A delegate with return type ref immutable(T) can be used to reassign an immutable(T) as follows:

alias DGm = ref           int  delegate() @safe;
alias DGi = ref immutable(int) delegate() @safe;

@safe void f()
{
    immutable int i = 0;
    DGi dgi = ref() => i;
    DGm[] dgms = [ dgi ]; // why?
    dgms[0]() = 1;
    assert(i == 1); // fails, because optimization
}

@safe void g()
{
    DGm[] dgms;
    foreach (immutable int i; [0])
    {
        assert(i == 0); // just to be sure
        DGi dgi = ref() => i;
        dgms ~= dgi; // why?
        dgms[0]() = 1;
        assert(i == 1); // passes, meaning immutable i was changed
    }
}

The lines marked with // why? should be rejected by the type system.