Forward ranges in Phobos v2

[Paul Backus]

On Wednesday, 3 November 2021 at 11:54:24 UTC, Dukc wrote:
> There's an easier solution: require all v2 ranges to have the 
> `inputRangeTag`. For forward ranges it must be set to false, 
> but if the tag does not exist at all then it isn't a v2 range.
>
> If we go for this, I'd rename the tag to `isReferenceRange` 
> though.
>
> It's going to require more manual usage of the `valueRange` 
> wrapper though, as the v1 ranges around aren't going to be v2 
> ranges as often.
>
> What do you say?

Sure, that would work. But that still leaves the issue of `const` 
ranges, which is the other thing `head` and `tail` are meant to 
address.

I think if we are going to make incompatible changes to the range 
API, we might as well do a proper redesign that fixes all of the 
known issues at once. And until we do that (maybe in v3?), it is 
probably better to hold off on incompatible changes.