Add unit tests for CPS marriage tax reforms (#737) (#1642)

* Add unit tests for CPS marriage tax reforms (#737)

Expose module-level Reform instances (`expanded_ma_reform`,
`marriage_neutral_it_reform`) so YAML tests can reference them by
dotted import path, matching the existing pattern used by
`scottish_child_payment_reform`. Both are built with no child-age or
education filters, so they apply to every married couple in tests.

Add three initial test cases covering the reform mechanics:

- Expanded MA: a single higher-rate earner has no Marriage Allowance
  even with the reform active (no spouse to transfer from).
- Expanded MA: a married higher-rate earner with a low-earning spouse
  receives Marriage Allowance under the reform (normally ineligible
  under ITA 2007 s. 55B because they're over the basic rate). With
  take-up forced to 1.0, the expected transfer is £1,260 — low
  earner's unused PA capped at PA × ma_rate, rounded up to the £10
  increment.
- Marriage-neutral IT: a single person's adjusted net income is
  unchanged because `has_spouse` is False and the optimal-split
  branch collapses to the original income.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* Cover marriage-neutral income splitting

---------

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Co-authored-by: Max Ghenis <mghenis@gmail.com>

Author: 3 people

changelog.d/737.md

@@ -0,0 +1 @@
+- Add unit tests for the two CPS marriage tax reforms (`expanded_ma_reform` and `marriage_neutral_it_reform`) and expose module-level Reform instances for them so they can be referenced from YAML tests by dotted import path. Tests cover: Marriage Allowance denied to a single higher-rate earner under the expanded-MA reform, Marriage Allowance granted to a higher-rate earner with a low-earning spouse under the expanded-MA reform, and adjusted net income unchanged for a single person under the marriage-neutral IT reform.

policyengine_uk/reforms/cps/marriage_tax_reforms.py

@@ -227,3 +227,10 @@ def create_marriage_tax_reform(parameters, period):
             return it_reform
         else:
             return None
+
+
+# Module-level reform instances for use in YAML tests (reference by dotted import path).
+# These build the reform classes with no child-age / education conditions, so the reform
+# applies to every married couple regardless of child presence.
+expanded_ma_reform = create_expanded_ma_reform()
+marriage_neutral_it_reform = create_marriage_neutral_income_tax_reform()

policyengine_uk/tests/policy/reforms/cps/marriage_tax_reforms.yaml

@@ -0,0 +1,102 @@
+# CPS Marriage Tax Reforms (Centre for Policy Studies)
+# Covers two structural reforms:
+#   - expanded_ma_reform: extends Marriage Allowance eligibility to higher/additional
+#     band earners (normally ineligible under ITA 2007 s. 55B).
+#   - marriage_neutral_it_reform: allows married couples to have income tax computed
+#     on equally split income if that yields a lower combined liability.
+#
+# The module-level reform instances are built with no child-age / education filters,
+# so they apply to every married couple in these tests.
+
+- name: Expanded MA — single higher-rate earner still has no Marriage Allowance
+  period: 2024
+  absolute_error_margin: 1
+  reforms:
+    - policyengine_uk.reforms.cps.marriage_tax_reforms.expanded_ma_reform
+  input:
+    gov.hmrc.income_tax.allowances.marriage_allowance.takeup_rate: 1.0
+    people:
+      adult:
+        age: 35
+        employment_income: 60_000
+    benunits:
+      benunit:
+        members: [adult]
+        is_married: false
+    households:
+      household:
+        members: [adult]
+  output:
+    marriage_allowance: 0
+
+- name: Expanded MA — married higher-rate earner gets Marriage Allowance from low-earner spouse
+  period: 2024
+  absolute_error_margin: 10
+  reforms:
+    - policyengine_uk.reforms.cps.marriage_tax_reforms.expanded_ma_reform
+  input:
+    gov.hmrc.income_tax.allowances.marriage_allowance.takeup_rate: 1.0
+    people:
+      high_earner:
+        age: 35
+        employment_income: 60_000
+      low_earner:
+        age: 33
+        employment_income: 10_000
+    benunits:
+      benunit:
+        members: [high_earner, low_earner]
+        is_married: true
+    households:
+      household:
+        members: [high_earner, low_earner]
+  output:
+    # Low earner's unused PA (12,570 − 10,000 = 2,570) is capped at PA × ma_rate
+    # = 12,570 × 0.1 = 1,257, then rounded up to the nearest £10 → 1,260.
+    # Low earner's partner has no unused PA to transfer, so low_earner's MA is 0.
+    marriage_allowance: [1260, 0]
+
+- name: Marriage-neutral IT — single person's adjusted net income is unchanged by the reform
+  period: 2024
+  absolute_error_margin: 1
+  reforms:
+    - policyengine_uk.reforms.cps.marriage_tax_reforms.marriage_neutral_it_reform
+  input:
+    people:
+      adult:
+        age: 35
+        employment_income: 50_000
+    benunits:
+      benunit:
+        members: [adult]
+        is_married: false
+    households:
+      household:
+        members: [adult]
+  output:
+    # has_spouse is False → split_income == original income → reform returns the
+    # unadjusted net income (taxable_employment_income ≈ employment_income here).
+    adjusted_net_income: 50_000
+
+- name: Marriage-neutral IT — married couple splits unequal income
+  period: 2024
+  absolute_error_margin: 1
+  reforms:
+    - policyengine_uk.reforms.cps.marriage_tax_reforms.marriage_neutral_it_reform
+  input:
+    people:
+      high_earner:
+        age: 35
+        employment_income: 60_000
+      low_earner:
+        age: 33
+        employment_income: 10_000
+    benunits:
+      benunit:
+        members: [high_earner, low_earner]
+        is_married: true
+    households:
+      household:
+        members: [high_earner, low_earner]
+  output:
+    adjusted_net_income: [35_000, 35_000]