[Cbc] Unbounded MIP problem returned as infeasible by CBCSolver

[Kish Shen]

Hi,

For the following (very simple) problem:

Minimize
obj: x0
Subject To
cons0:  x0 + x1 <= 3
cons1:  X0 = X1  = 0
Bounds
  x0 Free
  x1 Free
Integers
x0
End

which is unbounded, but when we use CBCSolver to solve this problem, we 
get the result that the problem is infeasible, specifically,

CbcModel* model;
.....
model->isProvenInfeasible() returns 1 (true)

Here is the relevant code we use to determine the status of a problem 
after calling CbcSolver (via CbcMain0 and CbcMain1) to solve the problem:


if (model->isInitialSolveProvenOptimal()) {
    if (model->isProvenInfeasible()) {
        ....

For the above problem, the above if .. succeeds, which I think should 
only be the case if the problem is proven infeasible at the root node, 
but the root node solve should be unbounded (and in fact if I solve the 
linear problem with CLP, the result is indeed unbounded), but the test

We actually have code to test if the root node solve is unbounded (in 
which case we classify the solve result as "Unknown", i.e. either 
infeasible or unbounded, which was the result I expected for the above 
problem), but

model->isContinuousUnbounded() returns 0 for the above problem, rather 
than 1 as I expected.

Is there something I am doing wrong, or is there a problem?

I am using Cbc 2.8, which was downloaded last month (14 Feb).

Thanks in advance for any help.

Cheers,

Kish