Index: lib/Headers/stdatomic.h
===================================================================
--- lib/Headers/stdatomic.h
+++ lib/Headers/stdatomic.h
@@ -31,6 +31,10 @@
 # include_next <stdatomic.h>
 #else
 
+#if !defined(__ALLOW_STDC_ATOMICS_IN_CXX__) && defined(__cplusplus)
+#error "<stdatomic.h> is incompatible with the C++ standard library; define __ALLOW_STDC_ATOMICS_IN_CXX__ to proceed."
+#endif
+
 #include <stddef.h>
 #include <stdint.h>
 
Index: test/Headers/stdatomic.cpp
===================================================================
--- test/Headers/stdatomic.cpp
+++ test/Headers/stdatomic.cpp
@@ -0,0 +1,10 @@
+// RUN: %clang_cc1 %s -verify
+// RUN: %clang_cc1 -D__ALLOW_STDC_ATOMICS_IN_CXX__ %s -verify
+
+#include <stdatomic.h>
+
+#ifndef __ALLOW_STDC_ATOMICS_IN_CXX__
+// expected-error@stdatomic.h:* {{<stdatomic.h> is incompatible with the C++ standard library}}
+#else
+// expected-no-diagnostics
+#endif