The protocol declaration for MTLJSONSerializing
is in MTLJSONAdapter.h
While you could fix this by adding another import (#import "MTLJSONAdapter.h"
), it's generally better to import a library or framework's umbrella header.
You should import Mantle's umbrella header to get access to all of the public interfaces in Mantle:
#import <Mantle/Mantle.h>
@interface TestMTL : MTLModel <MTLJSONSerializing>
@end