Compile error despite override

Sorry if this is such an easy question, there must be something I don't understand about inheritance, virtual and override in c++. In the following example, I get a compile-time error relative to a virtual method that I specifically override to avoid such error in a child class. Am I doing something wrong?

#include <array>
#include <deque>

template <class T, class C>
struct foo
{
    virtual const C& data() const =0;

    inline virtual T& operator[] ( unsigned n ) const
        { return const_cast<T&>( data()[n] ); }
};

template <class T>
struct bar
    : public foo<T,std::deque<T>>
{
    typedef std::deque<T> data_type;
    data_type m_data;

    inline const data_type& data() const 
        { return m_data; }
};

template <class T, unsigned N>
struct baz
    : public foo<T, std::deque<std::array<T,N>> > 
{
    typedef std::deque<std::array<T,N>> data_type;
    data_type m_data;

    inline const data_type& data() const 
        { return m_data; }
    inline T& operator[] ( unsigned n ) const override
        { return const_cast<T&>( data()[n/N][n%N] ); }
};

int main()
{
    bar<double> a;
    baz<double,3> b; // throws an error related to foo::operator[] depsite override
}

Here is the error:

clang++ -std=c++0x -Wall virtual_operator.cpp -o virtual_operator.o
virtual_operator.cpp:11:12: error: const_cast from 'const value_type' (aka 'const std::__1::array<double, 3>') to 'double &' is not allowed
                { return const_cast<T&>( data()[n] ); }
                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~
virtual_operator.cpp:26:8: note: in instantiation of member function 'foo<double, std::__1::deque<std::__1::array<double, 3>, std::__1::allocator<std::__1::array<double, 3> > > >::operator[]'
      requested here
struct baz
       ^
1 error generated.