nonnull.m

#import <Foundation/Foundation.h>

typedef void(^SomeBlock)(void);

@interface Foo: NSObject

// If this one isn't marked `_Nonnull`, a caller of the class
// won't warn.
- (void)doAThingWithCompletionHandler:(_Nonnull SomeBlock)handler;

@end

@implementation Foo

// If this one isn't marked  `_Nonnull`, the caller in `-test`
// won't warn.
- (void)doAThingWithCompletionHandler:(_Nonnull SomeBlock)handler {
	handler();
}

- (void)test {
	// warning
	[self doAThingWithCompletionHandler:nil];
}

@end

int main(int argc, char *argv[]) {
	@autoreleasepool {
		Foo *f = [Foo new];
		[f doAThingWithCompletionHandler:^(void) {
			NSLog(@"!");
		}];
		
		// warning... also segfaults, cuz it's just a warning!
		[f doAThingWithCompletionHandler:nil];
	}
}