[libc++] Always build c++experimental.a

[ldionne]

Summary

This is the first part of a plan to ship experimental features
by default while guarding them behind a compiler flag to avoid
users accidentally depending on them. Subsequent patches will
also encompass incomplete features (such as <format> and <ranges>)
in that categorization. Basically, the idea is that we always
build and ship the c++experimental library, however users can't
use what's in it unless they pass the -funstable flag to Clang.

Note that this patch intentionally does not start guarding
existing <experimental/FOO> content behind the flag, because
that would merely break users that might be relying on such
content being in the headers unconditionally. Instead, we
should start guarding new TSes behind the flag, and get rid
of the existing TSes we have by shipping their Standard
counterpart.

Also, this patch must jump through a few hoops like defining
_LIBCPP_ENABLE_EXPERIMENTAL because we still support compilers
that do not implement -funstable yet.

[ldionne]

@mstorsjo I might need some assistance figuring out what to do with the Windows parts of this change.

[Mordante]

Thanks for working on this! LGTM with some suggestions.

libcxx/docs/ReleaseNotes.rst

226 ↗

(On Diff #441428)

I agree this is a build system change, but I wonder how many users read this part and find the hidden gem.
Should we add a note at new features too? Maybe there we can mention format and ranges are available with this new switch.
(technically that actually belongs in the next patch, but I wouldn't mind to do it in this patch.)

libcxx/docs/UsingLibcxx.rst

42

I think it would be good to explain what experimental features are. The current wording makes it sound the features may not be part of an IS, WP, or TS.

How about something like

Libc++ provides implementations of some experimental features. Experimental features
are either a TS or a feature in the Standard whose implementation is incomplete. Those
features are disabled by default because they are neither API nor ABI stable. However,
the Clang flag ``-funstable`` can be used to turn those features on.

[ldionne]

Address review comments and try to fix some CI issues

libcxx/docs/ReleaseNotes.rst

226 ↗

(On Diff #441428)

I added a note to do this in the next patch, since I think it belongs there.

[mstorsjo]

In D128927#3622793, @ldionne wrote:

@mstorsjo I might need some assistance figuring out what to do with the Windows parts of this change.

I can try to have a look in a couple days or so - I'm vacationing and have been travelling, but I'm slowly trying to catch up on things here.

[Mordante]

I had a look at the changes and still LGTM!

[ldionne]

Rebase onto main and temporarily disable experimental tests on Windows

[mstorsjo]

It looks like the tests would work in CI in the clang-cl configurations with this modification:

diff --git a/libcxx/utils/ci/run-buildbot b/libcxx/utils/ci/run-buildbot
index 65155ba173ed..407e93ef1575 100755
--- a/libcxx/utils/ci/run-buildbot
+++ b/libcxx/utils/ci/run-buildbot
@@ -114,8 +114,6 @@ function generate-cmake-libcxx-win() {
           -DCMAKE_CXX_COMPILER=clang-cl \
           -DLIBCXX_ENABLE_FILESYSTEM=YES \
           -DLIBCXX_EXTRA_SITE_DEFINES="_LIBCPP_HAS_NO_INT128" \
-          -DLIBCXX_TEST_PARAMS="enable_experimental=False" \
-          -DLIBCXXABI_TEST_PARAMS="enable_experimental=False" \
           "${@}"
 }
 
@@ -521,7 +519,7 @@ armv7-noexceptions)
 ;;
 clang-cl-dll)
     clean
-    generate-cmake-libcxx-win
+    generate-cmake-libcxx-win -DLIBCXX_TEST_PARAMS="enable_experimental=False"
     echo "+++ Running the libc++ tests"
     ${NINJA} -vC "${BUILD_DIR}" check-cxx
 ;;
diff --git a/libcxx/utils/libcxx/test/params.py b/libcxx/utils/libcxx/test/params.py
index 6d878195fb8a..3b860b8538f2 100644
--- a/libcxx/utils/libcxx/test/params.py
+++ b/libcxx/utils/libcxx/test/params.py
@@ -67,7 +67,12 @@ def getUnstableFlag(cfg):
   if hasCompileFlag(cfg, '-funstable') and False: # TODO: Enable this once the design of `-funstable` is finished
     return '-funstable'
   else:
-    return '-D_LIBCPP_ENABLE_EXPERIMENTAL -lc++experimental'
+    # When linking in MSVC mode via the Clang driver, a -l<foo>
+    # maps to <foo>.lib, so we need to use -llibc++experimental here
+    # to make it link against the static libc++experimental.lib.
+    # We can't check for the feature 'msvc' in available_features
+    # as those features are added after processing parameters. 
+    return '-D_LIBCPP_ENABLE_EXPERIMENTAL ' + ('-llibc++experimental' if _isMSVC(cfg) else '-lc++experimental')
 
 DEFAULT_PARAMETERS = [
   Parameter(name='target_triple', type=str,

The current patch lost the ugly -llibc++experimental conditional, which is necessary for it to work in the clang-cl-static config (where libc++experimental is usable). For the clang-cl-dll config, we pretty much need to skip testing c++experimental because it's not usable in that configuration.

(For context, to make libc++experimental work in clang-cl-dll configs; we'd need to split all the _LIBCPP_TYPE_VIS and _LIBCPP_FUNC_VIS into a separate _LIBCPP_EXPERIMENTAL_FUNC_VIS, so that the headers would signal dllimport for the things that are provided by the main shared libc++ library, but signal static linkage for functions/types/templates that are provided by the statically linked libc++experimental. I'm not sure if we're willing to go there?)

libcxx/utils/libcxx/test/params.py

71

Actually, I'm not entirely convinced that this is a good way to handle linking against the library for tests.

When building tests, we don't rely on the compiler to implicitly link in our C++ library, but we link with -nostdlib++ (or -nodefaultlibs) and explicitly tell the compiler how to link in specifically the C++ library we've just built (in the test config files).

So in general, if linking with -nostdlib++ -fexperimental-library I kinda wouldn't expect the compiler driver to link against the library at all? It's kinda the same as if you'd do -stdlib=libstdc++ -fexperimental-library - we can't have that add -lc++experimental, as that only makes sense as long as we have -stdlib=libc++. So subsequently I don't think the compiler driver should be adding -lc++experimental as long as we're passing -nostdlib++ either?

But if libc++experimental is built unconditionally, wouldn't it be simplest to just always link against it in the test config files (just like how we force it to link against specifically the just-built libc++.a) - that would make it much more symmetrcial to how we handle the main -lc++?

