Box.m

#import <Foundation/Foundation.h>

@interface Box<__covariant T: __kindof NSObject *> : NSObject

@property (nonatomic, readonly) __nonnull T value;

- (nonnull instancetype)initWithValue:(nonnull T)value;

@end

@interface Box<T: __kindof NSObject *> ()

@property (nonatomic) T value;

@end

@implementation Box

@synthesize value = _value;

- (instancetype)initWithValue:(__kindof NSObject *)value
{
    if ((self = [super init])) {
        self.value = value;
    }
    return self;
}

- (NSString *)description
{
    return [NSString stringWithFormat:@"Box{%@}", self.value.description];
}

@end