When are associative arrays meant to throw a RangeError?

[Ben Davis]

I can't easily see what the rules are. Specifically, I'm finding:

Chunk[][char[4]] chunks;
//chunks["DATA"] is an array of Chunk objects, where Chunk is a class.
//I'm using this structure to remember positions of chunks in a file format.

//These both work:
chunks["AAAA"]~=new Chunk();
chunks["BBBB"]=chunks["BBBB"]~new Chunk();

//These all throw RangeErrors
Chunk[] tempVar=chunks["CCCC"]~new Chunk();
if (chunks["DDDD"].length!=1) throw new Exception("");
writefln("%s",chunks["EEEE"]);

//This works and formats the null as "[]"
writefln("%s",cast(Chunk[])null);

So, as far as I can tell, chunks[nonexistentKey] throws a RangeError in 
most cases, including with ~ in general, but not if the ~ is used for an 
in-place modification.

My initial expectation would be that every example above would throw a 
RangeError (except the one that doesn't use 'chunks' at all).

So here are my theories, in the approx order I came up with them:

1=. It's an optimisation accident, and it's meant to throw.

1=. It's a nasty bodge specifically to make assoc arrays usable with 
modify-assign expressions.

3. It could be a quirk of assignment expression evaluation order. 
Perhaps the assignment expression FIRST creates an entry in the assoc 
array using the value type's default value (an empty dynamic array), AND 
THEN evaluates the RHS, which is able to read the empty array that's 
just been inserted.

Whatever the case, I couldn't find any documentation on the subject. I 
looked under associative arrays (obviously), and also under assignment 
expressions, and also under expression statements just for good measure.

Would appreciate any comments :)

Thanks,

Ben :)