[ldionne]

Thanks @mstorsjo! Regarding _LIBCPP_EXPERIMENTAL_FUNC_VIS, yes I think it would make sense to use a different visibility macro for symbols that we know are provided only as part of a static library. I would not call it _LIBCPP_EXPERIMENTAL_FUNC_VIS though, I would call it something like _LIBCPP_STATIC_LIBRARY_FUNC_VIS or something like that.

libcxx/utils/libcxx/test/params.py

71

Interesting point. It does mean that we can never rely on -funstable adding -lc++experimental from our test suite -- I think that's OK.

However, I'd like to avoid linking against -lc++experimental in the base test configuration, simply because the base test configuration should mirror the minimal way of invoking Clang to use libc++.

[ldionne]

Windows fixes.

[mstorsjo]

In D128927#3638973, @ldionne wrote:

Thanks @mstorsjo! Regarding _LIBCPP_EXPERIMENTAL_FUNC_VIS, yes I think it would make sense to use a different visibility macro for symbols that we know are provided only as part of a static library. I would not call it _LIBCPP_EXPERIMENTAL_FUNC_VIS though, I would call it something like _LIBCPP_STATIC_LIBRARY_FUNC_VIS or something like that.

Sure, _LIBCPP_STATIC_LIBRARY_FUNC_VIS sounds good to me too.

libcxx/utils/libcxx/test/params.py

71

Yep. Also another example of why we can't really rely on the compiler driver adding it - technically I guess the compiler driver might even choose not to pass a generic -lc++ or -lc++experimental, but pass an absolute path to the libc++.{a,so,dylib} that is bundled with the compiler (clang does this for compiler-rt builtins) - so we really do need to rely on -nodefaultlibs and/or -nostdlib++ omitting the compiler-provided defaults.

Keeping adding -lc++experimental here instead of adding it in the base config files is fine with me.

[ldionne]

Rebase onto main and remove references to -funstable since the design is still changing.

[fhahn]

It looks like this is causing macOS builds to fail on GreenDragon. Please take a look and refer the commit if it takes longer to resolve, as the bot has been red for a while now.

[3495/5980] /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/c++ -DBUILD_EXAMPLES -D_LIBCPP_BUILDING_LIBRARY -D_LIBCPP_DISABLE_NEW_DELETE_DEFINITIONS -D_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -I/Users/buildslave/jenkins/workspace/clang-stage1-RA/clang-build/projects/libcxx/src -I/Users/buildslave/jenkins/workspace/clang-stage1-RA/llvm-project/libcxx/src -I/Users/buildslave/jenkins/workspace/clang-stage1-RA/clang-build/include -I/Users/buildslave/jenkins/workspace/clang-stage1-RA/llvm-project/llvm/include -fPIC -fvisibility-inlines-hidden -Werror=date-time -Werror=unguarded-availability-new -Wall -Wextra -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wmissing-field-initializers  -Wno-long-long -Wc++98-compat-extra-semi -Wimplicit-fallthrough -Wcovered-switch-default -Wno-noexcept-type -Wnon-virtual-dtor -Wdelete-non-virtual-dtor -Wstring-conversion -fdiagnostics-color -O2 -g  -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.15.sdk -mmacosx-version-min=10.9 -UNDEBUG -faligned-allocation -nostdinc++ -fvisibility-inlines-hidden -fvisibility=hidden -Wall -Wextra -W -Wwrite-strings -Wno-unused-parameter -Wno-long-long -Werror=return-type -Wextra-semi -Wundef -Wformat-nonliteral -Wno-user-defined-literals -Wno-covered-switch-default -Wno-error -std=c++2a -MD -MT projects/libcxx/src/CMakeFiles/cxx_experimental.dir/experimental/memory_resource.cpp.o -MF projects/libcxx/src/CMakeFiles/cxx_experimental.dir/experimental/memory_resource.cpp.o.d -o projects/libcxx/src/CMakeFiles/cxx_experimental.dir/experimental/memory_resource.cpp.o -c /Users/buildslave/jenkins/workspace/clang-stage1-RA/llvm-project/libcxx/src/experimental/memory_resource.cpp
FAILED: projects/libcxx/src/CMakeFiles/cxx_experimental.dir/experimental/memory_resource.cpp.o 
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/c++ -DBUILD_EXAMPLES -D_LIBCPP_BUILDING_LIBRARY -D_LIBCPP_DISABLE_NEW_DELETE_DEFINITIONS -D_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -I/Users/buildslave/jenkins/workspace/clang-stage1-RA/clang-build/projects/libcxx/src -I/Users/buildslave/jenkins/workspace/clang-stage1-RA/llvm-project/libcxx/src -I/Users/buildslave/jenkins/workspace/clang-stage1-RA/clang-build/include -I/Users/buildslave/jenkins/workspace/clang-stage1-RA/llvm-project/llvm/include -fPIC -fvisibility-inlines-hidden -Werror=date-time -Werror=unguarded-availability-new -Wall -Wextra -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wmissing-field-initializers  -Wno-long-long -Wc++98-compat-extra-semi -Wimplicit-fallthrough -Wcovered-switch-default -Wno-noexcept-type -Wnon-virtual-dtor -Wdelete-non-virtual-dtor -Wstring-conversion -fdiagnostics-color -O2 -g  -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.15.sdk -mmacosx-version-min=10.9 -UNDEBUG -faligned-allocation -nostdinc++ -fvisibility-inlines-hidden -fvisibility=hidden -Wall -Wextra -W -Wwrite-strings -Wno-unused-parameter -Wno-long-long -Werror=return-type -Wextra-semi -Wundef -Wformat-nonliteral -Wno-user-defined-literals -Wno-covered-switch-default -Wno-error -std=c++2a -MD -MT projects/libcxx/src/CMakeFiles/cxx_experimental.dir/experimental/memory_resource.cpp.o -MF projects/libcxx/src/CMakeFiles/cxx_experimental.dir/experimental/memory_resource.cpp.o.d -o projects/libcxx/src/CMakeFiles/cxx_experimental.dir/experimental/memory_resource.cpp.o -c /Users/buildslave/jenkins/workspace/clang-stage1-RA/llvm-project/libcxx/src/experimental/memory_resource.cpp
/Users/buildslave/jenkins/workspace/clang-stage1-RA/llvm-project/libcxx/src/experimental/memory_resource.cpp:9:10: fatal error: 'experimental/memory_resource' file not found
#include <experimental/memory_resource>
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 error generated.
[3496/5980] /Applications/Xc

[ldionne]

In D128927#3648616, @fhahn wrote:

It looks like this is causing macOS builds to fail on GreenDragon. Please take a look and refer the commit if it takes longer to resolve, as the bot has been red for a while now.

