Fix bug in elimination of constraints that disappear

1 files changed, 12 insertions, 6 deletions

diff --git a/sumofsquares/problems.py b/sumofsquares/problems.py
index fb25fd9..e6ca747 100644
--- a/sumofsquares/problems.py
+++ b/sumofsquares/problems.py
@@ -376,6 +376,17 @@ class InternalSOSProblem(Problem):
             constr = poly.to_affine(c.expression)
             nrows, ncols = constr.shape
 
+            # There is only one "variable" in the affine expression
+            if len(constr.slices.keys()) == 1:
+                # That variable is actually not a variable
+                if MonomialIndex.constant() in constr.slices.keys():
+                    # constraint has disappeared
+                    continue
+                    # FIXME: need to check for each type of constraint
+                    # that it is ok to remove constant value. Example of
+                    # a problematic edge case is NonNegative(a - a - 5)
+                    # where a is an optimization variable.
+
             if constr.degree > 1:
                 # If this error occurs an it is not the user's fault, there is
                 # a bug in SOSProblem.apply
@@ -385,12 +396,7 @@ class InternalSOSProblem(Problem):
             # Get constant and linear terms
             constant = constr.affine_coefficient(MonomialIndex.constant())
             linear = tuple(constr.affine_coefficient(v) for v in variable_indices)
-
-            # constraint has disappeared
-            if not (np.any(constant) or np.any(linear)):
-                # print("Constraint disappeared!")
-                continue
-            
+ 
             if isinstance(c, EqualToZero):
                 dims["z"].append(nrows)
                 constraints["z"].append((linear, constant))