[clang] [ARC][Documentation] Explicitly state that messaging weak objects keeps a strong reference during call lifetime (PR #72169)

[Akira Hatanaka via cfe-commits]

https://github.com/ahatanak created https://github.com/llvm/llvm-project/pull/72169

rdar://113636046

>From 99c6bac064fbb7eb8f5462b58389b33703faf3f2 Mon Sep 17 00:00:00 2001
From: Akira Hatanaka <ahatanaka at apple.com>
Date: Mon, 13 Nov 2023 14:27:28 -0800
Subject: [PATCH] [ARC][Documentation] Explicitly state that messaging weak
 objects keeps a strong reference during call lifetime

rdar://113636046
---
 clang/docs/AutomaticReferenceCounting.rst | 17 +++++++++++++++--
 1 file changed, 15 insertions(+), 2 deletions(-)

diff --git a/clang/docs/AutomaticReferenceCounting.rst b/clang/docs/AutomaticReferenceCounting.rst
index 820ad3978d5f206..bcac73215c9d32e 100644
--- a/clang/docs/AutomaticReferenceCounting.rst
+++ b/clang/docs/AutomaticReferenceCounting.rst
@@ -839,8 +839,21 @@ and non-ownership qualification.
 object lvalue.
 
 * For ``__weak`` objects, the current pointee is retained and then released at
-  the end of the current full-expression.  This must execute atomically with
-  respect to assignments and to the final release of the pointee.
+  the end of the current full-expression. In particular, messaging a ``__weak``
+  object keeps the object retained until the end of the full expression.
+
+  .. code-block:: objc
+
+    __weak MyObject *weakObj;
+
+    void foo() {
+      // weakObj is retained before the message send and released at the end of
+      // the full expression.
+      [weakObj m];
+    }
+
+  This must execute atomically with respect to assignments and to the final
+  release of the pointee.
 * For all other objects, the lvalue is loaded with primitive semantics.
 
 :arc-term:`Assignment` occurs when evaluating an assignment operator.  The