Index: cfe/trunk/lib/StaticAnalyzer/Checkers/CastToStructChecker.cpp
===================================================================
--- cfe/trunk/lib/StaticAnalyzer/Checkers/CastToStructChecker.cpp
+++ cfe/trunk/lib/StaticAnalyzer/Checkers/CastToStructChecker.cpp
@@ -84,6 +84,10 @@
     if (!VD || VD->getType()->isReferenceType())
       return true;
 
+    if (ToPointeeTy->isIncompleteType() ||
+        OrigPointeeTy->isIncompleteType())
+      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 +65,17 @@
   void *VP = P;
   Abc = (struct ABC *)VP;
 }
+
+// https://llvm.org/bugs/show_bug.cgi?id=31173
+void dontCrash1(struct AB X) {
+  struct UndefS *S = (struct UndefS *)&X;
+}
+
+struct S;
+struct T {
+  struct S *P;
+};
+extern struct S Var1, Var2;
+void dontCrash2() {
+  ((struct T *) &Var1)->P = &Var2;
+}