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 */
}

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)

On 24/09/15 07:10, Maor Ben Dayan wrote:
>      static assert(is(typeof((inout int = 0) { R r = R.init; h =
> r.front; })));  /* B fails */

What is `h`?