Flatten a range of static arrays

[Dennis]

On Friday, 7 February 2020 at 20:55:14 UTC, nullptr wrote:
> Depending on how your range is structured, it might be possible 
> to just mark front as returning by ref to make this work.

That's a good one. I can't make front() return by ref, but I can 
make front a member variable of the range struct. Only problem:
@safe function ... cannot call @system function 
std.algorithm.iteration.joiner!(...).joiner

I don't know why. I don't have time to delve into this at the 
moment but if anyone wants to try, here's a minimal testcase:

```
import std;

struct S {
     int[3] front = [10, 20, 30];
     bool empty = false;
     void popFront() {empty = true;}
}

void main() @safe {
     S.init.map!((return ref x) => x[]).joiner.writeln;
}
```
flags: -dip1000 -dip25