Thanks for the heads up. Those bots shouldn't be building libc++ at all. Let's take that offline.

Edit: CF https://github.com/llvm/llvm-zorg/pull/28

[hans]

In D128927#3648652, @ldionne wrote:

In D128927#3648616, @fhahn wrote:

It looks like this is causing macOS builds to fail on GreenDragon. Please take a look and refer the commit if it takes longer to resolve, as the bot has been red for a while now.

Thanks for the heads up. Those bots shouldn't be building libc++ at all. Let's take that offline.

Edit: CF https://github.com/llvm/llvm-zorg/pull/28

We're hitting the same "memory_resource.cpp:9:10: fatal error: 'experimental/memory_resource' file not found" error in Chromium (http://crbug.com/1345033).

The file exists, so the error is surprising. Since the GreenDragon bots also hit this, it seems there's a bug here that needs fixing?

[hans]

After some more digging, it seems we're hitting the same issue as we did back in https://reviews.llvm.org/D82702#2153130 and now the work-around of passing -DLIBCXX_ENABLE_EXPERIMENTAL_LIBRARY=OFF stopped working.

[hans]

After discussing with @thakis a bit, I've reverted this in 09cebfb978def7fa2a4460bca89690f8d3608216 to bring builds back to a green state while we figure out what's going on and how to fix this.

The fact that Greendragon broke (example build) seems like a strong indication that this would break others as well, and removing libc++ from those bots seems like avoiding rather than fixing the problem.

If it's the case that Clang and libc++ should no longer be built together (as indicated on https://github.com/llvm/llvm-zorg/pull/28) I think we need a better failure mode than "fatal error: 'experimental/memory_resource' file not found", some good documentation updates to explain what people should be doing instead, and time for people to fix their builds.

As for Chromium, we've been using the technique suggested by @ldionne in https://bugs.chromium.org/p/chromium/issues/detail?id=1067216#c7 plus -DLIBCXX_ENABLE_EXPERIMENTAL_LIBRARY=OFF. We would be grateful for ideas of what we should do instead moving forward.

(Also, this is probably a stupid question, but why can't this library use its own headers? I really don't understand the failure mode here to begin with.)

[ldionne]

The weird part here is that you're configuring libc++, but you are building neither the static nor the shared library. I don't understand why you do that, and that may hide some other more fundamental issue in your setup. Anyways, I think the issue should be resolved by 1d0f79558ca47f50119fb38c62ff5afd3160d260.

[hans]

In D128927#3662659, @ldionne wrote:

The weird part here is that you're configuring libc++, but you are building neither the static nor the shared library. I don't understand why you do that, and that may hide some other more fundamental issue in your setup.

Yes, I wish we weren't weird.
The reason libc++ is part of our build is that we want the headers so that our newly built Clang will be able to build c++ files. https://bugs.chromium.org/p/chromium/issues/detail?id=1067216#c7 has the background for our current situation.

In https://github.com/llvm/llvm-zorg/pull/28 you mentioned that "for a couple of years now, Clang and libc++ are entirely separate projects w.r.t. how they are shipped on Apple platforms, and it doesn't make sense to build libc++ at the same time as Clang anymore".
Does that mean the libc++ headers have moved to the SDK now, and if so from what version? If it's in the SDK version we use (I'd have to check), perhaps that would allow us to stop building like this.

Anyways, I think the issue should be resolved by 1d0f79558ca47f50119fb38c62ff5afd3160d260.

Yes, that fixed it for us. Thanks!

[ldionne]

In D128927#3665748, @hans wrote:

In D128927#3662659, @ldionne wrote:

The weird part here is that you're configuring libc++, but you are building neither the static nor the shared library. I don't understand why you do that, and that may hide some other more fundamental issue in your setup.

Yes, I wish we weren't weird.
The reason libc++ is part of our build is that we want the headers so that our newly built Clang will be able to build c++ files. https://bugs.chromium.org/p/chromium/issues/detail?id=1067216#c7 has the background for our current situation.

In https://github.com/llvm/llvm-zorg/pull/28 you mentioned that "for a couple of years now, Clang and libc++ are entirely separate projects w.r.t. how they are shipped on Apple platforms, and it doesn't make sense to build libc++ at the same time as Clang anymore".
Does that mean the libc++ headers have moved to the SDK now, and if so from what version? If it's in the SDK version we use (I'd have to check), perhaps that would allow us to stop building like this.

Yes, the libc++ headers are in the SDK. They have been since Xcode 12 if I'm not mistaken. They are *also* still shipped in the Clang toolchain, but that will change eventually -- there are a few things blocking us from doing that, but they will go away eventually.

[hans]

In https://github.com/llvm/llvm-zorg/pull/28 you mentioned that "for a couple of years now, Clang and libc++ are entirely separate projects w.r.t. how they are shipped on Apple platforms, and it doesn't make sense to build libc++ at the same time as Clang anymore".
Does that mean the libc++ headers have moved to the SDK now, and if so from what version? If it's in the SDK version we use (I'd have to check), perhaps that would allow us to stop building like this.

Yes, the libc++ headers are in the SDK. They have been since Xcode 12 if I'm not mistaken. They are *also* still shipped in the Clang toolchain, but that will change eventually -- there are a few things blocking us from doing that, but they will go away eventually.

We've removed libc++ from our toolchain build process (https://chromium-review.googlesource.com/c/chromium/src/+/3779521) so hopefully will see fewer problems of this nature in the future. Thanks!

