q about slices, strings

[Kevin Bailey]

I saw on the dlang.org homepage a sample of code that looks a 
little strange to me:

     int[char[2]] aa;
     auto arr = "ABBBA";

     foreach (i; 0 .. arr.length - 1)
         aa[arr[i .. $][0 .. 2]]++;

which I changed to:

         aa[arr[i .. i+2][0 .. 2]]++;

in the hopes of only doing:

         aa[arr[i .. i+2]]++;

The question is: Why is the [0..2] still necessary?

Without it, dmd (on that homepage) complains:

  cannot implicitly convert expression arr[i..i + 2LU] of type 
string to char[2]

The spec says that:

10.22 Slices
...
6. If the slice bounds can be known at compile time, the slice 
expression is implicitly convertible to an lvalue of static array.

7. The following forms of slice expression can be convertible to 
a static array type:

e     An expression that contains no side effects.
a, b  Integers (that may be constant-folded).

Form	       The length calculated at compile time
...
arr[e .. e+b]  b

So why isn't arr[i .. i+2] convertible? Is dmd behind the spec 
here?