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: 331379

libcxx/include/atomic

@@ -555,6 +555,9 @@ void atomic_signal_fence(memory_order m) noexcept;
 #if !defined(_LIBCPP_HAS_C_ATOMIC_IMP) && !defined(_LIBCPP_HAS_GCC_ATOMIC_IMP)
 #error <atomic> is not implemented
 #endif
+#ifdef __ALLOW_STDC_ATOMICS_IN_CXX__
+#error <stdatomic.h> is incompatible with the C++ standard library
+#endif
 
 #if _LIBCPP_STD_VER > 14
 # define __cpp_lib_atomic_is_always_lock_free 201603L

libcxx/test/libcxx/atomics/c_compatibility.fail.cpp

@@ -0,0 +1,28 @@
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// UNSUPPORTED: libcpp-has-no-threads
+//
+// <atomic>
+
+// Test that including <atomic> fails to compile when we want to use C atomics
+// in C++ and have corresponding macro defined.
+
+// MODULES_DEFINES: __ALLOW_STDC_ATOMICS_IN_CXX__
+#ifndef __ALLOW_STDC_ATOMICS_IN_CXX__
+#define __ALLOW_STDC_ATOMICS_IN_CXX__
+#endif
+
+#include <atomic>
+// expected-error@atomic:* {{<stdatomic.h> is incompatible with the C++ standard library}}
+
+int main()
+{
+}
+