[analyzer] Fix a crash regression on casting opaque symbolic pointers from unrelated base classes to derived classes.

[NoQ]

Summary

Commit D51191 causes a crash when a pointer to a completely unrelated type UnrelatedT (eg., opaque struct pattern) is being casted from base class BaseT to derived class DerivedT, which results in an ill-formed region Derived{SymRegion{$<UnrelatedT x>}, DerivedT}.

I guess we should prevent these from appearing somehow.

[NoQ]

Add a bit more discussion into the comments.

[NoQ]

Add a defensive check that prevents more crashes if the source type is not a class at all.

lib/StaticAnalyzer/Core/Store.cpp

if (Uncasted == MR) {
       // class. We must be casting the base to derived, so the cast should
       // fail.
       break;
     }
 
     MR = Uncasted;
   }
 
+  // If we're casting a symbolic base pointer to a derived class, use
+  // CXXDerivedObjectRegion to represent the cast. If it's a pointer to an
+  // unrelated type, it must be a weird reinterpret_cast and we have to
+  // be fine with ElementRegion. TODO: Should we instead make
+  // Derived{TargetClass, Element{SourceClass, SR}}?
   if (const auto *SR = dyn_cast<SymbolicRegion>(MR)) {
-    return loc::MemRegionVal(MRMgr.getCXXDerivedObjectRegion(TargetClass, SR));
+    QualType T = SR->getSymbol()->getType();
+    const CXXRecordDecl *SourceClass = T->getPointeeCXXRecordDecl();
+    if (TargetClass && SourceClass && TargetClass->isDerivedFrom(SourceClass))
+      return loc::MemRegionVal(
+          MRMgr.getCXXDerivedObjectRegion(TargetClass, SR));
+    return loc::MemRegionVal(GetElementZeroRegion(SR, TargetType));
   }
 
   // We failed if the region we ended up with has perfect type info.
   Failed = isa<TypedValueRegion>(MR);
   return UnknownVal();
 }
 
 /// CastRetrievedVal - Used by subclasses of StoreManager to implement

test/Analysis/casts.cpp

 struct C : A, B {};
 
 void foo(B *b) {
   C *c = (C *)b;
   b->y = 1;
   clang_analyzer_eval(c->x); // expected-warning{{UNKNOWN}}
   clang_analyzer_eval(c->y); // expected-warning{{TRUE}}
 }
+} // namespace base_to_derived_double_inheritance
+
+namespace base_to_derived_opaque_class {
+class NotInt {
+public:
+  operator int() { return !x; } // no-crash
+  int x;
+};
+
+typedef struct Opaque *OpaqueRef;
+typedef void *VeryOpaqueRef;
+
+class Transparent {
+public:
+  int getNotInt() { return NI; }
+  NotInt NI;
+};
+
+class SubTransparent : public Transparent {};
+
+SubTransparent *castToDerived(Transparent *TRef) {
+  return (SubTransparent *)TRef;
 }
 
+void foo(OpaqueRef ORef) {
+  castToDerived(reinterpret_cast<Transparent *>(ORef))->getNotInt();
+}
+
+void foo(VeryOpaqueRef ORef) {
+  castToDerived(reinterpret_cast<Transparent *>(ORef))->getNotInt();
+}
+} // namespace base_to_derived_opaque_class