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

This caused build failures when building Clang and libc++ together on Mac:

  fatal error: 'experimental/memory_resource' file not found

See the code review for details. Reverting until the problem and how to
solve it is better understood.

(Updates to some test files were not reverted, since they seemed
unrelated and were later updated by 340b48b.)

> 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

This reverts commit bb93993.

Author: zmodem

clang/cmake/caches/Fuchsia-stage2.cmake

@@ -80,6 +80,7 @@ if(WIN32)
   set(RUNTIMES_${target}_CMAKE_SYSTEM_NAME Windows CACHE STRING "")
   set(RUNTIMES_${target}_CMAKE_BUILD_TYPE RelWithDebInfo CACHE STRING "")
   set(RUNTIMES_${target}_LIBCXX_ABI_VERSION 2 CACHE STRING "")
+  set(RUNTIMES_${target}_LIBCXX_ENABLE_EXPERIMENTAL_LIBRARY OFF CACHE BOOL "")
   set(RUNTIMES_${target}_LIBCXX_ENABLE_FILESYSTEM OFF CACHE BOOL "")
   set(RUNTIMES_${target}_LIBCXX_ENABLE_ABI_LINKER_SCRIPT OFF CACHE BOOL "")
   set(RUNTIMES_${target}_LIBCXX_ENABLE_SHARED OFF CACHE BOOL "")

clang/cmake/caches/Fuchsia.cmake

@@ -51,6 +51,7 @@ endif()
 
 if(WIN32)
   set(LIBCXX_ABI_VERSION 2 CACHE STRING "")
+  set(LIBCXX_ENABLE_EXPERIMENTAL_LIBRARY OFF CACHE BOOL "")
   set(LIBCXX_ENABLE_FILESYSTEM OFF CACHE BOOL "")
   set(LIBCXX_ENABLE_ABI_LINKER_SCRIPT OFF CACHE BOOL "")
   set(LIBCXX_ENABLE_SHARED OFF CACHE BOOL "")

compiler-rt/cmake/Modules/AddCompilerRT.cmake

@@ -685,6 +685,7 @@ macro(add_custom_libcxx name prefix)
                -DLIBCXXABI_HERMETIC_STATIC_LIBRARY=ON
                -DLIBCXXABI_INCLUDE_TESTS=OFF
                -DLIBCXX_CXX_ABI=libcxxabi
+               -DLIBCXX_ENABLE_EXPERIMENTAL_LIBRARY=OFF
                -DLIBCXX_ENABLE_SHARED=OFF
                -DLIBCXX_HERMETIC_STATIC_LIBRARY=ON
                -DLIBCXX_INCLUDE_BENCHMARKS=OFF

libcxx/CMakeLists.txt

@@ -55,6 +55,7 @@ option(LIBCXX_ENABLE_ASSERTIONS
    by users in their own code regardless of this option." OFF)
 option(LIBCXX_ENABLE_SHARED "Build libc++ as a shared library." ON)
 option(LIBCXX_ENABLE_STATIC "Build libc++ as a static library." ON)
+option(LIBCXX_ENABLE_EXPERIMENTAL_LIBRARY "Build libc++experimental.a" ON)
 set(ENABLE_FILESYSTEM_DEFAULT ON)
 if (WIN32 AND NOT MINGW)
   # Filesystem is buildable for windows, but it requires __int128 helper
@@ -166,6 +167,9 @@ cmake_dependent_option(LIBCXX_INSTALL_STATIC_LIBRARY
 cmake_dependent_option(LIBCXX_INSTALL_SHARED_LIBRARY
   "Install the shared libc++ library." ON
   "LIBCXX_ENABLE_SHARED;LIBCXX_INSTALL_LIBRARY" OFF)
+cmake_dependent_option(LIBCXX_INSTALL_EXPERIMENTAL_LIBRARY
+        "Install libc++experimental.a" ON
+        "LIBCXX_ENABLE_EXPERIMENTAL_LIBRARY;LIBCXX_INSTALL_LIBRARY" OFF)
 
 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)
 if (LIBCXX_ABI_UNSTABLE)
@@ -926,7 +930,11 @@ add_subdirectory(include)
 add_subdirectory(src)
 add_subdirectory(utils)
 
