void delegate() → void delegate() immutable

@safe immutable(void delegate()) f(void delegate() d)
{
    return d;
}

@safe void delegate() immutable g(immutable(void delegate()) d)
{
    return delegate() immutable { d(); };
}

@safe void delegate() immutable very_bad(void delegate() d)
{
    return g(f(d));
}

void main()
{
}