[libc++][Android] Always redirect <stdatomic.h> to <atomic>

libcxx/include/atomic

@@ -598,7 +598,10 @@ template <class T>
 #    define _LIBCPP_STDATOMIC_H_HAS_DEFINITELY_BEEN_INCLUDED 0
 #  endif
 
-#  if _LIBCPP_STD_VER < 23 && _LIBCPP_STDATOMIC_H_HAS_DEFINITELY_BEEN_INCLUDED
+// The Android LLVM toolchain has historically allowed combining the <atomic>
+// and <stdatomic.h> headers in dialects before C++23, so for backwards
+// compatibility, preserve that ability when targeting Android.
+#  if _LIBCPP_STD_VER < 23 && !defined(__ANDROID__) && _LIBCPP_STDATOMIC_H_HAS_DEFINITELY_BEEN_INCLUDED
 #    error <atomic> is incompatible with <stdatomic.h> before C++23. Please compile with -std=c++23.
 #  endif
 

libcxx/include/stdatomic.h

@@ -126,7 +126,10 @@ using std::atomic_signal_fence                         // see below
 #    pragma GCC system_header
 #  endif
 
-#  if defined(__cplusplus) && _LIBCPP_STD_VER >= 23
+// The Android LLVM toolchain has historically allowed combining the <atomic>
+// and <stdatomic.h> headers in dialects before C++23, so for backwards
+// compatibility, preserve that ability when targeting Android.
+#  if defined(__cplusplus) && (_LIBCPP_STD_VER >= 23 || defined(__ANDROID__))
 
 #    include <atomic>
 #    include <version>

libcxx/test/libcxx/atomics/atomics.syn/compatible_with_stdatomic.compile.pass.cpp

@@ -7,7 +7,9 @@
 //===----------------------------------------------------------------------===//
 
 // UNSUPPORTED: no-threads
-// UNSUPPORTED: c++03, c++11, c++14, c++17, c++20
+
+// On Android, libc++'s <stdatomic.h> header always redirects to <atomic>, even before C++23.
+// UNSUPPORTED: (c++03 || c++11 || c++14 || c++17 || c++20) && !android
 
 // This test verifies that <stdatomic.h> redirects to <atomic>.
 

libcxx/test/libcxx/atomics/atomics.syn/incompatible_with_stdatomic.verify.cpp

@@ -9,6 +9,9 @@
 // UNSUPPORTED: no-threads
 // REQUIRES: c++03 || c++11 || c++14 || c++17 || c++20
 
+// On Android, libc++'s <stdatomic.h> header always redirects to <atomic>, even before C++23.
+// XFAIL: android
+
 // No diagnostic gets emitted when we build with modules.
 // XFAIL: clang-modules-build
 

libcxx/test/libcxx/atomics/stdatomic.h.syn/dont_hijack_header.cxx23.compile.pass.cpp

@@ -21,6 +21,9 @@
 // doesn't work at all if we don't use the <stdatomic.h> provided by libc++ in C++23 and above.
 // XFAIL: (c++11 || c++14 || c++17 || c++20) && gcc
 
+// On Android, libc++'s <stdatomic.h> header always redirects to <atomic>, even before C++23.
+// XFAIL: android
+
 #include <atomic>
 #include <stdatomic.h>
 #include <type_traits>