  HeapPrefix(uint16_t flg, size_t sz)
      : magic(kHeapMagic),
        flags(flg),
        size((sz == ((size_t)(uint32_t)sz)) ? static_cast<uint32_t>(sz) : 0) {}
  HeapPrefix(uint16_t flg, size_t sz)
      : magic(kHeapMagic),
        flags(flg),
        size((sz == ((size_t)(uint32_t)sz)) ? static_cast<uint32_t>(sz) : 0) {}