Fix use of static_assert macro with nested commas

llvm-svn: 245410
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 9a5e62bf75452d3a806c2bbc7f3de0cafa9407da
diff --git a/include/thread b/include/thread
index 6be1133..3dde390 100644
--- a/include/thread
+++ b/include/thread
@@ -141,7 +141,7 @@
 
      // Only __thread_local_data() may construct a __thread_specific_ptr
      // and only with _Tp == __thread_struct.
-    static_assert(is_same<_Tp, __thread_struct>::value, "");
+    static_assert((is_same<_Tp, __thread_struct>::value), "");
     __thread_specific_ptr();
     friend _LIBCPP_FUNC_VIS __thread_specific_ptr<__thread_struct>& __thread_local_data();