Warning: extractcounterexample is a new flag in Dafny and the command to extract it will likely change

Dafny is a program verifier made by Microsoft. The programmer specifies the pre and post conditions and Dafny will try to verify that the codebase meets those specifications. One of its coolest features is that Dafny will can provide a countermodel for why it things the code does not verify. Lets look at an example:

method arrayUpToN(n: int) returns (a: array<int>)
    requires n >= 0
    ensures a.Length == n
    ensures forall j :: 0 < j < n ==> a[j] >= 0
    ensures forall j, k : int :: 0 <= j <= k < n ==> a[j] <= a[k]
{
    var i := 0;
    a := new int[n];
    while i < n
        invariant 0 <= i <= n
        invariant forall k :: 0 <= k < i ==> a[k] >= 0
        invariant forall j, k :: 0 <= j <= k < i ==> a[j] <= a[k]
    {
        a[i] := i;
        i := i + 1;
    }
}

Say the content above is saved in example.dfy. To verify it and show a countermodel if it fails:

dafny example.dfy \
	/compile:4 /verifyAllModules \
	/proverOpt:O:model_compress=false \
	/proverOpt:O:model.completion=true \
	/warnShadowing /extractCounterexample \
	/errorTrace:0 /mv:example.model

The output can be a bit cryptic:

example.dfy(14,18): Error: This loop invariant might not be maintained by the loop.
example.dfy(14,18): Related message: loop invariant violation

Dafny program verifier finished with 1 verified, 1 error
Compiled assembly into example.dll
Program compiled successfully
Counterexample for first failing assertion: 
example.dfy(7,0): initial state:
        a : _System.array?<int> = ()
        n : int = 1237
        j : int = 196
        k : int = 1236
example.dfy(8,14):
        a : _System.array?<int> = ()
        n : int = 1237
        i : int = 0
        j : int = 196
        k : int = 1236
example.dfy(9,19):
        a : _System.array?<int> = (Length := 1237, [1236] := 1236, [196] := 1686)
        n : int = 1237
        i : int = 0
        j : int = 196
        k : int = 1236
example.dfy(10,4): after some loop iterations:
        a : _System.array?<int> = (Length := 1237, [196] := 1686)
        n : int = 1237
        i : int = k
        j : int = 196
        k : int = 1236
example.dfy(16,17):
        a : _System.array?<int> = (Length := 1237, [1236] := 1236, [196] := 1686)
        n : int = 1237
        i : int = k
        j : int = 196
        k : int = 1236
example.dfy(17,18):
        a : _System.array?<int> = (Length := 1237, [1236] := 1236, [196] := 1686)
        n : int = 1237
        i : int = n
        j : int = 196
        k : int = 1236

The important thing to note here is that it thinks a[196] = 1686 which is not true at all given the code. Turns out, we’ll have to add an extra loop invariant in order for it to verify correctly:

invariant forall k :: 0 <= k < i ==> a[k] == k

Running Dafny again will produce:

Dafny program verifier finished with 2 verified, 0 errors
Compiled assembly into example3.dll
Program compiled successfully