is std.algorithm.joiner lazy?

[Edwin van Leeuwen via Digitalmars-d-learn]

On Thursday, 7 April 2016 at 09:55:56 UTC, Puming wrote:
> When compiled, I get the error:
>
> Error: open path skips field __caches_field_0
> source/app.d(19, 36): Error: template instance 
> std.algorithm.iteration.cache!(MapResult!(__lambda1, int[])) 
> error instantiating

That seems like a bug to me and you might want to submit it to 
the bug tracker. Even converting it to an array first does not 
seem to work:

import std.stdio : writeln;
import std.algorithm : map, cache, joiner;
import std.array : array;

auto read(int a) {
    return [0, a]; // second level
}

auto mkarray(int a) {
   return [-a, a].map!(x=>read(x)).cache.joiner; // to avoid 
calling read twice
}

void main() {
   auto xs = [1,2 ,3, 4];
   auto r = xs.map!(x=>mkarray(x)).array;
	
   // Both lines below should be equal, but second does not compile
   [[0, -1, 0, 1], [0, -2, 0, 2], [0, -3, 0, 3], [0, -4, 0, 
4]].cache.joiner.writeln;
   r.cache.joiner.writeln;
}

Above results in following error:
/opt/compilers/dmd2/include/std/algorithm/iteration.d(326): 
Error: one path skips field __caches_field_0
/d617/f62.d(19): Error: template instance 
std.algorithm.iteration.cache!(Result[]) error instantiating