Unexpected readonly/readwrite @Property and @synthesize behavior

proptest.m

#import <Foundation/NSObject.h>

@interface Foo : NSObject

@property (readonly, assign) id bar;

@end

@interface Foo (Internal)

@property (readwrite, assign) id bar;

@end

@implementation Foo

@synthesize bar=_bar;

@end

int main(int argc, char **argv)
{
    @autoreleasepool {
        Foo *f = [[Foo alloc] init];
        f.bar = @"Hello";
        [f release];
    }

    return 0;
}

/*
 * % clang -v
 * Apple clang version 3.0 (tags/Apple/clang-211.12) (based on LLVM 3.0svn)
 * Target: x86_64-apple-darwin11.3.0
 * Thread model: posix
 * % clang -o proptest proptest.m -framework Foundation
 * % proptest
 * 2012-02-15 15:39:23.199 proptest[4617:707] -[Foo setBar:]: unrecognized selector sent to instance 0x10d513530
 * 2012-02-15 15:39:23.202 proptest[4617:707] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[Foo setBar:]: unrecognized selector sent to instance 0x10d513530'
 * *** First throw call stack:
 * (
 * 	0   CoreFoundation                      0x00007fff8892afc6 __exceptionPreprocess + 198
 * 	1   libobjc.A.dylib                     0x00007fff85460d5e objc_exception_throw + 43
 * 	2   CoreFoundation                      0x00007fff889b72ae -[NSObject doesNotRecognizeSelector:] + 190
 * 	3   CoreFoundation                      0x00007fff88917e73 ___forwarding___ + 371
 * 	4   CoreFoundation                      0x00007fff88917c88 _CF_forwarding_prep_0 + 232
 * 	5   proptest                            0x000000010d4aee86 main + 134
 * 	6   proptest                            0x000000010d4aedd4 start + 52
 * 	7   ???                                 0x0000000000000001 0x0 + 1
 * )
 */