Forward ranges in Phobos v2

[Adam D Ruppe]

On Wednesday, 3 November 2021 at 15:59:40 UTC, Steven 
Schveighoffer wrote:
> Oh, and one more thing, if we are going to do a tag, a UDA is 
> probably a more asthetic and functional tag. e.g. it doesn't 
> affect `__traits(allMembers, T)`.

Best way is to make the decoration also be a check. A UDA can do 
it:

---
template InputRange(Ty = void) {
         static if(is(Ty == void)) alias T = typeof(this); else 
alias T = Ty;
         static import std.range;
         static assert(std.range.isInputRange!T, "more informative 
message");
         enum isInputRange = true;
}

@InputRange!MyRange
struct MyRange {
         bool empty;
         int front;
         //void popFront() {}

         //mixin InputRange;

}
---



That silly one works as either a mixin or a UDA each with 
stylistic pros and cons, but the functionality benefit of a 
concrete check of intention and nice error messages before 
granting you the tag would legitimately be really quite nice.