Motive behind !empty() with front() instead of Optional front()

[Per Nordlöw]

On Tuesday, 6 April 2021 at 02:14:17 UTC, Paul Backus wrote:
>     auto ref T identity(T)(auto ref T arg)
>     {
>         import core.lifetime: forward;
>         return forward!arg;
>     }

When is the call to forward needed in this case?