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[libc++] Always build c++experimental.a
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. Differential Revision: https://reviews.llvm.org/D128927
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‎clang/cmake/caches/Fuchsia-stage2.cmake

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set(RUNTIMES_${target}_CMAKE_SYSTEM_NAME Windows CACHE STRING "")
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set(RUNTIMES_${target}_CMAKE_BUILD_TYPE RelWithDebInfo CACHE STRING "")
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set(RUNTIMES_${target}_LIBCXX_ABI_VERSION 2 CACHE STRING "")
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set(RUNTIMES_${target}_LIBCXX_ENABLE_EXPERIMENTAL_LIBRARY OFF CACHE BOOL "")
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set(RUNTIMES_${target}_LIBCXX_ENABLE_FILESYSTEM OFF CACHE BOOL "")
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set(RUNTIMES_${target}_LIBCXX_ENABLE_ABI_LINKER_SCRIPT OFF CACHE BOOL "")
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set(RUNTIMES_${target}_LIBCXX_ENABLE_SHARED OFF CACHE BOOL "")

‎clang/cmake/caches/Fuchsia.cmake

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if(WIN32)
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set(LIBCXX_ABI_VERSION 2 CACHE STRING "")
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set(LIBCXX_ENABLE_EXPERIMENTAL_LIBRARY OFF CACHE BOOL "")
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set(LIBCXX_ENABLE_FILESYSTEM OFF CACHE BOOL "")
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set(LIBCXX_ENABLE_ABI_LINKER_SCRIPT OFF CACHE BOOL "")
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set(LIBCXX_ENABLE_SHARED OFF CACHE BOOL "")

‎compiler-rt/cmake/Modules/AddCompilerRT.cmake

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-DLIBCXXABI_HERMETIC_STATIC_LIBRARY=ON
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-DLIBCXXABI_INCLUDE_TESTS=OFF
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-DLIBCXX_CXX_ABI=libcxxabi
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-DLIBCXX_ENABLE_EXPERIMENTAL_LIBRARY=OFF
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-DLIBCXX_ENABLE_SHARED=OFF
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-DLIBCXX_HERMETIC_STATIC_LIBRARY=ON
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-DLIBCXX_INCLUDE_BENCHMARKS=OFF

‎libcxx/CMakeLists.txt

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by users in their own code regardless of this option." OFF)
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option(LIBCXX_ENABLE_SHARED "Build libc++ as a shared library." ON)
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option(LIBCXX_ENABLE_STATIC "Build libc++ as a static library." ON)
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option(LIBCXX_ENABLE_EXPERIMENTAL_LIBRARY "Build libc++experimental.a" ON)
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set(ENABLE_FILESYSTEM_DEFAULT ON)
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if (WIN32 AND NOT MINGW)
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# Filesystem is buildable for windows, but it requires __int128 helper
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cmake_dependent_option(LIBCXX_INSTALL_SHARED_LIBRARY
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"Install the shared libc++ library." ON
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"LIBCXX_ENABLE_SHARED;LIBCXX_INSTALL_LIBRARY" OFF)
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cmake_dependent_option(LIBCXX_INSTALL_EXPERIMENTAL_LIBRARY
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"Install libc++experimental.a" ON
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"LIBCXX_ENABLE_EXPERIMENTAL_LIBRARY;LIBCXX_INSTALL_LIBRARY" OFF)
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option(LIBCXX_ABI_UNSTABLE "Use the unstable ABI of libc++. This is equivalent to specifying LIBCXX_ABI_VERSION=n, where n is the not-yet-stable version." OFF)
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if (LIBCXX_ABI_UNSTABLE)
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add_subdirectory(src)
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add_subdirectory(utils)
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set(LIBCXX_TEST_DEPS "")
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if (LIBCXX_ENABLE_EXPERIMENTAL_LIBRARY)
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list(APPEND LIBCXX_TEST_DEPS cxx_experimental)
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endif()
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set(LIBCXX_TEST_DEPS "cxx_experimental")
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if (LIBCXX_BUILD_EXTERNAL_THREAD_LIBRARY)
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list(APPEND LIBCXX_TEST_DEPS cxx_external_threads)

