const property don't wont return const reference to value

[Zhenya]

Hi!
Explain me please what's wrong with this code:

module main;

struct foo
{
        int m_bar;
        @property const ref int bar() const
        {
                return m_bar;
        }
}

void main()
{
}

Error: cast(int)this.m_bar is not an lvalue