From [object_.d](https://github.com/D-Programming-Language/druntime/blob/5f301ae426b57643c20a69c472b0fd6a1a82006d/src/object_.d#L836).

    //  1:                      // is IUnknown or is derived from IUnknown
    //  2:                      // has no possible pointers into GC memory
    //  4:                      // has offTi[] member
    //  8:                      // has constructors
    // 16:                      // has xgetMembers member
    // 32:                      // has typeinfo member
    // 64:                      // is not constructable