Problem overloading operator for a struct with an immutable member

[Nicolas Sicard via Digitalmars-d-learn]

I don't know if this is a bug or expected behaviour. The struct 
is mutable, assignable and pre-increment operator works. But 
post-increment doesn't compile because of the immutable member.

--
struct S
{
    int i;
    immutable(Object) o;
    S opUnary(string op)() { return this; }
    void opAssign(S other) {}
}

void main()
{
    S s, t;

    t = s; // OK
    ++s; // OK
    s++; // Error: cannot modify struct s S with immutable members
}
---