Figuring out the object that a non-string, compiler-verified key path refers to

gistfile1.m

// doesn't work with __block variables
#define keypath2(PATH) \
	do { \
		const char *_path = strchr(# PATH, '.') + 1; \
		\
		id _capturingBlock = ^{ \
			return PATH; \
		}; \
		\
		struct _literal { \
			void *isa; \
			int flags; \
			int reserved; \
			void (*invoke)(void *); \
			struct { \
				unsigned long reserved; \
				unsigned long blockSize; \
			} *descriptor; \
			const __unsafe_unretained id obj; \
		} *_literal = (__bridge struct _literal *)_capturingBlock; \
		\
		id _obj = _literal->obj; \
		NSLog(@"obj: %@ (key path \"%s\")", _obj, _path); \
	} while (0)

// Usage
keypath2(URL.port);
keypath2(URL.port.stringValue);
keypath2(obj.someUniqueProperty);