Explain migration for the removed couple carer-premium parameter

changelog.d/2137.fixed.md

@@ -0,0 +1 @@
+- Added migration directions for saved reforms using the removed couple carer premium parameter.

policyengine_uk/tests/test_removed_carer_premium_parameter.py

@@ -0,0 +1,83 @@
+"""Saved couple carer-premium policies fail with per-person migration help."""
+
+import pytest
+from policyengine_core.reforms import Reform
+
+from policyengine_uk import Simulation
+from policyengine_uk.system import system
+from policyengine_uk.utils.parameters import RemovedParameterNode
+from policyengine_uk.utils.scenario import Scenario
+
+REMOVED = "gov.dwp.carer_premium.couple"
+REPLACEMENT = "gov.dwp.carer_premium.single"
+SITUATION = {"people": {"person": {"age": {2026: 40}}}}
+
+
+@pytest.fixture(scope="module", autouse=True)
+def reuse_country_model():
+    with pytest.MonkeyPatch.context() as patch:
+        patch.setattr(
+            "policyengine_uk.simulation.CountryTaxBenefitSystem", system.clone
+        )
+        yield
+
+
+@pytest.fixture(scope="module")
+def baseline():
+    return Simulation(situation=SITUATION)
+
+
+@pytest.fixture
+def simulation(baseline):
+    return baseline.clone()
+
+
+@pytest.mark.parametrize(
+    "entry_point",
+    ["core_reform", "scenario_reform", "scenario_scalar", "scenario_saved_periods"],
+)
+def test_removed_carer_premium_reforms_give_migration_directions(
+    simulation, entry_point
+):
+    changes = {REMOVED: {"2026": 99}}
+    parameters = simulation.tax_benefit_system.parameters
+    original = parameters.get_child(REPLACEMENT)(2026)
+
+    with pytest.raises(ValueError) as error:
+        if entry_point == "core_reform":
+            Reform.from_dict(changes)(simulation.tax_benefit_system)
+        elif entry_point == "scenario_reform":
+            Scenario.from_reform(changes).apply(simulation)
+        elif entry_point == "scenario_scalar":
+            Scenario(parameter_changes={REMOVED: 99}).apply(simulation)
+        else:
+            Scenario(parameter_changes=changes).apply(simulation)
+
+    message = str(error.value)
+    assert f"{REMOVED} has been removed" in message
+    assert REPLACEMENT in message
+    assert "per qualifying claimant or partner" in message
+    assert "per-person" in message
+    assert parameters.get_child(REPLACEMENT)(2026) == original
+
+
+def test_removed_carer_premium_alias_uses_its_own_parent(baseline):
+    parameters = baseline.tax_benefit_system.parameters
+    premium = parameters.gov.dwp.carer_premium
+    alias = premium.children["couple"]
+
+    assert isinstance(alias, RemovedParameterNode)
+    assert alias.parent is premium
+    assert alias.metadata["removed"] is True
+    assert alias.metadata["household"] is False
+    assert alias.metadata["economy"] is False
+    assert REPLACEMENT in alias.description
+    assert "couple" not in parameters.gov.dwp.state_pension.age.children
+    with pytest.raises(ValueError, match="per-person"):
+        alias.update(period="2026", value=99)
+
+
+def test_per_person_carer_premium_remains_reformable(simulation):
+    Scenario(parameter_changes={REPLACEMENT: 99}).apply(simulation)
+
+    assert simulation.tax_benefit_system.parameters.get_child(REPLACEMENT)(2026) == 99

policyengine_uk/utils/parameters.py

@@ -160,6 +160,11 @@ class RemovedParameter(NamedTuple):
     "gov.dwp.state_pension.age.female": RemovedParameter(
         "removed female State Pension age parameter", _STATE_PENSION_AGE_REPLACEMENT
     ),
+    "gov.dwp.carer_premium.couple": RemovedParameter(
+        "removed couple carer premium parameter",
+        "The carer premium is calculated per qualifying claimant or partner. "
+        "Change gov.dwp.carer_premium.single (the per-person weekly amount) instead.",
+    ),
     "gov.simulation.labour_supply_responses.obr_elasticities": RemovedParameter(
         "removed OBR elasticities switch", _OBR_ELASTICITIES_REPLACEMENT
     ),