Open Radar

 
Summary:
Although Foundation/NSMutableDictionary.h declares -setObject:forKey: and -setObject:forKeyedSubscript: with a type signature that requires the KeyType to conform to NSCopying, this rich type information is lost at the call site:
Regardless of the dictionary’s actual KeyType, anything that conforms to NSCopying is accepted without warning by the compiler.

Steps to Reproduce:
Compile the code between the triple backticks using Xcode or command line clang:
```
#import <Foundation/Foundation.h>

@interface YUNOWarn : NSObject <NSCopying> @end

@implementation YUNOWarn

+ (void)pointlessMethod {
    NSMutableDictionary<NSString *, NSNumber *> *canary = [NSMutableDictionary dictionary];
    YUNOWarn *obviouslyInvalidKey = [[YUNOWarn alloc] init];

    canary[obviouslyInvalidKey] = @1234;
    [canary setObject:@1234 forKey:obviouslyInvalidKey];
}

- (id)copyWithZone:(NSZone *)zone {
    return self;
}

@end
```

Expected Results:
There are two warnings because of a type–mismatch for the key in line 11 and 12.

Actual Results:
This code compiles without a warning.

Regression:
??

Notes: