Index: cfe/trunk/lib/StaticAnalyzer/Checkers/CastToStructChecker.cpp
===================================================================
--- cfe/trunk/lib/StaticAnalyzer/Checkers/CastToStructChecker.cpp
+++ cfe/trunk/lib/StaticAnalyzer/Checkers/CastToStructChecker.cpp
@@ -84,6 +84,13 @@
     if (!VD || VD->getType()->isReferenceType())
       return true;
 
+    // Don't warn when target type has no definition.
+    if (const RecordType *RD = dyn_cast<RecordType>(ToPointeeTy.getTypePtr())) {
+      if (!RD->getDecl()->getDefinition()) {
+        return true;
+      }
+    }
+
     // Warn when there is widening cast.
     unsigned ToWidth = Ctx.getTypeInfo(ToPointeeTy).Width;
     unsigned OrigWidth = Ctx.getTypeInfo(OrigPointeeTy).Width;
Index: cfe/trunk/test/Analysis/cast-to-struct.cpp
===================================================================
--- cfe/trunk/test/Analysis/cast-to-struct.cpp
+++ cfe/trunk/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;
+}