avcodec_open2 overwrites flags contrary to what the doc says

[Jean-Michaël Celerier]

The avcodec_open2 documentation states, about the options parameter:

 * @param options A dictionary filled with AVCodecContext and codec-private
 *                options, which are set on top of the options already set in
 *                avctx, can be NULL. On return this object will be filled with
 *                options that were not found in the avctx codec context.

Yet given the following program, the final assert does not pass: AV_CODEC_FLAG_GLOBAL_HEADER gets overwritten in the codec context's flags.

extern "C" {
#include <libavcodec/avcodec.h>
}

#include <cassert>
int main()
{
  auto codec = avcodec_find_encoder_by_name("libx264");
  auto c = avcodec_alloc_context3(codec);

  c->width = 1280;
  c->height = 720;
  c->time_base = (AVRational){100000, int(100000 * 30)};
  c->framerate = AVRational{c->time_base.den, c->time_base.num};
  c->gop_size = 0;
  c->max_b_frames = 0;
  c->pix_fmt = AV_PIX_FMT_YUV420P;
  c->strict_std_compliance = FF_COMPLIANCE_NORMAL;
  c->flags |= AV_CODEC_FLAG_GLOBAL_HEADER;

  AVDictionary* opt{};
  av_dict_set(&opt, "flags", "low_delay", 0);

  avcodec_open2(c, codec, &opt);
  assert(c->flags & AV_CODEC_FLAG_GLOBAL_HEADER); 
}

[Gyan]

I think you'll find that the flag is overwritten by av_dict_set. Use the value "+low_delay" and then check.