Forward ranges in Phobos v2

[Paul Backus]

On Tuesday, 2 November 2021 at 21:58:20 UTC, Andrei Alexandrescu 
wrote:
> On 2021-11-02 17:44, Dukc wrote:
>> 
>>> What's a value range?
>> 
>> Opposite of a reference range - copying implies `save()`.
>
> Yah, one simple improvement we could make is to assume all 
> forward ranges copy their iteration state when copying the 
> range. Then input ranges do NOT do that, i.e. all copies of an 
> input range refer to the same stream and iterate it together 
> (advancing one advances all).
>
> The differentiation can be made with a nested enum tag:
>
> struct MyInputRange {
>     enum inputRangeTag = true;
>     ...
> }
>
> Client code can inspect R.inputRangeTag to figure whether the 
> range is input (if present) or forward (if missing).

Not sure this is the best idea--it means new-style algorithms 
will silently treat old-style input ranges as though they were 
forward ranges, which could lead to incorrect behavior at 
runtime. If we are going to make incompatible changes to the 
range API, we should do it in such a way that version mismatches 
are caught at compile time.