Clang: type checking of `__kindof id<SomeProtocol>` is too loose

Originator:daniel
Number:rdar://22782165 Date Originated:2015-09-21
Status:open Resolved:
Product:Developer Tools Product Version:Xcode Version 7.0 (7A220)
Classification:Other Bug Reproducible:Always
 
Summary:
When annotating API using the construct mentioned above, Clang’s type checker appears to be too lax:
It looks as if the `__kindof` keyword effectively removes the protocol constraint.

An example of a scenario where this matters can be found in the Dummy.m file of the attached sample project.

Steps to Reproduce:
Build the attached sample project, and open the file Dummy.m.

Expected Results:
Compiling results in the following warning:
${PROJECT_DIR}/Dummy.m:32:20: warning: initializing 'NSArray *const __strong' with an expression of incompatible type 'id<D12DBColumnRepresentable> _Nullable'
    NSArray *const array = row[@"bogus"];
                   ^       ~~~~~~~~~~~~~


Actual Results:
*crickets*

No really: the build completes without any diagnostics

Version:
Xcode Version 7.0 (7A220)

Notes:
The warning provided in “Expected Results” section is the diagnostic obtained when removing the `__kindof` in the typedef.

In that case, however, there’s also a warning on the line that gets the number.
That’s okay because there’s no guarantee that an arbitrary id<SomeProtocol> is some concrete type that conforms to said protocol, but that means one needs to rely on `__kindof` not removing the protocol requirement.

Configuration:


Attachments:
'WarningSign.zip' was successfully uploaded.

Comments

Contents of Dummy.m -- add as source file to a new framework or library project

@import Foundation;

// Empty protocol to say get an “any of …” type
@protocol D12DBColumnRepresentable <NSObject> @end

// Convenience that uses this “any of …” type
typedef NSDictionary<NSString *, id<D12DBColumnRepresentable>> D12DBRowDictionary;

// “Populating” the list of that makes up the “any of …” type
@interface NSData (D12DBColumnRepresentable) <D12DBColumnRepresentable> @end
@interface NSNull (D12DBColumnRepresentable) <D12DBColumnRepresentable> @end
@interface NSNumber (D12DBColumnRepresentable) <D12DBColumnRepresentable> @end
@interface NSString (D12DBColumnRepresentable) <D12DBColumnRepresentable> @end

static inline void ExemplaryFunction(NSString *columnName, D12DBRowDictionary *row) {
    // Good: NSNumber has been marked as <D12DBColumnRepresentable>
    // Therefore, this line shouldn’t warn
    NSNumber *const number = row[columnName];
    NSCParameterAssert([number isKindOfClass:[NSNumber class]]);

// Bad: NSArray clearly doesn’t conform to <D12DBColumnRepresentable>
    // Therefore, this line **must** warn since the protocol constraint isn’t satisfied
    NSArray *const array = row[@"bogus"];
    NSCParameterAssert([array isKindOfClass:[NSArray class]]);
}
By daniel at Sept. 21, 2015, 4:16 p.m. (reply...)

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