diff --git a/llvm/include/llvm/Support/RWMutex.h b/llvm/include/llvm/Support/RWMutex.h
--- a/llvm/include/llvm/Support/RWMutex.h
+++ b/llvm/include/llvm/Support/RWMutex.h
@@ -94,14 +94,14 @@
 template <bool mt_only> class SmartRWMutex {
   // shared_mutex (C++17) is more efficient than shared_timed_mutex (C++14)
   // on Windows and always available on MSVC except with libc++.
+#if !defined(LLVM_USE_RW_MUTEX_IMPL)
 #if (defined(_MSC_VER) && !defined(_LIBCPP_VERSION)) || __cplusplus > 201402L
   std::shared_mutex impl;
 #else
-#if !defined(LLVM_USE_RW_MUTEX_IMPL)
   std::shared_timed_mutex impl;
+#endif
 #else
   RWMutexImpl impl;
-#endif
 #endif
   unsigned readers = 0;
   unsigned writers = 0;