[PATCH] D67737: [clang-tidy] Add check for classes missing -hash ⚠️

[Stephane Moore via Phabricator via cfe-commits]

stephanemoore created this revision.
Herald added subscribers: cfe-commits, xazax.hun, mgorny.
Herald added a project: clang.

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.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D67737

Files:
  clang-tools-extra/clang-tidy/objc/CMakeLists.txt
  clang-tools-extra/clang-tidy/objc/MissingHashCheck.cpp
  clang-tools-extra/clang-tidy/objc/MissingHashCheck.h
  clang-tools-extra/clang-tidy/objc/ObjCTidyModule.cpp
  clang-tools-extra/docs/ReleaseNotes.rst
  clang-tools-extra/docs/clang-tidy/checks/list.rst
  clang-tools-extra/docs/clang-tidy/checks/objc-missing-hash.rst
  clang-tools-extra/test/clang-tidy/objc-missing-hash.m

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