Generality creep

[Andrei Alexandrescu]

On 3/28/19 9:16 AM, ag0aep6g wrote:
> On 28.03.19 14:05, Andrei Alexandrescu wrote:
>> Then some ranges are not meant to be assignable.
> 
> Should Phobos be compatible with those ranges?

A variety of algorithm and data structures in Phobos are relying on 
assignment. Presumably a good part of them can be converted with ease to 
use single assignment, but not all.

The overall message is we got bogged down on the "wrong" side of 
generality - cross-cutting and nonscalable code additions to support 
unprincipled and low-impact corner cases.

Part of that is we've been cagey about defining copy and assignment 
semantics of ranges in a simple and comprehensive manner. It seems to me 
going with these is the right thing:

* Input ranges are copyable and assignable, and have pointer semantics 
(all copies refer to the same underlying position, and advancing one 
advances all others).

* Forward ranges are copyable and assignable, but distinct copies refer 
to distinct positions in the range such that advancing one does not 
advance the others.

* We don't support other semantics.