Open Radar

 
Summary:

The Objective-C generics syntax is non-ambiguous because types must be pointer types and protocols do not have a trailing asterisk. The compiler fails to disambiguate the case where a protocol and class have the same name and both are adopted - ie. when a collection class is known to fulfil a particular protocol and you also want to use generics with that collection.

Steps to Reproduce:

Compile:

@protocol Countable
- (NSUInteger)count;
@end

@interface Countable : NSObject <Countable>
@end

@interface MyModel : NSObject
@property (nonatomic, strong) NSArray<Countable,Countable*> * objects;
@end


Expected Results:

Compiles, with compile time generics applied to ensure only objects of class “Countable” are added, and a property that at runtime can be introspected to show that it conforms to the protocol “Countable”.

Actual Results:

Compile error:

error: type argument 'Countable' is neither an Objective-C object nor a block type

Regression:
nil

Notes:

Demo project attached. Note that objects and protocols of the same name are not uncommon, eg. NSObject and we can see that the name conflict does not in itself introduce a type since we can adopt either the protocol or the generic class, just not both at once - which makes the error even more mystifying since we know that the compiler does indeed know that “Countable” is an Objective-C object.