Explicit type cast of zero to OpenCL event_t type

Hi all,

I have a question. Clang allows implicit type cast of zero to OpenCL event_t type to support OpenCL async_work_group_copy() function. But explicit type cast generates the error. Could someone explain about this why clang prevent explicit type cast of zero to OpenCL event_t type? I think OpenCL spec does not mention about implicit or explicit type cast to event_t type. I have attached the simple patch to support explicit type cast on the assumption that it is needed.

Please review this patch.

Thanks,
JinGu Kang