-set(LIBCXX_TEST_DEPS "cxx_experimental")
+set(LIBCXX_TEST_DEPS "")
+
+if (LIBCXX_ENABLE_EXPERIMENTAL_LIBRARY)
+  list(APPEND LIBCXX_TEST_DEPS cxx_experimental)
+endif()
 
 if (LIBCXX_BUILD_EXTERNAL_THREAD_LIBRARY)
   list(APPEND LIBCXX_TEST_DEPS cxx_external_threads)

libcxx/appveyor.yml

@@ -45,7 +45,7 @@ build_script:
   #############################################################################
   - cmake -G "%GENERATOR%" %CMAKE_OPTIONS%
     "-DCMAKE_BUILD_TYPE=%configuration%"
-    "-DLLVM_PATH=C:\projects\deps\llvm"
+    "-DLLVM_PATH=C:\projects\deps\llvm" -DLIBCXX_ENABLE_EXPERIMENTAL_LIBRARY=OFF
     -DLLVM_LIT_ARGS="-sv --show-xfail --show-unsupported"
     %APPVEYOR_BUILD_FOLDER%
 

libcxx/cmake/caches/AIX.cmake

@@ -9,6 +9,7 @@ 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_ABI_LINKER_SCRIPT OFF CACHE BOOL "")
+set(LIBCXX_ENABLE_EXPERIMENTAL_LIBRARY OFF CACHE BOOL "")
 set(LIBCXX_ENABLE_SHARED ON CACHE BOOL "")
 set(LIBCXX_ENABLE_STATIC OFF CACHE BOOL "")
 set(LIBCXXABI_ENABLE_SHARED ON CACHE BOOL "")

libcxx/cmake/caches/Apple.cmake

@@ -4,6 +4,7 @@ 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 "")

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

@@ -1,3 +1,2 @@
-set(LIBCXX_TEST_PARAMS "enable_experimental=False" CACHE STRING "")
-set(LIBCXXABI_TEST_PARAMS "${LIBCXX_TEST_PARAMS}" CACHE STRING "")
+set(LIBCXX_ENABLE_EXPERIMENTAL_LIBRARY OFF CACHE BOOL "")
 set(LIBCXX_ENABLE_INCOMPLETE_FEATURES OFF CACHE BOOL "")

libcxx/docs/BuildingLibcxx.rst

@@ -99,7 +99,8 @@ it is the simplest way to build.
           -T "ClangCL"                                    ^
           -DLLVM_ENABLE_RUNTIMES=libcxx                   ^
           -DLIBCXX_ENABLE_SHARED=YES                      ^
-          -DLIBCXX_ENABLE_STATIC=NO
+          -DLIBCXX_ENABLE_STATIC=NO                       ^
+          -DLIBCXX_ENABLE_EXPERIMENTAL_LIBRARY=NO
   > cmake --build build
 
 CMake + ninja (MSVC)
@@ -130,7 +131,8 @@ In either case, then run:
   > cmake -G Ninja -S runtimes -B build                                               ^
           -DCMAKE_C_COMPILER=clang-cl                                                 ^
           -DCMAKE_CXX_COMPILER=clang-cl                                               ^
-          -DLLVM_ENABLE_RUNTIMES=libcxx
+          -DLLVM_ENABLE_RUNTIMES=libcxx                                               ^
+          -DLIBCXX_ENABLE_EXPERIMENTAL_LIBRARY=NO
   > ninja -C build cxx
   > ninja -C build check-cxx
 
@@ -296,6 +298,23 @@ libc++ specific options
   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:
 

libcxx/docs/UsingLibcxx.rst

@@ -34,20 +34,22 @@ matches that Standard in the library.
   library until the standard has been ratified.
 
 
-Enabling experimental C++ Library features
-==========================================
+Using libc++experimental and ``<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.
+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
 
 .. warning::
   Experimental libraries are Experimental.
-    * The contents of the ``<experimental/...>`` headers and the associated static
+    * The contents of the ``<experimental/...>`` headers and ``libc++experimental.a``
       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,