Allow Auto-Caption Subtitles to download as fallback option only for YouTube

[danfiscus]

Checklist

• [x] I'm reporting a site feature request
• [x] I've verified that I'm running youtube-dl version 2020.03.01
• [x] I've searched the bugtracker for similar site feature requests including closed ones

Description

I would like to be able to specify that if none of the requested languages I specify in --sub-lang are available, the program should automatically download and use the automatically generated captions, but if the language(s) I specify are available, the auto-generated captions will be ignored (neither downloaded nor embedded).

The solution provided in a related issue #14831 does not work for this, as it will always write the auto-generated subs, regardless of the availability of the requested subs.

For example, if I specify --sub-lang en,en-GB and the video has neither of these subs available, only then should the program proceed to the automatically-generated subtitles. If one or both of those subs are available, however, I would not want the program to also download the automatic ones.

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I have been trying to find a work around for this for days now. Saw you posted this days ago, was hoping it would have a fix by now.

The issue for me is with youtube livestream videos. ydl will not recognize the auto subs until the video is finished streaming AND once youtube has converted it to a non HLS video container, usually taking hours for longer videos on youtube's end.

Have you found any useful way to grab them in the interim?