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 set(LIBCXXABI_ENABLE_STATIC OFF CACHE BOOL "")
 set(LIBCXX_CXX_ABI libcxxabi CACHE STRING "")
 set(LIBCXXABI_USE_LLVM_UNWINDER ON CACHE BOOL "")
 set(LIBUNWIND_ENABLE_SHARED ON CACHE BOOL "")
 set(LIBUNWIND_ENABLE_STATIC OFF CACHE BOOL "")

libcxx/cmake/caches/Apple.cmake

 set(CMAKE_BUILD_TYPE MinSizeRel CACHE STRING "")
 set(CMAKE_POSITION_INDEPENDENT_CODE OFF CACHE BOOL "")
 
 set(LIBCXX_USE_COMPILER_RT ON CACHE BOOL "")
 set(LIBCXX_ENABLE_ASSERTIONS OFF CACHE BOOL "")
 set(LIBCXX_ABI_VERSION "1" CACHE STRING "")
-set(LIBCXX_ENABLE_EXPERIMENTAL_LIBRARY OFF CACHE BOOL "")
 set(LIBCXX_ENABLE_STATIC ON CACHE BOOL "")
 set(LIBCXX_ENABLE_SHARED ON CACHE BOOL "")
 set(LIBCXX_CXX_ABI libcxxabi CACHE STRING "")
 set(LIBCXX_ENABLE_BACKWARDS_COMPATIBILITY_DEBUG_MODE_SYMBOLS OFF CACHE BOOL "")
 set(LIBCXX_ENABLE_VENDOR_AVAILABILITY_ANNOTATIONS ON CACHE BOOL "")
 set(LIBCXX_ENABLE_INCOMPLETE_FEATURES OFF CACHE BOOL "")
 
 set(LIBCXX_HERMETIC_STATIC_LIBRARY ON CACHE BOOL "")

libcxx/cmake/caches/Generic-no-experimental.cmake

-set(LIBCXX_ENABLE_EXPERIMENTAL_LIBRARY OFF CACHE BOOL "")
+set(LIBCXX_TEST_PARAMS "enable_experimental=False" CACHE STRING "")
+set(LIBCXXABI_TEST_PARAMS "${LIBCXX_TEST_PARAMS}" CACHE STRING "")
 set(LIBCXX_ENABLE_INCOMPLETE_FEATURES OFF CACHE BOOL "")

libcxx/docs/BuildingLibcxx.rst

 it is the simplest way to build.
 
 .. code-block:: batch
 
   > cmake -G "Visual Studio 16 2019" -S runtimes -B build ^
           -T "ClangCL"                                    ^
           -DLLVM_ENABLE_RUNTIMES=libcxx                   ^
           -DLIBCXX_ENABLE_SHARED=YES                      ^
-          -DLIBCXX_ENABLE_STATIC=NO                       ^
-          -DLIBCXX_ENABLE_EXPERIMENTAL_LIBRARY=NO
+          -DLIBCXX_ENABLE_STATIC=NO
   > cmake --build build
 
 CMake + ninja (MSVC)
 --------------------
 
 Building with ninja is required for development to enable tests.
 A couple of tests require Bash to be available, and a couple dozens
 of tests require other posix tools (cp, grep and similar - LLVM's tests

 
 In either case, then run:
 
 .. code-block:: batch
 
   > cmake -G Ninja -S runtimes -B build                                               ^
           -DCMAKE_C_COMPILER=clang-cl                                                 ^
           -DCMAKE_CXX_COMPILER=clang-cl                                               ^
-          -DLLVM_ENABLE_RUNTIMES=libcxx                                               ^
-          -DLIBCXX_ENABLE_EXPERIMENTAL_LIBRARY=NO
+          -DLLVM_ENABLE_RUNTIMES=libcxx
   > ninja -C build cxx
   > ninja -C build check-cxx
 
 If you are running in an MSYS2 shell and you have installed the
 MSYS2-provided clang package (which defaults to a non-MSVC target), you
 should add e.g. ``-DCMAKE_CXX_COMPILER_TARGET=x86_64-windows-msvc`` (replacing
 ``x86_64`` with the architecture you're targeting) to the ``cmake`` command
 line above. This will instruct ``check-cxx`` to use the right target triple

 .. option:: LIBCXX_INSTALL_INCLUDE_TARGET_DIR:PATH
 
   **Default**: ``include/c++/v1`` or
   ``include/${LLVM_DEFAULT_TARGET_TRIPLE}/c++/v1``
 
   Path where target-specific libc++ headers should be installed. If a relative
   path, relative to ``CMAKE_INSTALL_PREFIX``.
 
-.. _libc++experimental options:
-
-libc++experimental Specific Options
-------------------------------------
-
-.. option:: LIBCXX_ENABLE_EXPERIMENTAL_LIBRARY:BOOL
-
-  **Default**: ``ON``
-
-  Build and test libc++experimental.a.
-
-.. option:: LIBCXX_INSTALL_EXPERIMENTAL_LIBRARY:BOOL
-
-  **Default**: ``LIBCXX_ENABLE_EXPERIMENTAL_LIBRARY AND LIBCXX_INSTALL_LIBRARY``
-
-  Install libc++experimental.a alongside libc++.
-
 
 .. _ABI Library Specific Options:
 
 ABI Library Specific Options
 ----------------------------
 
 .. option:: LIBCXX_CXX_ABI:STRING
 

libcxx/docs/UsingLibcxx.rst

.. code-block:: bash
   $ clang++ -std=c++17 test.cpp
 
 .. warning::
   Using ``-std=c++XY`` with a version of the Standard that has not been ratified yet
   is considered unstable. Libc++ reserves the right to make breaking changes to the
   library until the standard has been ratified.
 
 
-Using libc++experimental and ``<experimental/...>``
-===================================================
+Enabling experimental C++ Library features
+==========================================
 
-Libc++ provides implementations of experimental technical specifications
-in a separate library, ``libc++experimental.a``. Users of ``<experimental/...>``
-headers may be required to link ``-lc++experimental``. Note that not all
-vendors ship ``libc++experimental.a``, and as a result, you may not be
-able to use those experimental features.
-
-.. code-block:: bash
-
-  $ clang++ test.cpp -lc++experimental
+Libc++ provides implementations of some experimental features. Experimental features
+are either Technical Specifications (TSes) or official features that were voted to
+the Standard but whose implementation is not complete or stable yet in libc++. Those
+are disabled by default because they are neither API nor ABI stable. However, the
+``_LIBCPP_ENABLE_EXPERIMENTAL`` macro can be defined to turn those features on. Note
+that you will also need to link to the appropriate ``libc++experimental.a`` static
+archive.
 
 .. warning::
   Experimental libraries are Experimental.
-    * The contents of the ``<experimental/...>`` headers and ``libc++experimental.a``
+    * The contents of the ``<experimental/...>`` headers and the associated static
       library will not remain compatible between versions.
     * No guarantees of API or ABI stability are provided.
     * When the standardized version of an experimental feature is implemented,
       the experimental feature is removed two releases after the non-experimental
       version has shipped. The full policy is explained :ref:`here <experimental features>`.
 
