[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, 16 insertions, 0 deletions

diff --git a/clang-tools-extra/docs/clang-tidy/checks/objc-missing-hash.rst b/clang-tools-extra/docs/clang-tidy/checks/objc-missing-hash.rst
new file mode 100644
index 00000000000..ea8f775897c
--- /dev/null
+++ b/clang-tools-extra/docs/clang-tidy/checks/objc-missing-hash.rst
@@ -0,0 +1,16 @@
+.. title:: clang-tidy - objc-missing-hash
+
+objc-missing-hash
+=================
+
+Finds Objective-C implementations that implement ``-isEqual:`` without also
+appropriately implementing ``-hash``.
+
+Apple documentation highlights that objects that are equal must have the same
+hash value:
+https://developer.apple.com/documentation/objectivec/1418956-nsobject/1418795-isequal?language=objc
+
+Note that the check only verifies the presence of ``-hash`` in scenarios where
+its omission could result in unexpected behavior. The verification of the
+implementation of ``-hash`` is the responsibility of the developer, e.g.,
+through the addition of unit tests to verify the implementation.