Fix empty constraints after dropping parameters (#448)

When dropping parameters due to the use of cardinality constraints, it
can happen that an attempt is made to create a Constraint without any
parameters, causing the code to crash.

This PR introduces an explicit test for this scenario as well as a fix
by only including constraints that still have parameters after dropping
them.

CHANGELOG.md

@@ -33,6 +33,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - `qNIPV` not working with single `MIN` targets
 - Passing a `TargetTransformation` without passing `bounds` when createing a 
   `NumericalTarget` now raises an error
+- Crash when using `ContinuousCardinalityConstraint` caused by an unintended interplay
+  between constraints and dropped parameters yielding empty parameter sets
 
 ### Deprecations
 - Passing a dataframe via the `data` argument to `Objective.transform` is no longer

baybe/searchspace/continuous.py

@@ -298,10 +298,14 @@ def _drop_parameters(self, parameter_names: Collection[str]) -> SubspaceContinuo
         return SubspaceContinuous(
             parameters=[p for p in self.parameters if p.name not in parameter_names],
             constraints_lin_eq=[
-                c._drop_parameters(parameter_names) for c in self.constraints_lin_eq
+                c._drop_parameters(parameter_names)
+                for c in self.constraints_lin_eq
+                if set(c.parameters) - set(parameter_names)
             ],
             constraints_lin_ineq=[
-                c._drop_parameters(parameter_names) for c in self.constraints_lin_ineq
+                c._drop_parameters(parameter_names)
+                for c in self.constraints_lin_ineq
+                if set(c.parameters) - set(parameter_names)
             ],
         )
 

tests/constraints/test_cardinality_constraint_continuous.py

@@ -169,3 +169,36 @@ def test_random_recommender_with_cardinality_constraint(
     _validate_samples(
         recommendations, max_cardinality=2, min_cardinality=1, batch_size=batch_size
     )
+
+
+def test_empty_constraints_after_cardinality_constraint():
+    """Constraints that have no more parameters left due to activated
+    cardinality constraints do not cause crashes."""  # noqa
+
+    N_PARAMETERS = 2
+
+    parameters = [
+        NumericalContinuousParameter(name=f"x_{i+1}", bounds=(0, 1))
+        for i in range(N_PARAMETERS)
+    ]
+    constraints = [
+        ContinuousLinearConstraint(
+            parameters=["x_1"],
+            operator="=",
+            coefficients=[1.0],
+            rhs=0.3,
+        ),
+        ContinuousLinearConstraint(
+            parameters=["x_2"],
+            operator="<=",
+            coefficients=[1.0],
+            rhs=0.6,
+        ),
+        ContinuousCardinalityConstraint(
+            parameters=["x_1", "x_2"],
+            max_cardinality=1,
+            min_cardinality=1,
+        ),
+    ]
+    subspace = SubspaceContinuous.from_product(parameters, constraints)
+    subspace.sample_uniform(1)