Index: llvm/include/llvm/Support/Parallel.h
===================================================================
--- llvm/include/llvm/Support/Parallel.h
+++ llvm/include/llvm/Support/Parallel.h
@@ -21,6 +21,13 @@
 #if defined(_MSC_VER) && LLVM_ENABLE_THREADS
 #pragma warning(push)
 #pragma warning(disable : 4530)
+
+// With Microsoft's Standard Library, all C++ standard library headers include
+// yvals.h.  With libc++, that isn't the case but ppl.h depends on having
+// yvals.h be included first, and it only includes a few standard library
+// headers. To make things work, explicitly include it here.
+#include <yvals.h>
+
 #include <concrt.h>
 #include <ppl.h>
 #pragma warning(pop)