How "id" type understands the receiver of method without casting?

Question

After merging master to my working branch I got compiler error on the line, which wasn't be changed. The error looks like

id test;
[test count];

Multiple methods named 'count' found with mismatched result.

At first it looks clear, because compiler doesn't know which concrete type the "test" variable is. But I don't understand why it worked before.

1. If I create a new file this line works, assuming that is a NSArray's method. Why compiler doesn't show error in this case?

2. While showing error message, there is several possible receivers of count method are shown. (NSArray, NSDictionary, NSSet) Does it search all classes that can receive that message and show error if there are multiple?

3. I noticed that error occurs when I import "-Swift.h" file. How it depends?

Answer 1

Compiler doesn't cast or check your id type. It just provides you all possible selectors. You said that this issue connected with importing "-Swift.h" file. In this case check you Swift code, probably you have count function visible for Objective C which returns something else than Int.

Also, you can check the issue in Issue navigator, select it and it will show all count calls visible in Objective C. Check them all, most of them will return NSUInteger, but there should be one that returns something else, for example:

SWIFT_CLASS("_TtC3dev19YourClass")
@interface YourClass : NSObject
- (int32_t)count SWIFT_WARN_UNUSED_RESULT;
@end