Index: lib/StaticAnalyzer/Checkers/CastToStructChecker.cpp
===================================================================
--- lib/StaticAnalyzer/Checkers/CastToStructChecker.cpp
+++ lib/StaticAnalyzer/Checkers/CastToStructChecker.cpp
@@ -55,6 +55,12 @@
   if (!ToPointeeTy->isStructureOrClassType())
     return true;
 
+  if (const RecordType *RD = dyn_cast<RecordType>(ToPointeeTy.getTypePtr())) {
+    if (!RD->getDecl()->getDefinition()) {
+      return true;
+    }
+  }
+
   // We allow cast from void*.
   if (OrigPointeeTy->isVoidType())
     return true;
Index: test/Analysis/cast-to-struct.cpp
===================================================================
--- test/Analysis/cast-to-struct.cpp
+++ test/Analysis/cast-to-struct.cpp
@@ -65,3 +65,8 @@
   void *VP = P;
   Abc = (struct ABC *)VP;
 }
+
+// https://llvm.org/bugs/show_bug.cgi?id=31173
+void dontCrash(struct AB X) {
+  struct UndefS *S = (struct UndefS *)&X;
+}