 
 Using libc++ when it is not the system default

libcxx/src/CMakeLists.txt

if (LIBCXX_ENABLE_STATIC)
   if (LIBCXX_STATICALLY_LINK_ABI_IN_STATIC_LIBRARY)
     target_link_libraries(cxx_static PRIVATE libcxx-abi-static-objects)
   endif()
 endif()
 
 # Add a meta-target for both libraries.
 add_custom_target(cxx DEPENDS ${LIBCXX_BUILD_TARGETS})
 
-if (LIBCXX_ENABLE_EXPERIMENTAL_LIBRARY)
-  set(LIBCXX_EXPERIMENTAL_SOURCES
-    experimental/memory_resource.cpp
-    )
-  add_library(cxx_experimental STATIC ${LIBCXX_EXPERIMENTAL_SOURCES})
-  target_link_libraries(cxx_experimental PUBLIC cxx-headers)
-  if (LIBCXX_ENABLE_SHARED)
-    target_link_libraries(cxx_experimental PRIVATE cxx_shared)
-  else()
-    target_link_libraries(cxx_experimental PRIVATE cxx_static)
-  endif()
+
 
-  set_target_properties(cxx_experimental
-    PROPERTIES
-      COMPILE_FLAGS "${LIBCXX_COMPILE_FLAGS}"
-      OUTPUT_NAME   "c++experimental"
-  )
-  cxx_add_common_build_flags(cxx_experimental)
+if (LIBCXX_HERMETIC_STATIC_LIBRARY)
+  # _LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS can be defined in __config_site
+  # too. Define it in the same way here, to avoid redefinition conflicts.
+  target_compile_definitions(cxx_experimental PRIVATE _LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS=)
+endif()
+
 
-  if (LIBCXX_HERMETIC_STATIC_LIBRARY)
-    # _LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS can be defined in __config_site
-    # too. Define it in the same way here, to avoid redefinition conflicts.
-    target_compile_definitions(cxx_experimental PRIVATE _LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS=)
-  endif()
-endif()
-
 
 if (LIBCXX_BUILD_EXTERNAL_THREAD_LIBRARY)
   set(LIBCXX_EXTERNAL_THREADING_SUPPORT_SOURCES
       "${CMAKE_CURRENT_SOURCE_DIR}/../test/support/external_threads.cpp")
 
   if (LIBCXX_ENABLE_SHARED)
     add_library(cxx_external_threads SHARED ${LIBCXX_EXTERNAL_THREADING_SUPPORT_SOURCES})
   else()

 
 if (LIBCXX_INSTALL_STATIC_LIBRARY)
   install(TARGETS cxx_static
     ARCHIVE DESTINATION ${LIBCXX_INSTALL_LIBRARY_DIR} COMPONENT cxx
     LIBRARY DESTINATION ${LIBCXX_INSTALL_LIBRARY_DIR} COMPONENT cxx
     RUNTIME DESTINATION ${LIBCXX_INSTALL_RUNTIME_DIR} COMPONENT cxx)
 endif()
 
-if(LIBCXX_INSTALL_EXPERIMENTAL_LIBRARY)
+if (LIBCXX_INSTALL_LIBRARY)
   install(TARGETS cxx_experimental
     LIBRARY DESTINATION ${LIBCXX_INSTALL_LIBRARY_DIR} COMPONENT cxx
     ARCHIVE DESTINATION ${LIBCXX_INSTALL_LIBRARY_DIR} COMPONENT cxx
     RUNTIME DESTINATION ${LIBCXX_INSTALL_RUNTIME_DIR} COMPONENT cxx)
 endif()
 
 # NOTE: This install command must go after the cxx install command otherwise
 # it will not be executed after the library symlinks are installed.
 if (LIBCXX_ENABLE_SHARED AND LIBCXX_ENABLE_ABI_LINKER_SCRIPT)
   install(FILES "$<TARGET_LINKER_FILE:cxx_shared>"
     DESTINATION ${LIBCXX_INSTALL_LIBRARY_DIR}
     COMPONENT libcxx)
 endif()
 
 if (NOT CMAKE_CONFIGURATION_TYPES)
     if(LIBCXX_INSTALL_LIBRARY)
-      set(lib_install_target cxx)
-    endif()
-    if (LIBCXX_INSTALL_EXPERIMENTAL_LIBRARY)
-      set(experimental_lib_install_target cxx_experimental)
+      set(lib_install_target "cxx;cxx_experimental")
     endif()
     if(LIBCXX_INSTALL_HEADERS)
       set(header_install_target install-cxx-headers)
     endif()
     if (LIBCXX_ENABLE_PARALLEL_ALGORITHMS)
       set(pstl_install_target install-pstl)
     endif()
     add_custom_target(install-cxx
                       DEPENDS ${lib_install_target}
-                              ${experimental_lib_install_target}
+                              cxx_experimental
                               ${header_install_target}
                               ${pstl_install_target}
                       COMMAND "${CMAKE_COMMAND}"
                       -DCMAKE_INSTALL_COMPONENT=cxx
                       -P "${LIBCXX_BINARY_DIR}/cmake_install.cmake")
     add_custom_target(install-cxx-stripped
                       DEPENDS ${lib_install_target}
-                              ${experimental_lib_install_target}
+                              cxx_experimental
                               ${header_install_target}
                               ${pstl_install_target}
                       COMMAND "${CMAKE_COMMAND}"
                       -DCMAKE_INSTALL_COMPONENT=cxx
                       -DCMAKE_INSTALL_DO_STRIP=1
                       -P "${LIBCXX_BINARY_DIR}/cmake_install.cmake")
 endif()

libcxx/test/CMakeLists.txt

 macro(serialize_lit_param param value)
   string(APPEND SERIALIZED_LIT_PARAMS "config.${param} = ${value}\n")
 endmacro()
 
 if (NOT LIBCXX_ENABLE_EXCEPTIONS)
   serialize_lit_param(enable_exceptions False)
 endif()
 
-if (NOT LIBCXX_ENABLE_EXPERIMENTAL_LIBRARY)
-  serialize_lit_param(enable_experimental False)
-endif()
-
 if (NOT LIBCXX_ENABLE_RTTI)
   serialize_lit_param(enable_rtti False)
 endif()
 
 if (LIBCXX_ENABLE_ASSERTIONS)
   serialize_lit_param(enable_assertions True)
 endif()
 

libcxx/test/libcxx/experimental/memory/memory.resource.global/global_memory_resource_lifetime.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 //
 //===----------------------------------------------------------------------===//
 
