gistfile1.txt

#import <Foundation/Foundation.h>

@interface MYClass : NSObject
+ (void)errorMethod:(NSObject *(^)(NSObject *))block;
+ (void)method:(id(^)(id))block;
@end

int main(int argc, const char * argv[])
{
    @autoreleasepool
    {
        NSNumber *(^blk)(NSString *) = ^(NSString *str) {
            return @(str.doubleValue);
        };

        [MYClass method:blk];
        [MYClass errorMethod:blk]; // Incompatible block pointer types sending 'NSNumber *(^__strong)(NSString *__strong)' to parameter of type 'NSObject *(^)(NSObject *__strong)'
    }
    return 0;
}