Emit an error when mixing <stdatomic.h> and <atomic>

Atomics in C and C++ are incompatible at the moment and mixing the
headers can result in confusing error messages.

Emit an error explicitly telling about the incompatibility. Introduce
the macro `__ALLOW_STDC_ATOMICS_IN_CXX__` that allows to choose in C++
between C atomics and C++ atomics.

rdar://problem/27435938

Reviewers: rsmith, EricWF, mclow.lists

Reviewed By: mclow.lists

Subscribers: jkorous-apple, christof, bumblebritches57, JonChesterfield, smeenai, cfe-commits

Differential Revision: https://reviews.llvm.org/D45470

llvm-svn: 331378

clang/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>
 

clang/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