Function lua_upvaluejoin

depends/lua/src/lapi.c:1286–1297

LUA_API void lua_upvaluejoin (lua_State *L, int fidx1, int n1,
                                            int fidx2, int n2) {
  UpVal **up1 = getupvalref(L, fidx1, n1);
  UpVal **up2 = getupvalref(L, fidx2, n2);
  if (*up1 == *up2)
    return;
  luaC_upvdeccount(L, *up1);
  *up1 = *up2;
  (*up1)->refcount++;
  if (upisopen(*up1)) (*up1)->u.open.touched = 1;
  luaC_upvalbarrier(L, *up1);
}