diff --git a/lib/Support/Windows/ThreadLocal.inc b/lib/Support/Windows/ThreadLocal.inc
index 3914cf72fa9..14ce61933cb 100644
--- a/lib/Support/Windows/ThreadLocal.inc
+++ b/lib/Support/Windows/ThreadLocal.inc
@@ -23,7 +23,7 @@ namespace llvm {
 using namespace sys;
 
 ThreadLocalImpl::ThreadLocalImpl() : data() {
-  typedef int SIZE_TOO_BIG[sizeof(DWORD) <= sizeof(data) ? 1 : -1];
+  static_assert(sizeof(DWORD) <= sizeof(data), "size too big");
   DWORD* tls = reinterpret_cast<DWORD*>(&data);
   *tls = TlsAlloc();
   assert(*tls != TLS_OUT_OF_INDEXES);