Take this program:

struct S
{
public:

    this(int[] function(const int[])func)
    {
        _func = func;
    }

    @property int[] function(const int[]) func()
    {
        return func;
    }

private:
    int[] function(const int[]) _func;
}

int[] add1(int[] num)
{
    return num ~ 1;
}

void main()
{
    auto s = S(&add1);
}

It shouldn't compile, but it does. add1() does not take a const int[] and therefore could theoretically modify its arguments (though it doesn't here), violating the type of the function pointer. Using in and immutable instead of const has the same problem. They compile when they shouldn't.