Forward ranges in Phobos v2

[jmh530]

On Wednesday, 3 November 2021 at 15:03:53 UTC, Andrei 
Alexandrescu wrote:
> [snip]
>
> Good point. Maybe have all ranges define that enum with values 
> true and false respectively?

Maybe I just haven't been following this discussion closely 
enough, but I'm a little confused by this. We have 
`__traits(hasMember, T, inputRangeTag)` after all. There should 
be default behavior when that tag isn't there and more 
sophisticated behavior when it is (if it is needed).