Possible issue with isInputRange

[Alex Parrill via Digitalmars-d]

On Wednesday, 23 September 2015 at 23:10:21 UTC, Maor Ben Dayan 
wrote:
> isInputRange will always return true for a range returning ref 
> to non-copyable type.
> This is a problem when trying to work with chain etc. together 
> with such ranges.
> The problem is that the test in isInputRange should have been 
> similar to A below instead of B (no need to try and assign the 
> return value of front for the range to be an input range).
> Below is a reduced code example.
>
> Am I correct in assuming that this is a phobos bug ?
>
> code example:
>
> void main()
> {
>     import std.range;
>     import std.traits;
>
>     struct Snowflake {
>         int x;
>         @disable this(this);
>     }
>
>     Snowflake[12] flakes;
>     foreach(uint i; 0..flakes.length) {
>         flakes[i].x = i;
>     }
>     alias R = Snowflake[];
>
>     foreach(ref s; flakes[0..$]) { /* works just fine, I guess 
> it is a valid input range */
>         // do something
>     }
>
>     static assert(is(typeof((inout int = 0) { R r = R.init; 
> })));
>     static assert(is(typeof((inout int = 0) { R r = R.init; if 
> (r.empty) {} })));
>     static assert(is(typeof((inout int = 0) { R r = R.init; 
> r.popFront(); })));
>     static assert(is(typeof((inout int = 0) { R r = R.init; 
> r.front; })));        /* A passes */
>     static assert(is(typeof((inout int = 0) { R r = R.init; h = 
> r.front; })));  /* B fails */
>     static assert(isInputRange!(Snowflake[]));
>               /* fails */
> }

It's because you disabled the copy constructor of `Snowflake`. 
Apparently `isInputRange` requires copyable elements (it does 
`auto h = r.front;` in its check).

Also, just because it's compatible with foreach loops doesn't 
mean it's a range; it may be an object with `opApply` (such as 
`std.parallelism.parallel` 
http://dlang.org/phobos/std_parallelism.html#.parallel)