[llvm-branch-commits] [libcxx] release/18.x: [libc++] Enable availability based on the compiler instead of __has_extension (#84065) (PR #84374)

[via llvm-branch-commits]

https://github.com/llvmbot created https://github.com/llvm/llvm-project/pull/84374

Backport 292a28df6c55679fad0589dea35278a8c66b2ae1

Requested by: @ldionne

>From 68fc36a1e40b498c6103cbec0f5614bfce0fa3e7 Mon Sep 17 00:00:00 2001
From: Louis Dionne <ldionne.2 at gmail.com>
Date: Thu, 7 Mar 2024 15:12:21 -0500
Subject: [PATCH] [libc++] Enable availability based on the compiler instead of
 __has_extension (#84065)

__has_extension(...) doesn't work as intended when -pedantic-errors is
used with Clang. With that flag, __has_extension(...) is equivalent to
__has_feature(...), which means that checks like

    __has_extension(pragma_clang_attribute_external_declaration)

will return 0. In turn, this has the effect of disabling availability
markup in libc++, which is undesirable.

rdar://124078119
(cherry picked from commit 292a28df6c55679fad0589dea35278a8c66b2ae1)
---
 libcxx/include/__availability                 |  7 +++---
 ...lity-with-pedantic-errors.compile.pass.cpp | 22 +++++++++++++++++++
 2 files changed, 25 insertions(+), 4 deletions(-)
 create mode 100644 libcxx/test/libcxx/vendor/apple/availability-with-pedantic-errors.compile.pass.cpp

diff --git a/libcxx/include/__availability b/libcxx/include/__availability
index c5069a027750ec..b8b2da9bb12265 100644
--- a/libcxx/include/__availability
+++ b/libcxx/include/__availability
@@ -72,11 +72,10 @@
 #  endif
 #endif
 
-// Availability markup is disabled when building the library, or when the compiler
+// Availability markup is disabled when building the library, or when a non-Clang
+// compiler is used because only Clang supports the necessary attributes.
 // doesn't support the proper attributes.
-#if defined(_LIBCPP_BUILDING_LIBRARY) || defined(_LIBCXXABI_BUILDING_LIBRARY) ||                                       \
-    !__has_feature(attribute_availability_with_strict) || !__has_feature(attribute_availability_in_templates) ||       \
-    !__has_extension(pragma_clang_attribute_external_declaration)
+#if defined(_LIBCPP_BUILDING_LIBRARY) || defined(_LIBCXXABI_BUILDING_LIBRARY) || !defined(_LIBCPP_COMPILER_CLANG_BASED)
 #  if !defined(_LIBCPP_HAS_NO_VENDOR_AVAILABILITY_ANNOTATIONS)
 #    define _LIBCPP_HAS_NO_VENDOR_AVAILABILITY_ANNOTATIONS
 #  endif
diff --git a/libcxx/test/libcxx/vendor/apple/availability-with-pedantic-errors.compile.pass.cpp b/libcxx/test/libcxx/vendor/apple/availability-with-pedantic-errors.compile.pass.cpp
new file mode 100644
index 00000000000000..c55a0a4d6e5d1b
--- /dev/null
+++ b/libcxx/test/libcxx/vendor/apple/availability-with-pedantic-errors.compile.pass.cpp
@@ -0,0 +1,22 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// REQUIRES: stdlib=apple-libc++
+
+// Test that using -pedantic-errors doesn't turn off availability annotations.
+// This used to be the case because we used __has_extension(...) to enable the
+// availability annotations, and -pedantic-errors changes the behavior of
+// __has_extension(...) in an incompatible way.
+
+// ADDITIONAL_COMPILE_FLAGS: -pedantic-errors
+
+#include <__availability>
+
+#if defined(_LIBCPP_HAS_NO_VENDOR_AVAILABILITY_ANNOTATIONS)
+#  error Availability annotations should be enabled on Apple platforms in the system configuration!
+#endif