Supporting foreach (k, v; T.init) for a user-defined (container) type

[Per Nordlöw via Digitalmars-d-learn]

What property of a container (type) `T` enables iteration as
```d
foreach (k, v; T.init)
{
    ...
}
```

? I thought it sufficed to define `T.byKeyValue` but its presence 
seem to have no effect.