[libc++] Re-introduce _LIBCPP_DISABLE_AVAILABILITY

The _LIBCPP_DISABLE_AVAILABILITY macro was removed in afae1a5
as an intended no-op. It turns out that some projects are making use
of that macro to work around a Clang bug with availability annotations
that still exists: llvm#134151.

Since that Clang bug still hasn't been fixed, I feel that we must
sill honor that unfortunate macro until we've figured out how to
get rid of it without breaking code.

Author: ldionne

libcxx/docs/ReleaseNotes/20.rst

@@ -153,9 +153,6 @@ Deprecations and Removals
   headers as an extension and only deprecates them. The ``_LIBCPP_DISABLE_DEPRECATION_WARNINGS`` macro can be defined to
   suppress deprecation for these headers.
 
-- The ``_LIBCPP_DISABLE_AVAILABILITY`` macro that was used to force-disable availability markup has now been removed.
-  Whether availability markup is used by the library is now solely controlled at configuration-time.
-
 - The pointer safety functions ``declare_reachable``, ``declare_no_pointers``, ``undeclare_no_pointers`` and
   ``__undeclare_reachable`` have been removed from the library. These functions were never implemented in a non-trivial
   way, making it very unlikely that any binary depends on them.

libcxx/include/__configuration/availability.h

@@ -69,7 +69,13 @@
 
 // Availability markup is disabled when building the library, or when a non-Clang
 // compiler is used because only Clang supports the necessary attributes.
-#if defined(_LIBCPP_BUILDING_LIBRARY) || defined(_LIBCXXABI_BUILDING_LIBRARY) || !defined(_LIBCPP_COMPILER_CLANG_BASED)
+//
+// We also allow users to force-disable availability markup via the `_LIBCPP_DISABLE_AVAILABILITY`
+// macro because that is the only way to work around a Clang bug related to availability
+// attributes: https://github.com/llvm/llvm-project/issues/134151.
+// Once that bug has been fixed, we should remove the macro.
+#if defined(_LIBCPP_BUILDING_LIBRARY) || defined(_LIBCXXABI_BUILDING_LIBRARY) ||                                       \
+    !defined(_LIBCPP_COMPILER_CLANG_BASED) || defined(_LIBCPP_DISABLE_AVAILABILITY)
 #  undef _LIBCPP_HAS_VENDOR_AVAILABILITY_ANNOTATIONS
 #  define _LIBCPP_HAS_VENDOR_AVAILABILITY_ANNOTATIONS 0
 #endif

libcxx/test/libcxx/vendor/apple/disable-availability.sh.cpp

@@ -0,0 +1,43 @@
+//===----------------------------------------------------------------------===//
+//
+// 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++
+
+// This test ensures that we retain a way to disable availability markup on Apple platforms
+// in order to work around Clang bug https://github.com/llvm/llvm-project/issues/134151.
+//
+// Once that bug has been fixed or once we've made changes to libc++'s use of availability
+// that render that workaround unnecessary, the macro and this test can be removed.
+//
+// The test works by creating a final linked image that refers to a function marked with
+// both an availability attribute and with _LIBCPP_HIDE_FROM_ABI. We then check that this
+// generates a weak reference to the function -- without the bug, we'd expect a strong
+// reference or no reference at all instead.
+
+// First, test the test. Make sure that we do (incorrectly) produce a weak definition when we
+// don't define _LIBCPP_DISABLE_AVAILABILITY. Otherwise, something may have changed in libc++
+// and this test might not work anymore.
+// RUN: %{cxx} %s %{flags} %{compile_flags} %{link_flags} -fvisibility=hidden -fvisibility-inlines-hidden -shared -o %t.1.dylib
+// RUN: nm -omg %t.1.dylib | c++filt | grep value | grep weak
+
+// Now, make sure that 'weak' goes away when we define _LIBCPP_DISABLE_AVAILABILITY.
+// In fact, all references to the function might go away, so we just check that we don't emit
+// any weak reference.
+// RUN: %{cxx} %s %{flags} %{compile_flags} %{link_flags} -fvisibility=hidden -fvisibility-inlines-hidden -D_LIBCPP_DISABLE_AVAILABILITY -shared -o %t.2.dylib
+// RUN: nm -omg %t.2.dylib | c++filt | grep -v weak
+
+#include <version>
+
+template <class T>
+struct optional {
+  T val_;
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_INTRODUCED_IN_LLVM_11_ATTRIBUTE T value() const { return val_; }
+};
+
+using PMF = int (optional<int>::*)() const;
+PMF f() { return &optional<int>::value; }