Youtube-dl fails to mux video with bestvideo+bestaudio format selection

[mlindner]

When downloading https://www.youtube.com/watch?v=kkd8A3QGP7M with -f bestvideo+bestaudio the muxing of the formats fails. This appears to be because youtube-dl is selecting the same container format as the video portion (which is VP9 webm) and trys to mux an audio stream of aac format.

The command run was:
youtube-dl -v -f bestvideo+bestaudio kkd8A3QGP7M

The error created is:
ERROR: Could not write header for output file #0 (incorrect codec parameters ?): Invalid argument
When running again with verbosity the ffmpeg command run is shown. Running that ffmpeg command generates:
[webm @ 0x7fe332042600] Only VP8 or VP9 video and Vorbis or Opus audio and WebVTT subtitles are supported for WebM.

A suggested fix would be to default to either an mp4 or an mkv format if webm format is not possible because of muxing of non-webm compatible codecs.

[dstftw]

Duplicate of #4906.
You can use filters as workaround, e.g. -f bestvideo[ext=mp4]+bestaudio.