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+===================
+Availability Markup
+===================
+
+.. contents::
+   :local:
+
+Overview
+========
+
+Libc++ is used as a system library on macOS and iOS (amongst others). In order
+for users to be able to compile a binary that is intended to be deployed to an
+older version of the platform, clang provides the
+`availability attribute <https://clang.llvm.org/docs/AttributeReference.html#availability>`_
+that can be placed on declarations to describe the lifecycle of a symbol in the
+library.
+
+Design
+======
+
+When a new feature is introduced that requires dylib support, a macro should be
+created in include/__config to mark this feature as unavailable for all the
+systems. For example::
+
+    // Define availability macros.
+    #if defined(_LIBCPP_USE_AVAILABILITY_APPLE)
+    # define _LIBCPP_AVAILABILITY_BAD_OPTIONAL_ACCESS __attribute__((unavailable))
+    #else  if defined(_LIBCPP_USE_AVAILABILITY_SOME_OTHER_VENDOR)
+    # define _LIBCPP_AVAILABILITY_BAD_OPTIONAL_ACCESS __attribute__((unavailable))
+    #else
+    # define _LIBCPP_AVAILABILITY_BAD_OPTIONAL_ACCESS
+    #endif
+
+When the library is updated by the platform vendor, the markup can be updated.
+For example::
+
+    #define _LIBCPP_AVAILABILITY_SHARED_MUTEX                                  \
+      __attribute__((availability(macosx,strict,introduced=10.12)))            \
+      __attribute__((availability(ios,strict,introduced=10.0)))                \
+      __attribute__((availability(tvos,strict,introduced=10.0)))               \
+      __attribute__((availability(watchos,strict,introduced=3.0)))
+
+In the source code, the macro can be added on a class if the full class requires
+type info from the library for example::
+
+    _LIBCPP_BEGIN_NAMESPACE_EXPERIMENTAL
+    class _LIBCPP_EXCEPTION_ABI _LIBCPP_AVAILABILITY_BAD_OPTIONAL_ACCESS bad_optional_access
+      : public std::logic_error {
+
+or on a particular symbol:
+
+    _LIBCPP_OVERRIDABLE_FUNC_VIS _LIBCPP_AVAILABILITY_SIZED_NEW_DELETE void  operator delete(void* __p, std::size_t __sz) _NOEXCEPT;
+
+Furthermore, a lit feature should be added to match that availability macro,
+so that tests depending on that feature can be marked to XFAIL if the feature
+is not supported. This way, the test suite will work on platforms that have
+not shipped the feature yet. This can be done by adding the appropriate lit
+feature in test/config.py.
+
+
+Testing
+=======
+
+Some parameters can be passed to lit to run the test-suite and exercise the
+availability.
+
+* The `platform` parameter controls the deployment target. For example lit can
+  be invoked with `--param=platform=macosx10.8`. Default is the current host.
+* The `use_system_cxx_lib` parameter indicates to use another library than the
+  just built one. Invoking lit with `--param=use_system_cxx_lib=true` will run
+  the test-suite against the host system library. Alternatively a path to the
+  directory containing a specific prebuilt libc++ can be used, for example:
+  `--param=use_system_cxx_lib=/path/to/macOS/10.8/`.
+
+Tests can be marked as XFAIL based on multiple features made available by lit:
+
+
+* if `--param=platform=macosx10.8` is passed, the following features will be available:
+
+  - availability
+  - availability=x86_64
+  - availability=macosx
+  - availability=x86_64-macosx
+  - availability=x86_64-apple-macosx10.8
+  - availability=macosx10.8
+
+  This feature is used to XFAIL a test that *is* using a class or a method marked
+  as unavailable *and* that is expected to *fail* if deployed on an older system.
+
+* if `use_system_cxx_lib` and `--param=platform=macosx10.8` are passed to lit,
+  the following features will also be available:
+
+  - with_system_cxx_lib
+  - with_system_cxx_lib=x86_64
+  - with_system_cxx_lib=macosx
+  - with_system_cxx_lib=x86_64-macosx
+  - with_system_cxx_lib=x86_64-apple-macosx10.8
+  - with_system_cxx_lib=macosx10.8
+
+  This feature is used to XFAIL a test that is *not* using a class or a method
+  marked as unavailable *but* that is expected to fail if deployed on an older
+  system. For example, if the test exhibits a bug in the libc on a particular
+  system version, or if the test uses a symbol that is not available on an
+  older version of the dylib (but for which there is no availability markup,
+  otherwise the XFAIL should use `availability` above).