isInputRange is false for non-copyable types

[Shachar Shemesh]

import std.range;
import std.stdio;

struct S {
     int a;

     @disable this(this);
}

void main() {
     writeln(isInputRange!(S[])); // Prints false
}

The reason, as far as I can tell, is that an input range requires that 
we can do:

auto a = ir.front; // Assuming ir isn't empty

That doesn't work for non-copyable types, for obvious reasons.

With that said, that seems more like a deficiency in the input range 
definition than anything. There is no reason we shouldn't be able to 
iterator type operations over non-copyable types.

Shachar