Objective-C overriding _Nullable parameter with _Nonnull parameter

Question

Why this code generates the following warning?

@interface Foo : NSObject 
- (void)m:(id _Nullable)p;
@end

@interface Bar : Foo
- (void)m:(id _Nonnull)p;
@end

conflicting nullability specifier on parameter types, '_Nonnull' conflicts with existing specifier '_Nullable' [-Wnullability]

But the other way does not generate warnings:

@interface Foo : NSObject 
- (void)m:(id _Nonnull)p;
@end

@interface Bar : Foo
- (void)m:(id _Nullable)p;
@end

Is it because in the second case we going from more restrictive to less restrictive?

Answer 1

It's all because of Liskov substitution principle

According to this principle:

if S is a subtype of T, then objects of type T may be replaced with objects of type S without altering any of the desirable properties of the program.

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In your example: Bar is subtype of Foo. So you should be able to replace instance of Foo with instance of Bar.

In other words, imagine method

- (void)doSomethingWith:(Foo *)object {
    [object m:nil];
}

If you pass to this method instance of Foo - everything is ok. But you may not pass to it instance of Bar - because you may not call [Bar m:nil]

Here you can see, that this code breaks Liskov substitution principle.

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Otherwise, in the second example, everything is fine.

Imagine method:

- (void)doSomethingWith:(Foo *)object {
    [object m:@""]; // _Nonnull requirement
}

As you can see, you can pass to this method instances of both Foo and Bar classes.

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NOTE: Here it is important to note that in Objc (unlike Swift) these rules are not so strict and you can easily deceive the compiler.