Array slice length confusion

[Tim Matthews]

First off if this has been asked so many times please just ignore this. 
You should be able to collapse this whole subject in most good new 
readers plus it is in d.learn out of the way so I expect no "ahh here we 
go again" sort of replies.

Anyway I like how you can get a slice of array that is just a view into 
the original. If I modify the slice, it modifies the full original array 
which can be very useful. If however I modify the length of the slice 
which I was doing in an attempt to extend the view, it becomes it's own 
array.

This example should help explain what I mean:

module test;

import std.stdio;

void main()
{
     char[5] a = "Hello";
     char[] b;
     b = a[1..3]; //b is a view into a
     writeln(a); //(Hello)
     writeln(b); //(el)
     b[0] = 'a'; //no copy on write
     writeln(a); //(Hallo)
     writeln(b); //(al)
     b.length = b.length+2; //COW on length change?
     b[0] = 'z'; //does not modify a
     writeln(a); //(Hallo)
     writeln(b); //(zl)
}


Was this a design choice, bug, undefined behavior, etc...?