As a first step towards fixing PR9641, add a CK_DynamicToNull cast kind which

represents a dynamic cast where we know that the result is always null.

For example:

struct A {
  virtual ~A();
};
struct B final : A { };
struct C { };

bool f(B* b) {
  return dynamic_cast<C*>(b);
}

llvm-svn: 129256

1 files changed, 8 insertions, 0 deletions

diff --git a/clang/lib/Sema/SemaCXXCast.cpp b/clang/lib/Sema/SemaCXXCast.cpp
index 31a772a5d8e..52a13ef5b29 100644
--- a/clang/lib/Sema/SemaCXXCast.cpp
+++ b/clang/lib/Sema/SemaCXXCast.cpp
@@ -522,6 +522,14 @@ CheckDynamicCast(Sema &Self, ExprResult &SrcExpr, QualType DestType,
     return;
   }
 
+  // If the source class is marked 'final', and the destination class does not
+  // derive from the source class, then we know that the result is always null.
+  if (SrcRecord->getDecl()->hasAttr<FinalAttr>() &&
+      !Self.IsDerivedFrom(DestPointee, SrcPointee)) {
+    Kind = CK_DynamicToNull;
+    return;
+  }
+
   // C++ 5.2.7p5
   // Upcasts are resolved statically.
   if (DestRecord && Self.IsDerivedFrom(SrcPointee, DestPointee)) {