Storing nullable values to local variables defeats -Wnullable-to-nonnull-conversion

[Quuxplusone]

Bugzilla Link	PR30804
Status	NEW
Importance	P normal
Reported by	Timothy J. Wood (tjw@me.com)
Reported on	2016-10-26 18:42:10 -0700
Last modified on	2016-10-26 18:42:10 -0700
Version	trunk
Hardware	Macintosh MacOS X
CC	llvm-bugs@lists.llvm.org
Fixed by commit(s)
Attachments	nonnull-conversion.m (1132 bytes, application/octet-stream)
Blocks
Blocked by
See also

Created attachment 17499
sample case

Adding nullability annotations to ObjC source for correct interoperability with
Swift is made more difficult by the inability of clang to detect if a method
might return nil even if it says it doesn't.

clang apparently has a -Wnullable-to-nonnull-conversion flag to detect cases
like this. While it works for code like:

- (nonnull NSString *)ok:(nullable NSString *)arg;
{
    return arg;
}

it is trivially confused by:

- (nonnull NSString *)bad:(nullable NSString *)arg;
{
    // BAD: nullable value returned to a non-nullable result
    NSString *result = arg;
    return result;
}

Example file attached with a couple variants.

[Quuxplusone]

Attached nonnull-conversion.m (1132 bytes, application/octet-stream): sample case