dafny non aliased memory weird behavior - alias

Question

I have a dafny defined graph ADT (from this SO question) brought here again for completeness:
class Graph
{
var adjList : seq<seq<int>>;
constructor (adjListInput : seq<seq<int>>)
ensures adjList == adjListInput
{
adjList := adjListInput;
}
}
function ValidGraph(G : Graph) : bool
reads G
{
(forall u :: 0 <= u < |G.adjList| ==> forall v :: 0 <= v < |G.adjList[u]| ==> 0 <= G.adjList[u][v] < |G.adjList|) &&
(forall u :: 0 <= u < |G.adjList| ==> forall v,w :: 0 <= v < w < |G.adjList[u]| ==> G.adjList[u][v] != G.adjList[u][w])
}
method main()
{
var G : Graph := new Graph([[1,2],[0,2],[0,1]]);
var nonRelatedArray := new int[8];
var i := 0; while (i < 14)
{
// nonRelatedArray[3] := 55;
i := i + 1;
}
assert (ValidGraph(G));
}
If I remove the write comment to nonRelatedArray at index 3, I get an assertion violation, which is a bit weird because it seems reasonable that the memory model would be able to determine that nonRelatedArray is (well) non related to G.

Answer 1

You can fix this by adding modifies nonRelatedArray to the loop. The key to this modifies clause is that it does not mention G. So then Dafny knows that G will not be modified by the loop, so it will still be a valid graph.
It is a little confusing what happens if you leave off a modifies clause from a loop. If you don't do any writes to the heap (like when you comment out the write above), then Dafny (actually, Boogie) is able to automatically see that nothing is changed at all. But if you do any writes into the heap, Dafny's default modifies clause all of a sudden becomes "anything the surrounding scope is allowed to modify". If you want something other than these two defaults, you need to ask for it explicitly by giving a modifies clause.