‎libcxx/appveyor.yml

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#############################################################################
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- cmake -G "%GENERATOR%" %CMAKE_OPTIONS%
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"-DCMAKE_BUILD_TYPE=%configuration%"
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"-DLLVM_PATH=C:\projects\deps\llvm" -DLIBCXX_ENABLE_EXPERIMENTAL_LIBRARY=OFF
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"-DLLVM_PATH=C:\projects\deps\llvm"
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-DLLVM_LIT_ARGS="-sv --show-xfail --show-unsupported"
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%APPVEYOR_BUILD_FOLDER%
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‎libcxx/cmake/caches/AIX.cmake

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set(LIBCXX_ENABLE_ASSERTIONS OFF CACHE BOOL "")
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set(LIBCXX_ABI_VERSION "1" CACHE STRING "")
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set(LIBCXX_ENABLE_ABI_LINKER_SCRIPT OFF CACHE BOOL "")
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set(LIBCXX_ENABLE_EXPERIMENTAL_LIBRARY OFF CACHE BOOL "")
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set(LIBCXX_ENABLE_SHARED ON CACHE BOOL "")
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set(LIBCXX_ENABLE_STATIC OFF CACHE BOOL "")
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set(LIBCXXABI_ENABLE_SHARED ON CACHE BOOL "")

‎libcxx/cmake/caches/Apple.cmake

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set(LIBCXX_USE_COMPILER_RT ON CACHE BOOL "")
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set(LIBCXX_ENABLE_ASSERTIONS OFF CACHE BOOL "")
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set(LIBCXX_ABI_VERSION "1" CACHE STRING "")
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set(LIBCXX_ENABLE_EXPERIMENTAL_LIBRARY OFF CACHE BOOL "")
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set(LIBCXX_ENABLE_STATIC ON CACHE BOOL "")
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set(LIBCXX_ENABLE_SHARED ON CACHE BOOL "")
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set(LIBCXX_CXX_ABI libcxxabi CACHE STRING "")

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

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

‎libcxx/docs/BuildingLibcxx.rst

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-T "ClangCL" ^
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-DLLVM_ENABLE_RUNTIMES=libcxx ^
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-DLIBCXX_ENABLE_SHARED=YES ^
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-DLIBCXX_ENABLE_STATIC=NO ^
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-DLIBCXX_ENABLE_EXPERIMENTAL_LIBRARY=NO
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-DLIBCXX_ENABLE_STATIC=NO
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> cmake --build build
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CMake + ninja (MSVC)
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> cmake -G Ninja -S runtimes -B build ^
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-DCMAKE_C_COMPILER=clang-cl ^
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-DCMAKE_CXX_COMPILER=clang-cl ^
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-DLLVM_ENABLE_RUNTIMES=libcxx ^
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-DLIBCXX_ENABLE_EXPERIMENTAL_LIBRARY=NO
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-DLLVM_ENABLE_RUNTIMES=libcxx
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> ninja -C build cxx
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> ninja -C build check-cxx
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Path where target-specific libc++ headers should be installed. If a relative
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path, relative to ``CMAKE_INSTALL_PREFIX``.
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.. _libc++experimental options:
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libc++experimental Specific Options
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------------------------------------
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.. option:: LIBCXX_ENABLE_EXPERIMENTAL_LIBRARY:BOOL
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**Default**: ``ON``
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Build and test libc++experimental.a.
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.. option:: LIBCXX_INSTALL_EXPERIMENTAL_LIBRARY:BOOL
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**Default**: ``LIBCXX_ENABLE_EXPERIMENTAL_LIBRARY AND LIBCXX_INSTALL_LIBRARY``
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Install libc++experimental.a alongside libc++.
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.. _ABI Library Specific Options:
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‎libcxx/docs/UsingLibcxx.rst

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

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