emerge -1va =libvpx-1.8.0-r1 --verbose-conflicts

These are the packages that would be merged, in order:

Calculating dependencies... done!
[ebuild     U #] media-libs/libvpx-1.8.0-r1:0/6::media-kit [1.7.0:0/5::media-kit] USE="highbitdepth postproc svc threads -abi_riscv_lp64 -abi_riscv_lp64d -doc -static-libs -test" CPU_FLAGS_X86="(-avx%*) (-avx2%*) (-mmx%*) (-sse%*) (-sse2%*) (-sse3%*) (-sse4_1%*) (-ssse3%*)" 0 KiB

Total: 1 package (1 upgrade), Size of downloads: 0 KiB

!!! Multiple package instances within a single package slot have been pulled
!!! into the dependency graph, resulting in a slot conflict:

media-libs/libvpx:0

  (media-libs/libvpx-1.8.0-r1:0/6::media-kit, ebuild scheduled for merge) pulled in by
    =media-libs/libvpx-1.8.0-r1 (Argument)
    >=media-libs/libvpx-1.5:0/6=[svc] required by (dev-qt/qtwebengine-5.12.3:5/5.12::core-ui-kit, installed)
                           ^^^^^                                                                                                                

  (media-libs/libvpx-1.7.0:0/5::media-kit, installed) pulled in by
    media-libs/libvpx:0/5= required by (media-video/vlc-3.0.7.1:0/5-9::media-kit, installed)
                     ^^^^^                                                                                                      
    >=media-libs/libvpx-1.3.0:0/5=[-mgorny(-)] required by (media-plugins/gst-plugins-vpx-1.14.4:1.0/1.0::media-kit, installed)
                             ^^^^^                                                                                                                                 
    >=media-libs/libvpx-1.4.0:0/5=[-mgorny(-)] required by (media-video/ffmpeg-3.4.6:0/55.57.57::media-kit, installed)