fix: compute a single IIS in Xpress infeasibility path (#658)

Author: FabianHofmann

doc/release_notes.rst

@@ -5,6 +5,7 @@ Version 0.6.7
 -------------
 
 * Fix Xpress IIS label mapping for masked constraints and add a regression test for matching infeasible coordinates.
+* Fix ``Model.compute_infeasibilities`` returning a flattened, deduplicated union of all IIS when Xpress found more than one. The Xpress path now computes a single IIS (via ``firstIIS``), matching the Gurobi path.
 * Use ``xarray.Dataset.copy`` instead of constructor for compatibility with the latest xarray version.
 * Blacklist highspy 1.14.0 which produces wrong results due to broken presolve and crashes on Windows (`HiGHS#2964 <https://github.com/ERGO-Code/HiGHS/issues/2964>`_).
 

linopy/model.py

@@ -1601,19 +1601,17 @@ def _compute_infeasibilities_xpress(self, solver_model: Any) -> list[int]:
         be skipped (e.g., labels [0, 2, 4] with gaps instead of sequential
         [0, 1, 2]).
         """
-        # Compute all IIS
+        # Compute a single IIS (matches Gurobi behavior; multiple IIS would
+        # otherwise get flattened into an ambiguous union). Mode 2 prioritises
+        # a fast IIS search over minimality.
         try:  # Try new API first
-            solver_model.IISAll()
+            solver_model.firstIIS(2)
         except AttributeError:  # Fallback to old API
-            solver_model.iisall()
+            solver_model.iisfirst(2)
 
-        # Get the number of IIS found
-        num_iis = solver_model.attributes.numiis
-        if num_iis == 0:
+        if solver_model.attributes.numiis == 0:
             return []
 
-        labels = set()
-
         clabels = self.matrices.clabels
         constraint_position_map = {}
         for position, constraint_obj in enumerate(solver_model.getConstraint()):
@@ -1622,17 +1620,12 @@ def _compute_infeasibilities_xpress(self, solver_model: Any) -> list[int]:
                 if constraint_label >= 0:
                     constraint_position_map[constraint_obj] = constraint_label
 
-        # Retrieve each IIS
-        for iis_num in range(1, num_iis + 1):
-            iis_constraints = self._extract_iis_constraints(solver_model, iis_num)
-
-            for constraint_obj in iis_constraints:
-                if constraint_obj in constraint_position_map:
-                    labels.add(constraint_position_map[constraint_obj])
-                # Note: Silently skip constraints not found in mapping
-                # This can happen if the model structure changed after solving
+        labels = set()
+        for constraint_obj in self._extract_iis_constraints(solver_model, 1):
+            if constraint_obj in constraint_position_map:
+                labels.add(constraint_position_map[constraint_obj])
 
-        return sorted(list(labels))
+        return sorted(labels)
 
     def _extract_iis_constraints(self, solver_model: Any, iis_num: int) -> list[Any]:
         """