 // UNSUPPORTED: c++03
 
+// Aligned allocation is required by std::experimental::pmr, but it was not provided
+// before macosx10.13 and as a result we get linker errors when deploying to older than
+// macosx10.13.
+// XFAIL: use_system_cxx_lib && target={{.+}}-apple-macosx10.{{9|10|11|12}}
+
 // <experimental/memory_resource>
 
 // memory_resource * new_delete_resource()
 
 // The lifetime of the value returned by 'new_delete_resource()' should
 // never end, even very late into program termination. This test constructs
 // attempts to use 'new_delete_resource()' very late in program termination
 // to detect lifetime issues.

libcxx/test/libcxx/experimental/memory/memory.resource.global/new_delete_resource_lifetime.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 //
 //===----------------------------------------------------------------------===//
 
 // UNSUPPORTED: c++03
 
+// Aligned allocation is required by std::experimental::pmr, but it was not provided
+// before macosx10.13 and as a result we get linker errors when deploying to older than
+// macosx10.13.
+// XFAIL: use_system_cxx_lib && target={{.+}}-apple-macosx10.{{9|10|11|12}}
+
 // <experimental/memory_resource>
 
 // memory_resource * new_delete_resource()
 
 // The lifetime of the value returned by 'new_delete_resource()' should
 // never end, even very late into program termination. This test constructs
 // attempts to use 'new_delete_resource()' very late in program termination
 // to detect lifetime issues.

libcxx/test/std/experimental/memory/memory.polymorphic.allocator.class/memory.polymorphic.allocator.mem/construct_pair.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 //
 //===----------------------------------------------------------------------===//
 
 // UNSUPPORTED: c++03
 
+// Aligned allocation is required by std::experimental::pmr, but it was not provided
+// before macosx10.13 and as a result we get linker errors when deploying to older than
+// macosx10.13.
+// XFAIL: use_system_cxx_lib && target={{.+}}-apple-macosx10.{{9|10|11|12}}
+
 // <experimental/memory_resource>
 
 // template <class T> class polymorphic_allocator
 
 // template <class U1, class U2>
 // void polymorphic_allocator<T>::construct(pair<U1, U2>*)
 
 #include <experimental/memory_resource>

libcxx/test/std/experimental/memory/memory.polymorphic.allocator.class/memory.polymorphic.allocator.mem/construct_piecewise_pair_evil.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 //
 //===----------------------------------------------------------------------===//
 
 // UNSUPPORTED: c++03
 
+// Aligned allocation is required by std::experimental::pmr, but it was not provided
+// before macosx10.13 and as a result we get linker errors when deploying to older than
+// macosx10.13.
+// XFAIL: use_system_cxx_lib && target={{.+}}-apple-macosx10.{{9|10|11|12}}
+
 // <memory_resource>
 
 // template <class T> class polymorphic_allocator
 
 // template <class U1, class U2, class ...Args1, class ...Args2>
 // void polymorphic_allocator<T>::construct(pair<U1, U2>*, piecewise_construct_t
 //                                          tuple<Args1...>, tuple<Args2...>)
 

libcxx/test/std/experimental/memory/memory.polymorphic.allocator.class/memory.polymorphic.allocator.mem/destroy.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 //
 //===----------------------------------------------------------------------===//
 
 // UNSUPPORTED: c++03
 
+// Aligned allocation is required by std::experimental::pmr, but it was not provided
+// before macosx10.13 and as a result we get linker errors when deploying to older than
+// macosx10.13.
+// XFAIL: use_system_cxx_lib && target={{.+}}-apple-macosx10.{{9|10|11|12}}
+
 // <experimental/memory_resource>
 
 // template <class T> class polymorphic_allocator
 
 // template <class U>
 // void polymorphic_allocator<T>::destroy(U * ptr);
 
 #include <experimental/memory_resource>

libcxx/test/std/experimental/memory/memory.polymorphic.allocator.class/memory.polymorphic.allocator.mem/resource.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 //
 //===----------------------------------------------------------------------===//
 
 // UNSUPPORTED: c++03
 
+// Aligned allocation is required by std::experimental::pmr, but it was not provided
+// before macosx10.13 and as a result we get linker errors when deploying to older than
+// macosx10.13.
+// XFAIL: use_system_cxx_lib && target={{.+}}-apple-macosx10.{{9|10|11|12}}
+
 // <experimental/memory_resource>
 
 // template <class T> class polymorphic_allocator
 
 // memory_resource *
 // polymorphic_allocator<T>::resource() const
 
 #include <experimental/memory_resource>

libcxx/test/std/experimental/memory/memory.polymorphic.allocator.class/memory.polymorphic.allocator.mem/select_on_container_copy_construction.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 //
 //===----------------------------------------------------------------------===//
 
 // UNSUPPORTED: c++03
 
+// Aligned allocation is required by std::experimental::pmr, but it was not provided
+// before macosx10.13 and as a result we get linker errors when deploying to older than
+// macosx10.13.
+// XFAIL: use_system_cxx_lib && target={{.+}}-apple-macosx10.{{9|10|11|12}}
+
 // <experimental/memory_resource>
 
 // template <class T> class polymorphic_allocator
 
 // polymorphic_allocator
 // polymorphic_allocator<T>::select_on_container_copy_construction() const
 
 #include <experimental/memory_resource>

libcxx/test/std/experimental/memory/memory.resource.aliases/header_deque_synop.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 //
 //===----------------------------------------------------------------------===//
 
 // UNSUPPORTED: c++03
 
+// Aligned allocation is required by std::experimental::pmr, but it was not provided
+// before macosx10.13 and as a result we get linker errors when deploying to older than
+// macosx10.13.
+// XFAIL: use_system_cxx_lib && target={{.+}}-apple-macosx10.{{9|10|11|12}}
+
 // <experimental/deque>
 
 // namespace std { namespace experimental { namespace pmr {
 // template <class T>
 // using deque =
 //     ::std::deque<T, polymorphic_allocator<T>>
 //
 // }}} // namespace std::experimental::pmr

libcxx/test/std/experimental/memory/memory.resource.aliases/header_forward_list_synop.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 //
 //===----------------------------------------------------------------------===//
 
