What is assign-enum warning in clang?

What is assign-enum warning in clang?

I was going through AFNetworking implementation and I found this
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wassign-enum"
[request setHTTPBody:[NSJSONSerialization dataWithJSONObject:parameters
options:0 error:&error]];
#pragma clang diagnostic pop
(AFHTTPClient:489-492)
The assign-enum warning is obviously being turned off, but I wonder what
does it mean.
What is the warning thrown by clang in that case?