Static verification limitations when using pure functions in C# code contracts?

Question

I'm trying to statically verify the following partial implementation of an array-based stack with code contracts. The method Pop() uses the pure function IsNotEmpty() to ensure that the subsequent array access will be at/above the lower bound. The static verifier fails and suggests that I add the precondition Contract.Requires(0 <= this.top).

Can anyone explain why the verifier cannot prove the array access is valid with respect to the lower bound given the contract IsNotEmpty()?

At first I thought the Contract.Requires(IsNotEmpty()) approach might be incorrect because a subclass could override IsNotEmpty(). However, the verifier still can't prove the array access is valid if I mark the class as sealed.

Update: If I change IsNotEmpty() to a read-only property, the verification succeeds as expected. This raises the question: how / why are read-only properties and pure functions treated differently?

class StackAr<T>
{
    private T[] data;
    private int capacity;

    /// <summary>
    /// the index of the top array element
    /// </summary>
    private int top;

    [ContractInvariantMethod]
    private void ObjectInvariant()
    {
        Contract.Invariant(data != null);
        Contract.Invariant(top < capacity);
        Contract.Invariant(top >= -1);
        Contract.Invariant(capacity == data.Length);
    }

    public StackAr(int capacity)
    {
        Contract.Requires(capacity > 0);
        this.capacity = capacity;
        this.data = new T[capacity];
        top = -1;
    }

    [Pure]
    public bool IsNotEmpty()
    {
        return 0 <= this.top;
    }

    public T Pop()
    {
        Contract.Requires(IsNotEmpty());

        //CodeContracts: Suggested precondition: 
        //Contract.Requires(0 <= this.top);

        T element = data[top];
        top--;
        return element;
    }
}

Answer 1

This should solve the problem:

[Pure]
public bool IsNotEmpty() {
    Contract.Ensures(Contract.Result<bool>() && 0 <= this.top || !Contract.Result<bool>() && 0 > this.top);
    return 0 <= this.top;
}

See this thread on the code contracts forum for more information: Calling a method in Contract.Requires

Edit:

Another solution to this problem, as mentioned in the thread linked above, is the -infer command line option:

Now, inferring the post condition of this method is possible, and in fact we do have the option to do so: try to add -infer ensures on the extra options line in the contract property pane for the static checker.

I have checked, and this does resolve the problem. If you look in the code contracts manual, you'll see that the default option is to only infer property post-conditions. By using this switch, you can tell it to attempt to infer the post-conditions of methods too:

-infer (requires + propertyensures + methodensures) (default=propertyensures)