In HEVC SCC, in case that the DPB size is N and two versions of current pictures are different, there should be at most N-2 temporal reference pictures. When two versions of current pictures are the same (loop filters are off), there should be at most N-1 temporal reference pictures.

In SCM software, the removal of unnecessary pictures in the DPB is done after current picture is decoded and after the extra version of current picture (as reference picture) is removed. If the encoder has assigned N-1 temporal reference pictures to current picture, while two versions of current pictures are different, the decoder can still decode such an illegal bitstream.

In the provided patch, an assertion is made such that after removing unnecessary pictures from the DPB for current picture (and after decoding current picture), the DPB fullness including the current decoded picture itself should be at most N - TwoVersionsCurrDecPicFlag.