static check, VisitBinaryOperator() function's problem

Hi, All!

I'm using a RecursiveASTVisitor and implementing its VisitBinaryOperator()
method to check the assign operation using "sizeof()".
    My test code like this:
    void main()
    {
        int size = sizeof(MSG); //init
        size = sizeof(MSG); //assign
    }

MSG is a struct; But i find that clang can't access VisitBinaryOperator().
And variable size does't have Init Expr in VisitVarDecl(). I have checked
AST,there is no BinaryOperator or other operation. I dont know why.
<http://clang-developers.42468.n3.nabble.com/file/n4031252/1.jpg>