 // UNSUPPORTED: c++03
 
+// Aligned allocation is required by std::experimental::pmr, but it was not provided
+// before macosx10.13 and as a result we get linker errors when deploying to older than
+// macosx10.13.
+// XFAIL: use_system_cxx_lib && target={{.+}}-apple-macosx10.{{9|10|11|12}}
+
 // <experimental/forward_list>
 
 // namespace std { namespace experimental { namespace pmr {
 // template <class T>
 // using forward_list =
 //     ::std::forward_list<T, polymorphic_allocator<T>>
 //
 // }}} // namespace std::experimental::pmr

libcxx/test/std/experimental/memory/memory.resource.aliases/header_list_synop.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 //
 //===----------------------------------------------------------------------===//
 
 // UNSUPPORTED: c++03
 
+// Aligned allocation is required by std::experimental::pmr, but it was not provided
+// before macosx10.13 and as a result we get linker errors when deploying to older than
+// macosx10.13.
+// XFAIL: use_system_cxx_lib && target={{.+}}-apple-macosx10.{{9|10|11|12}}
+
 // <experimental/list>
 
 // namespace std { namespace experimental { namespace pmr {
 // template <class T>
 // using list =
 //     ::std::list<T, polymorphic_allocator<T>>
 //
 // }}} // namespace std::experimental::pmr

libcxx/test/std/experimental/memory/memory.resource.aliases/header_map_synop.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 //
 //===----------------------------------------------------------------------===//
 
 // UNSUPPORTED: c++03
 
+// Aligned allocation is required by std::experimental::pmr, but it was not provided
+// before macosx10.13 and as a result we get linker errors when deploying to older than
+// macosx10.13.
+// XFAIL: use_system_cxx_lib && target={{.+}}-apple-macosx10.{{9|10|11|12}}
+
 // <experimental/map>
 
 // namespace std { namespace experimental { namespace pmr {
 // template <class K, class V, class Compare = less<Key> >
 // using map =
 //     ::std::map<K, V, Compare, polymorphic_allocator<pair<const K, V>>>
 //
 // template <class K, class V, class Compare = less<Key> >

libcxx/test/std/experimental/memory/memory.resource.aliases/header_regex_synop.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 //
 //===----------------------------------------------------------------------===//
 
 // UNSUPPORTED: c++03
 // UNSUPPORTED: no-localization
 
+// Aligned allocation is required by std::experimental::pmr, but it was not provided
+// before macosx10.13 and as a result we get linker errors when deploying to older than
+// macosx10.13.
+// XFAIL: use_system_cxx_lib && target={{.+}}-apple-macosx10.{{9|10|11|12}}
+
 // <experimental/regex>
 
 // namespace std { namespace experimental { namespace pmr {
 //
 //  template <class BidirectionalIterator>
 //  using match_results =
 //    std::match_results<BidirectionalIterator,
 //                       polymorphic_allocator<sub_match<BidirectionalIterator>>>;

libcxx/test/std/experimental/memory/memory.resource.aliases/header_set_synop.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 //
 //===----------------------------------------------------------------------===//
 
 // UNSUPPORTED: c++03
 
+// Aligned allocation is required by std::experimental::pmr, but it was not provided
+// before macosx10.13 and as a result we get linker errors when deploying to older than
+// macosx10.13.
+// XFAIL: use_system_cxx_lib && target={{.+}}-apple-macosx10.{{9|10|11|12}}
+
 // <experimental/set>
 
 // namespace std { namespace experimental { namespace pmr {
 // template <class V, class Compare = less<V> >
 // using set =
 //     ::std::set<V, Compare, polymorphic_allocator<V>>
 //
 // template <class V, class Compare = less<V> >

libcxx/test/std/experimental/memory/memory.resource.aliases/header_string_synop.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 //
 //===----------------------------------------------------------------------===//
 
 // UNSUPPORTED: c++03
 
+// Aligned allocation is required by std::experimental::pmr, but it was not provided
+// before macosx10.13 and as a result we get linker errors when deploying to older than
+// macosx10.13.
+// XFAIL: use_system_cxx_lib && target={{.+}}-apple-macosx10.{{9|10|11|12}}
+
 // <experimental/string>
 
 // namespace std { namespace experimental { namespace pmr {
 // template <class Char, class Traits = ...>
 // using basic_string =
 //     ::std::basic_string<Char, Traits, polymorphic_allocator<Char>>
 //
 // typedef ... string

libcxx/test/std/experimental/memory/memory.resource.aliases/header_unordered_map_synop.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 //
 //===----------------------------------------------------------------------===//
 
 // UNSUPPORTED: c++03
 
+// Aligned allocation is required by std::experimental::pmr, but it was not provided
+// before macosx10.13 and as a result we get linker errors when deploying to older than
+// macosx10.13.
+// XFAIL: use_system_cxx_lib && target={{.+}}-apple-macosx10.{{9|10|11|12}}
+
 // <experimental/unordered_map>
 
 // namespace std { namespace experimental { namespace pmr {
 // template <class K, class V, class H = hash<K>, class P = equal_to<K> >
 // using unordered_map =
 //     ::std::unordered_map<K, V, H, P, polymorphic_allocator<pair<const K, V>>>
 //
 // template <class K, class V,  class H = hash<K>, class P = equal_to<K> >

libcxx/test/std/experimental/memory/memory.resource.aliases/header_unordered_set_synop.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 //
 //===----------------------------------------------------------------------===//
 
 // UNSUPPORTED: c++03
 
+// Aligned allocation is required by std::experimental::pmr, but it was not provided
+// before macosx10.13 and as a result we get linker errors when deploying to older than
+// macosx10.13.
+// XFAIL: use_system_cxx_lib && target={{.+}}-apple-macosx10.{{9|10|11|12}}
+
 // <experimental/unordered_set>
 
 // namespace std { namespace experimental { namespace pmr {
 // template <class V, class H = hash<V>, class P = equal_to<V> >
 // using unordered_set =
 //     ::std::unordered_set<V, H, P, polymorphic_allocator<V>>
 //
 // template <class V,  class H = hash<V>, class P = equal_to<V> >

libcxx/test/std/experimental/memory/memory.resource.aliases/header_vector_synop.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 //
 //===----------------------------------------------------------------------===//
 
 // UNSUPPORTED: c++03
 
+// Aligned allocation is required by std::experimental::pmr, but it was not provided
+// before macosx10.13 and as a result we get linker errors when deploying to older than
+// macosx10.13.
+// XFAIL: use_system_cxx_lib && target={{.+}}-apple-macosx10.{{9|10|11|12}}
+
 // <experimental/vector>
 
