Test that the single-threaded lit feature is available iff the corresponding guard is #defined

[jroelofs]

Summary

Inspired by the test @EricWF added in r220722.

[EricWF]

Why not make it fail at compile time?

#ifdef _LIBCPP_HAS_NO_THREADS
#error <error message>
#endif

int main() {}

[jroelofs]

Making it an execute test, IMHO, gives stronger guarantees that it's failing the way we want it to. I don't really have any other reasons to prefer one version over the other.

[EricWF]

I wasn't suggesting making it a .pass.cpp test. Just making it fail during compilation instead. I would prefer using a #error directive so the output of the test failure is clearer, but it's up to you.

Otherwise, LGTM.

[jroelofs]

Nevermind... I'm being silly. Ignore my previous comment.

I don't really have a preference either way.

[jroelofs]

Implement @EricWF's suggestion.

[jroelofs]

r220729

test/atomics/libcpp-has-no-threads.pass.cpp

+//===----------------------------------------------------------------------===//
+//
+//                     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.
+//
+//===----------------------------------------------------------------------===//
+// XFAIL: libcpp-has-no-threads
+
+#ifdef _LIBCPP_HAS_NO_THREADS
+#error This should be XFAIL'd for the purpose of detecting that the LIT feature\
+ 'libcpp-has-no-threads' is available iff _LIBCPP_HAS_NO_THREADS is defined
+#endif
+
+int main()
+{
+}