HEVC with alpha to PNG does not maintain alpha

[Jozef Chutka]

When converting hevc.mp4 with alpha into PNG, the alpha information is not respected, producing opaque PNG.

How to reproduce:

% ffmpeg -i hevc.mp4 -vframes 1 output.png

hevc.mp4 was created from source image with alpha channel and seems to have the alpha properly encoded:

ffmpeg -i source.png -vcodec hevc_videotoolbox -r 1 -t 1 -vtag hvc1 -alpha_quality 1 -y hevc.mp4

Following ​https://trac.ffmpeg.org/ticket/7965 which refers to ​https://patchwork.ffmpeg.org/project/ffmpeg/patch/tencent_5F1F4DA9CFDF5521A752B8C9161A4D23A007@qq.com/ it seems hevcdec has some kind of decoding support for alpha channel... is it hidden behind some command line flag?