 // namespace std { namespace experimental { namespace pmr {
 // template <class T>
 // using vector =
 //     ::std::vector<T, polymorphic_allocator<T>>
 //
 // }}} // namespace std::experimental::pmr

libcxx/test/std/experimental/memory/memory.resource.global/new_delete_resource.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 //
 //===----------------------------------------------------------------------===//
 
 // UNSUPPORTED: c++03
 
+// Aligned allocation is required by std::experimental::pmr, but it was not provided
+// before macosx10.13 and as a result we get linker errors when deploying to older than
+// macosx10.13.
+// XFAIL: use_system_cxx_lib && target={{.+}}-apple-macosx10.{{9|10|11|12}}
+
 // <experimental/memory_resource>
 
 // memory_resource * new_delete_resource()
 
 #include <experimental/memory_resource>
 #include <type_traits>
 #include <cassert>
 

libcxx/test/std/experimental/memory/memory.resource.global/null_memory_resource.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // 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
 //
 //===----------------------------------------------------------------------===//
 
 // UNSUPPORTED: c++03
 
+// Aligned allocation is required by std::experimental::pmr, but it was not provided
+// before macosx10.13 and as a result we get linker errors when deploying to older than
+// macosx10.13.
+// XFAIL: use_system_cxx_lib && target={{.+}}-apple-macosx10.{{9|10|11|12}}
+
 // <experimental/memory_resource>
 
 // memory_resource * null_memory_resource()
 
 #include <experimental/memory_resource>
 #include <new>
 #include <type_traits>
 #include <cassert>

libcxx/utils/ci/run-buildbot

 function generate-cmake-libcxx-win() {
     # TODO: Clang-cl in MSVC configurations don't have access to compiler_rt
     # builtins helpers for int128 division. See
     # https://reviews.llvm.org/D91139#2429595 for a comment about longterm
     # intent for handling the issue. In the meantime, define
     # -D_LIBCPP_HAS_NO_INT128 (both when building the library itself and
     # when building tests) to allow enabling filesystem for running tests,
     # even if it uses a non-permanent ABI.
-
     generate-cmake-base \
           -DLLVM_ENABLE_RUNTIMES="libcxx" \
           -DCMAKE_C_COMPILER=clang-cl \
           -DCMAKE_CXX_COMPILER=clang-cl \
           -DLIBCXX_ENABLE_FILESYSTEM=YES \
           -DLIBCXX_EXTRA_SITE_DEFINES="_LIBCPP_HAS_NO_INT128" \
           "${@}"
 }

 clang-cl-dll)
     clean
     # TODO: Currently, building with the experimental library breaks running
     # tests (the test linking look for the c++experimental library with the
     # wrong name, and the statically linked c++experimental can't be linked
     # correctly when libc++ visibility attributes indicate dllimport linkage
     # anyway), thus just disable the experimental library. Remove this
     # setting when cmake and the test driver does the right thing automatically.
-    generate-cmake-libcxx-win -DLIBCXX_ENABLE_EXPERIMENTAL_LIBRARY=OFF
+    generate-cmake-libcxx-win -DLIBCXX_TEST_PARAMS="enable_experimental=False"
     echo "+++ Running the libc++ tests"
     ${NINJA} -vC "${BUILD_DIR}" check-cxx
 ;;
 clang-cl-static)
     clean
     generate-cmake-libcxx-win -DLIBCXX_ENABLE_SHARED=OFF
     echo "+++ Running the libc++ tests"
     ${NINJA} -vC "${BUILD_DIR}" check-cxx

libcxx/utils/libcxx/test/params.py

if hasCompileFlag(cfg, '-std='+std):
     return '-std='+std
   if std in fallbacks and hasCompileFlag(cfg, '-std='+fallbacks[std]):
     return '-std='+fallbacks[std]
   return None
 
 DEFAULT_PARAMETERS = [
   Parameter(name='target_triple', type=str,
             help="The target triple to compile the test suite for. This must be "
                  "compatible with the target that the tests will be run on.",
             actions=lambda triple: filter(None, [
               AddFeature('target={}'.format(triple)),
               AddFlagIfSupported('--target={}'.format(triple)),
               AddSubstitution('%{triple}', triple)
             ])),
 
   Parameter(name='std', choices=_allStandards, type=str,
             help="The version of the standard to compile the test suite with.",
Parameter(name='use_sanitizer', choices=['', 'Address', 'Undefined', 'Memory', 'MemoryWithOrigins', 'Thread', 'DataFlow', 'Leaks'], type=str, default='',
 
               AddFlag('-fsanitize=dataflow') if sanitizer == 'DataFlow' else None,
               AddFlag('-fsanitize=leaks') if sanitizer == 'Leaks' else None,
 
               AddFeature('sanitizer-new-delete') if sanitizer in ['Address', 'Memory', 'MemoryWithOrigins', 'Thread'] else None,
             ])),
 
   Parameter(name='enable_experimental', choices=[True, False], type=bool, default=True,
-            help="Whether to enable tests for experimental C++ libraries (typically Library Fundamentals TSes).",
+            help="Whether to enable tests for experimental C++ Library features.",
             actions=lambda experimental: [] if not experimental else [
-              AddFeature('c++experimental'),
               # When linking in MSVC mode via the Clang driver, a -l<foo>
               # maps to <foo>.lib, so we need to use -llibc++experimental here
               # to make it link against the static libc++experimental.lib.
               # We can't check for the feature 'msvc' in available_features
               # as those features are added after processing parameters.
-              PrependLinkFlag(lambda config: '-llibc++experimental' if _isMSVC(config) else '-lc++experimental')
+              #
+              # TODO: Switch to using the appropriate experimental compiler flag once
+              #       all compilers we support implement that flag.
+              AddFeature('c++experimental'),
+              PrependLinkFlag(lambda cfg: '-llibc++experimental' if _isMSVC(cfg) else '-lc++experimental'),
+              AddCompileFlag('-D_LIBCPP_ENABLE_EXPERIMENTAL'),
             ]),
 
   Parameter(name='long_tests', choices=[True, False], type=bool, default=True,
             help="Whether to enable tests that take longer to run. This can be useful when running on a very slow device.",
             actions=lambda enabled: [] if not enabled else [
               AddFeature('long_tests')
             ]),