It might be a good thing, but ...
Why not just return a proxy type upon each indexing?
The proxy type will have createIndex that will forward to others in turn.

Here a prof of concept I belive it could be generalized and polished.
For simplicity sake it's for n-dim arrays:

import std.stdio, std.exception;

struct Proxy(T)
{
		T* _this;
		int idx;
		void opAssign(X)(X value){
                    debug writeln("Proxy.opAssign"); 
		    createIndex(idx) = value;			
		}
	static if(typeof(*_this).dimension >= 2)
	{

		// somewhere io expression ...a[idx][jdx]... is create all, except last one
                auto opIndex(int jdx){
			return proxy(&_this.createIndex(idx), jdx);
		}
			//a[idx][jdx] = y; is create if non-existent
                auto opIndexAssign(X)(X val, int jdx){  //TODO: constraints!
			debug writeln("Proxy.opIndexAssign");
			_this.createIndex(idx).createIndex(jdx) = val;
		}
	}

		@property ref expr(){
                    debug writeln("Proxy.expr"); 
		    return _this.normalIndex(idx);
		}

		alias expr this;
}

auto proxy(T)(T* x, int idx){ return Proxy!(T)(x,idx); }



struct M(size_t dim)
{
	static if(dim == 1){
		alias int Val;
	}
	else{
		alias M!(dim-1) Val;
	}
	enum dimension = dim;

	
	Val[] arr;
        

	ref createIndex(int idx){
		debug writeln("Created ", typeof(this).stringof);
		if(arr.length < idx)
			arr.length = idx+1;
		return arr[idx];
	}
        ref normalIndex(int idx){
		debug writeln("Indexed ", typeof(this).stringof);
		return arr[idx];
	}
	auto opIndex(int idx){
		return Proxy!(M)(&this, idx);
	}
        alias arr this;
}

unittest{
	M!(3) d3arr;	
	d3arr[1][2] = [2, 3, 4];
	assert(d3arr[1][2][2] == 4);
	int[] x = d3arr[1][2];                                              
	assert(d3arr[1][2].length == 3); 
	assert(d3arr[1][1] == null); //inited 
	//booom used before explicit = 
	assert(collectException!Error(d3arr[2][2][1] + 1 == 1) !is null);
}

That's a pretty neat idea. Can it be made to work with containers that contain other containers (possibly of a different type)? E.g., a linked list of arrays of AA's?

Well, linked list is, for sure, not indexed so not a problem ;)
As for sets I don't see a problem, I can extend this idea to arbitrary set easily. In fact it's even cleaner for sets (maps) then arrays.
If you need a headstart I can scratch up a simple version for integer sets.

Ahm. So Q was about geterogenious stuff like arrays of sets(maps) or maps of arrays ? 
I think the 2 mentioned situations cover it all, thus you can parametrize this idea on basis of:
a) contigous container, to get item with index X you need to allocted all elements up X. Here X is obviously can be only integer of some sort.
b)non-contigous container, to get item with index X you check/create only slot indexed by X.

Whoa, this feature is weird indeed. Given the declaration int[int][int] a; a[0][0]=0 shouldn't work because the AA has no entry with key 0, so a[0] should throw RangeError similar to how b=a[0] throws RangeError.

Igor Stepanov suggested [¹] an opIndex extension.

ref Foo opIndex(bool lvalue)(size_t idx)

Where the compiler would call opIndex!true when an lvalue is required and opIndex!false when an rvalue suffices.

[¹]: http://forum.dlang.org/post/vorfqumugibjcztdrezb@forum.dlang.org

(In reply to Martin Nowak from comment #6)
> Igor Stepanov suggested [¹] an opIndex extension.
> 
> ref Foo opIndex(bool lvalue)(size_t idx)

A separate opIndexCreate might be better, b/c it allows you to 
have a ref return for one and an rvalue return for the other 
function. It also allows to use those operands as polymorphic 
functions in classes.

Just wanted to post something here.

The call a[b][c][d] = 0 results in different calls (_aaGetY now) than x = a[b][c][d] (_aaGetRvalueX).

So I think H S Teoh is onto the right path. Having a different opIndex available for assignment (or lvalue manipluation) makes sense and should be straightforward to define.

Now, to define this a little more concretely, I think opIndexCreate should ONLY be used when the entire chain of indexing results in a definite lvalue requirement. This means ONLY opIndexCreate (or AA usage) available in the expression, and the final "call" should be an opAssign or opOpAssign or opIndexOpAssign. I would also throw in opUnary that expects mutation (i.e. ++ or --) because it's currently supported by AAs.

Unfortunately, the existing behavior is somewhat inconsistent:

struct S {
  int x;
  void opUnary(string s : "++")() {++x;}
}

S[int][int] aa;

aa[1][1] = S(1); // ok
aa[2][2].x = 5; // range violation
++aa[3][3]; // range violation

int[int] aa2;

aa2[4] += 3; // ok
++aa2[5]; // ok

void foo(ref int x)

foo(aa2[6]); // range violation

So there is not 100% consistency here. The most rational logical implementation would just require lvalue usage. But the reality is different.

Also note that ++aa[5] does not match ANY operator that would be on the type of aa. There is no opIndexOpUnary akin to opIndexOpAssign. And it doesn't work if your underlying type supports ++. That is an inconsistency that will be tough to duplicate.

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