours]}H"     if parts.key?(:hours)
    time << "#{parts[:minutes]}M"   if parts.key?(:minutes)
    if parts.key?(:seconds)
      time << "#{sprintf(@precision ? "%0.0#{@precision}f" : '%g', parts[:seconds])}S"
    end
    output << "T#{time}" unless time.empty?
    "#{sign}#{output}"
  end

There was a pull request: #34683, to solve this issue but it never got merged.