commit 50070d8602a07160cece5890899929e9f210244d
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Apr 22 11:10:06 2021

    libstdc++: Remove #error from <semaphore> implementation [PR 100179]
    
    This removes the #error from <bits/semaphore_base.h> for the case where
    neither __atomic_semaphore nor __platform_semaphore is defined.
    
    Also rename the _GLIBCXX_REQUIRE_POSIX_SEMAPHORE macro to
    _GLIBCXX_USE_POSIX_SEMAPHORE for consistency with the similar
    _GLIBCXX_USE_CXX11_ABI macro that can be used to request an alternative
    (ABI-changing) implementation.
    
    libstdc++-v3/ChangeLog:
    
            PR libstdc++/100179
            * include/bits/semaphore_base.h: Remove #error.
            * include/std/semaphore: Do not define anything unless one of
            the implementations is available.

diff --git a/libstdc++-v3/include/bits/semaphore_base.h b/libstdc++-v3/include/bits/semaphore_base.h
index 84b33423fff..4948f0fd0bc 100644
--- a/libstdc++-v3/include/bits/semaphore_base.h
+++ b/libstdc++-v3/include/bits/semaphore_base.h
@@ -263,14 +263,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   };
 #endif // __cpp_lib_atomic_wait
 
-// Note: the _GLIBCXX_REQUIRE_POSIX_SEMAPHORE macro can be used to force the
+// Note: the _GLIBCXX_USE_POSIX_SEMAPHORE macro can be used to force the
 // use of Posix semaphores (sem_t). Doing so however, alters the ABI.
-#if defined __cpp_lib_atomic_wait && !_GLIBCXX_REQUIRE_POSIX_SEMAPHORE
+#if defined __cpp_lib_atomic_wait && !_GLIBCXX_USE_POSIX_SEMAPHORE
   using __semaphore_impl = __atomic_semaphore;
 #elif _GLIBCXX_HAVE_POSIX_SEMAPHORE
   using __semaphore_impl = __platform_semaphore;
-#else
-#  error "No suitable semaphore implementation available"
 #endif
 
 _GLIBCXX_END_NAMESPACE_VERSION
diff --git a/libstdc++-v3/include/std/semaphore b/libstdc++-v3/include/std/semaphore
index a1560915d83..52addca742c 100644
--- a/libstdc++-v3/include/std/semaphore
+++ b/libstdc++-v3/include/std/semaphore
@@ -34,6 +34,7 @@
 #if __cplusplus > 201703L
 #include <bits/semaphore_base.h>
 
+#if __cpp_lib_atomic_wait || _GLIBCXX_HAVE_POSIX_SEMAPHORE
 namespace std _GLIBCXX_VISIBILITY(default)
 {
 _GLIBCXX_BEGIN_NAMESPACE_VERSION
@@ -89,5 +90,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
 _GLIBCXX_END_NAMESPACE_VERSION
 } // namespace
+#endif // cpp_lib_atomic_wait || _GLIBCXX_HAVE_POSIX_SEMAPHORE
 #endif // C++20
 #endif // _GLIBCXX_SEMAPHORE