[18.x][Docs] Add release note about Clang-defined target OS macros

[zixu-w]

The change is included in the 18.x release. Move the release note to the release branch and reformat.

(cherry picked from commit b40d5b1)

[llvmbot]

@llvm/pr-subscribers-clang

Author: Zixu Wang (zixu-w)

Changes

The change is included in the 18.x release. Move the release note to the release branch and reformat.

(cherry picked from commit b40d5b1)

---

Full diff: https://github.com/llvm/llvm-project/pull/80044.diff

1 Files Affected:

• (modified) clang/docs/ReleaseNotes.rst (+24)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 060bc7669b72a..015578a6d688e 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -171,6 +171,22 @@ AST Dumping Potentially Breaking Changes
       "qualType": "foo"
     }
 
+Clang Frontend Potentially Breaking Changes
+-------------------------------------------
+- Target OS macros extension
+  A new Clang extension (see :ref:`here <target_os_detail>`) is enabled for
+  Darwin (Apple platform) targets. Clang now defines ``TARGET_OS_*`` macros for
+  these targets, which could break existing code bases with improper checks for
+  the ``TARGET_OS_`` macros. For example, existing checks might fail to include
+  the ``TargetConditionals.h`` header from Apple SDKs and therefore leaving the
+  macros undefined and guarded code unexercised.
+
+  Affected code should be checked to see if it's still intended for the specific
+  target and fixed accordingly.
+
+  The extension can be turned off by the option ``-fno-define-target-os-macros``
+  as a workaround.
+
 What's New in Clang |release|?
 ==============================
 Some of the major new features and improvements to Clang are listed
@@ -344,6 +360,14 @@ New Compiler Flags
   (like other functions) with respect to visibility attributes, pragmas and
   options (e.g ``--fvisibility=``).
 
+.. _target_os_detail:
+* ``-fdefine-target-os-macros`` and its complement
+  ``-fno-define-target-os-macros``. Enables or disables the Clang extension to
+  provide built-in definitions of a list of ``TARGET_OS_*`` macros based on the
+  target triple.
+
+  The extension is enabled by default for Darwin (Apple platform) targets.
+
 Deprecated Compiler Flags
 -------------------------
 

[tstellar]

Looks like this patch caused the documentation build to fail.

[zixu-w]

Looks like this patch caused the documentation build to fail.

Fixed. Thanks for checking @tstellar !