Is it BUG or not in SDL_BlitCopy?

SDL Development
1

I found blit error after calling SDL_BlitSurface. In detail, when src and
dst are same surface, and start pixel pointer of src_rect is less than start
pixel of dst_rect, and src_rect overlapps dst_rect, SDL_BlitSurface results
to only some top lines right, other bottom lines display error.

Track logic of SDL_BlitCopy, I think SDL_BlitCopy results to it.

…

if (overlap) {

     while (h--) {

               SDL_memmove(dst, src, w);

               src += srcskip;

               dst += dstskip;

     }

    return;

}

…

In that case, overlap is true, then execute while loop.

For example, line#0 of src is copied to line#0 of dst, but src and dst are
same surface, line#0 of dst is line#2 of src in fact. It will only copy data
of line#0-#2 to dst repeatly!

I attempt to modify above code:

…

if (overlap) {

     if (src > dst) {

               while (h--) {

                        SDL_memmove(dst, src, w);

                        src += srcskip;

                        dst += dstskip;

               }

     } else if (src < dst) {

               src += (h - 1) * srcskip;

               dst += (h - 1) * dstskip;

               while (h --) {

                        SDL_memmove(dst, src, w);

                        src -= srcskip;

                        dst -= dstskip;

               }

     }

return;

}

…

Is it BUG or not in SDL_BlitCopy?