diff --git a/clang/lib/AST/ExprConstant.cpp b/clang/lib/AST/ExprConstant.cpp
--- a/clang/lib/AST/ExprConstant.cpp
+++ b/clang/lib/AST/ExprConstant.cpp
@@ -11736,9 +11736,9 @@
     bool Ret = HandleSizeof(Info, ExprLoc, Ty, Result);
     if (Ty->isStructureType() &&
         Ty->getAsStructureType()->getDecl()->hasFlexibleArrayMember()) {
-      const auto *VD =
-          cast<VarDecl>(LVal.getLValueBase().get<const ValueDecl *>());
-      Result += VD->getFlexibleArrayInitChars(Info.Ctx);
+      if (const auto *V = LVal.getLValueBase().dyn_cast<const ValueDecl *>())
+        if (const auto *VD = dyn_cast<VarDecl>(V))
+          Result += VD->getFlexibleArrayInitChars(Info.Ctx);
     }
     return Ret;
   };
diff --git a/clang/test/CodeGen/object-size.c b/clang/test/CodeGen/object-size.c
--- a/clang/test/CodeGen/object-size.c
+++ b/clang/test/CodeGen/object-size.c
@@ -551,6 +551,10 @@
   // CHECK: ret i64 16
   return OBJECT_SIZE_BUILTIN(&D34, 1);
 }
+unsigned long test35(void) {
+  // CHECK: ret i64 16
+  return OBJECT_SIZE_BUILTIN(&(struct DynStructVar){}, 1);
+}
 
 // CHECK-LABEL: @PR30346
 void PR30346(void) {