[libc++] Proper fix for libc++'s modulemap after D68480

[ldionne]

Summary

In libc++, we normally #ifdef out header content instead of #erroring
out when the Standard in use is insufficient for the requirements of
the header.

[ldionne]

@EricWF I'm unsure about this change -- I think we might want to keep the requires in the modulemap but also get rid of the #error behavior.

[mclow.lists]

This looks fine to me.

[EricWF]

In D75074#1889993, @ldionne wrote:

@EricWF I'm unsure about this change -- I think we might want to keep the requires in the modulemap but also get rid of the #error behavior.

I would agree with that.

LGTM.

[ldionne]

Add a test and don't touch the modulemap

[teemperor]

LGTM, thanks for the test!

libcxx/include/barrier

 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
 #endif
 
 #ifdef _LIBCPP_HAS_NO_THREADS
 # error <barrier> is not supported on this single threaded system
 #endif
 
-#if _LIBCPP_STD_VER < 14
-# error <barrier> requires C++14 or later
-#endif
+#if _LIBCPP_STD_VER >= 14
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 struct __empty_completion
 {
     inline _LIBCPP_INLINE_VISIBILITY
     void operator()() noexcept
     {
_LIBCPP_INLINE_VISIBILITY
     void arrive_and_drop()
     {
         __b.arrive_and_drop();
     }
 };
 
 _LIBCPP_END_NAMESPACE_STD
 
+#endif // _LIBCPP_STD_VER >= 14
+
 #endif //_LIBCPP_BARRIER

libcxx/include/latch

 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
 #endif
 
 #ifdef _LIBCPP_HAS_NO_THREADS
 # error <latch> is not supported on this single threaded system
 #endif
 
-#if _LIBCPP_STD_VER < 14
-# error <latch> requires C++14 or later
-#endif
+#if _LIBCPP_STD_VER >= 14
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 class latch
 {
     __atomic_base<ptrdiff_t> __a;
 
 public:
public:
     {
         count_down(__update);
         wait();
     }
 };
 
 _LIBCPP_END_NAMESPACE_STD
 
+#endif // _LIBCPP_STD_VER >= 14
+
 #endif //_LIBCPP_LATCH

libcxx/include/semaphore

 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
 #endif
 
 #ifdef _LIBCPP_HAS_NO_THREADS
 # error <semaphore> is not supported on this single threaded system
 #endif
 
-#if _LIBCPP_STD_VER < 14
-# error <semaphore> is requires C++14 or later
-#endif
+#if _LIBCPP_STD_VER >= 14
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 /*
 
 __atomic_semaphore_base is the general-case implementation, to be used for
 user-requested least-max values that exceed the OS implementation support
 (incl. when the OS has no support of its own) and for binary semaphores.
bool try_acquire_until(chrono::time_point<Clock, Duration> const& __abs_time)
             return try_acquire_for(__abs_time - current);
     }
 };
 
 using binary_semaphore = counting_semaphore<1>;
 
 _LIBCPP_END_NAMESPACE_STD
 
+#endif // _LIBCPP_STD_VER >= 14
+
 #endif //_LIBCPP_SEMAPHORE

libcxx/test/libcxx/modules/stds_include.sh.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
+//
+//===----------------------------------------------------------------------===//
+
+// Test that we can include libc++ headers when building with modules
+// enabled in various Standard modes. This is a common source of breakage
+// since the 'std' module will include all headers, so if something in a
+// header fails under a standard mode, importing anything will fail.
+
+// This test fails on Windows because the underlying libc headers on Windows
+// are not modular
+// XFAIL: LIBCXX-WINDOWS-FIXME
+
+// FIXME: The <atomic> header is not supported for single-threaded systems,
+// but still gets built as part of the 'std' module, which breaks the build.
+// XFAIL: libcpp-has-no-threads
+
+// REQUIRES: modules-support
+
+// NOTE: The -std=XXX flag is present in %flags, so we overwrite it by passing it after %flags.
+// RUN: %cxx %flags %compile_flags -fmodules -fcxx-modules -fsyntax-only -std=c++98 %s
+// RUN: %cxx %flags %compile_flags -fmodules -fcxx-modules -fsyntax-only -std=c++03 %s
+// RUN: %cxx %flags %compile_flags -fmodules -fcxx-modules -fsyntax-only -std=c++11 %s
+// RUN: %cxx %flags %compile_flags -fmodules -fcxx-modules -fsyntax-only -std=c++14 %s
+// RUN: %cxx %flags %compile_flags -fmodules -fcxx-modules -fsyntax-only -std=c++17 %s
+// RUN: %cxx %flags %compile_flags -fmodules -fcxx-modules -fsyntax-only -std=c++2a %s
+
+#include <vector>
+
+int main(int, char**) {
+  return 0;
+}