Index: lib/Sema/SemaDeclCXX.cpp
===================================================================
--- lib/Sema/SemaDeclCXX.cpp
+++ lib/Sema/SemaDeclCXX.cpp
@@ -13319,8 +13319,9 @@
                                          Expr *AssertExpr,
                                          Expr *AssertMessageExpr,
                                          SourceLocation RParenLoc) {
-  StringLiteral *AssertMessage =
-      AssertMessageExpr ? cast<StringLiteral>(AssertMessageExpr) : nullptr;
+  auto *AssertMessage = cast_or_null<StringLiteral>(AssertMessageExpr);
+  if (AssertMessage && !AssertMessage->getLength())
+    AssertMessage = nullptr;
 
   if (DiagnoseUnexpandedParameterPack(AssertExpr, UPPC_StaticAssertExpression))
     return nullptr;