foreach doesn't seem to know about invariant

[Janice Caron]

It'll be interesting to see how we're supposed to overload that with
opApply, taking const into account, in the next version of const. Do
we have to write the function twice? (Four times if you also have to
write Value-only versions).

const int opApply(int delegate(Key key, Value value) dg)
{
    /* whatever */
}

int opApply(int delegate(Key key, ref Value value) dg)
{
    /* whatever */
}

I don't think you can say "ref const(Value) value".