Binding rvalues to ref [WAS: I close BIP27. I won't be pursuing BIPs anymore]

[Nicholas Wilson via Digitalmars-d]

On Thursday, 20 October 2016 at 01:04:35 UTC, Jonathan M Davis 
wrote:
> The transitivity of const shoot stuff in the foot pretty 
> thoroughly in a number of cases. A prime example would be 
> ranges, because they have to be mutated to be iterated over. If 
> the function actually took a range directly, you wouldn't 
> bother with const ref, but it could be an object that contains 
> a range, and because you can't normally get a tail-const range 
> from a const range (aside from dynamic arrays), it can become 
> quite difficult to actually iterate over the range. e.g.
>
> auto foo(ref const(Bar) bar)
> {
>     auto range = bar.getSomeRange();
>     ...
> }
>

Is it legal to `.save` a const range, and then use it (provided 
it does not mutate any object reachable from bar)?

> Because getSomeRange would have to be const (or inout) in order 
> to be called, the return type is going to have be const if 
> there are any indirections in it. And if it's const, and it's a 
> range, it's useless.
>
> If this were C++, const wouldn't be transitive, so it would be 
> trivial to get an iterator or range which wasn't const and did 
> not violate const or attempt to work around it in any way and 
> thus could be iterated. But in D, const is far too restrictive 
> for that to work.
>
> This is the kind of problem that quickly makes you start 
> wondering whether it's even worth trying to use const in D at 
> all.
>
> - Jonathan M Davis