[clang-tidy] Add check for classes missing -hash ⚠️

Summary:
Apple documentation states that:
"If two objects are equal, they must have the same hash value. This last
point is particularly important if you define isEqual: in a subclass and
intend to put instances of that subclass into a collection. Make sure
you also define hash in your subclass."
https://developer.apple.com/documentation/objectivec/1418956-nsobject/1418795-isequal?language=objc

In many or all versions of libobjc, -[NSObject isEqual:] is a pointer
equality check and -[NSObject hash] returns the messaged object's
pointer. A relatively common form of developer error is for a developer to
override -isEqual: in a subclass without overriding -hash to ensure that
hashes are equal for objects that are equal.

It is assumed that an override of -isEqual: is a strong signal for
changing the object's equality operator to something other than pointer
equality which implies that a missing override of -hash could result in
distinct objects being equal but having distinct hashes because they are
independent instances. This added check flags classes that override
-isEqual: but inherit NSObject's implementation of -hash to warn of the
potential for unexpected behavior.

The proper implementation of -hash is the responsibility of the
developer and the check will only verify that the developer made an
effort to properly implement -hash. Developers can set up unit tests
to verify that their implementation of -hash is appropriate.

Test Notes:
Ran check-clang-tools.

Reviewers: aaron.ballman, benhamilton

Reviewed By: aaron.ballman

Subscribers: Eugene.Zelenko, mgorny, xazax.hun, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D67737

llvm-svn: 372445

1 files changed, 3 insertions, 0 deletions

diff --git a/clang-tools-extra/clang-tidy/objc/ObjCTidyModule.cpp b/clang-tools-extra/clang-tidy/objc/ObjCTidyModule.cpp
index 636e2c0..c9b57d5 100644
--- a/clang-tools-extra/clang-tidy/objc/ObjCTidyModule.cpp
+++ b/clang-tools-extra/clang-tidy/objc/ObjCTidyModule.cpp
@@ -12,6 +12,7 @@
 #include "AvoidNSErrorInitCheck.h"
 #include "AvoidSpinlockCheck.h"
 #include "ForbiddenSubclassingCheck.h"
+#include "MissingHashCheck.h"
 #include "PropertyDeclarationCheck.h"
 #include "SuperSelfCheck.h"
 
@@ -30,6 +31,8 @@ public:
         "objc-avoid-spinlock");
     CheckFactories.registerCheck<ForbiddenSubclassingCheck>(
         "objc-forbidden-subclassing");
+    CheckFactories.registerCheck<MissingHashCheck>(
+        "objc-missing-hash");
     CheckFactories.registerCheck<PropertyDeclarationCheck>(
         "objc-property-declaration");
     CheckFactories.registerCheck<SuperSelfCheck>(