A quick look at variance with Objective-C generics. `NSSet` is covariant on its `ObjectType`, while `NSHashTable` is invariant. Contravariance is also supported.

covariance.m

@import Foundation;

@interface ContravariantCollection<__contravariant ObjectType> : NSObject
@end

@implementation ContravariantCollection
@end

void stringSetThing(NSSet<NSString *> *set) {
}

void mutableStringSetThing(NSSet<NSMutableString *> *set) {
}

void stringHashTableThing(NSHashTable<NSString *> *hashTable) {
}

void mutableStringHashTableThing(NSHashTable<NSMutableString *> *hashTable) {
}

void stringContraThing(ContravariantCollection<NSString *> *set) {
}

void mutableStringContraThing(ContravariantCollection<NSMutableString *> *set) {
}

int main(int argc, char *argv[]) {
	@autoreleasepool {
		NSSet<NSString *> *setOfStrings;
		NSSet<NSMutableString *> *setOfMutableStrings;

		stringSetThing(setOfStrings);
		stringSetThing(setOfMutableStrings); // Okay, because NSSet<NSMutableString *> is a subtype of NSSet<NSString *>
		mutableStringSetThing(setOfStrings); // Warning: Incompatible pointer types passing 'NSSet<NSString *> *' to parameter of type 'NSSet<NSMutableString *> *'
		mutableStringSetThing(setOfMutableStrings);

		NSHashTable<NSString *> *tableOfStrings;
		NSHashTable<NSMutableString *> *tableOfMutableStrings;

		stringHashTableThing(tableOfStrings);
		stringHashTableThing(tableOfMutableStrings); // Warning: Incompatible pointer types passing 'NSHashTable<NSMutableString *> *' to parameter of type 'NSHashTable<NSString *> *'
		mutableStringHashTableThing(tableOfStrings); // Warning: Incompatible pointer types passing 'NSHashTable<NSString *> *' to parameter of type 'NSHashTable<NSMutableString *> *'
		mutableStringHashTableThing(tableOfMutableStrings);

		ContravariantCollection<NSString *> *collectionOfStrings;
		ContravariantCollection<NSMutableString *> *collectionOfMutableStrings;

		stringContraThing(collectionOfStrings);
		stringContraThing(collectionOfMutableStrings); // Warning: Incompatible pointer types passing 'ContravariantCollection<NSMutableString *> *' to parameter of type 'ContravariantCollection<NSString *> *'
		mutableStringContraThing(collectionOfStrings); // Okay, because MyCollection<NSString *> is a subtype of MyCollection<NSMutableString *>
		mutableStringContraThing(collectionOfMutableStrings);
	}
	return 0;
}

The Wikipedia article on Covariance and contravariance is good.

bob’s comment about variance on this Objective C Generics article clearly explains why Apple’s immutable collections are covariant, and their mutable collections are invariant.