Parametrise Marriage Allowance eligible bands and fix NONE case (#1624)

* Parametrise Marriage Allowance eligible bands and fix NONE case

Fixes #395. The previous formula hard-coded BASIC / STARTER /
INTERMEDIATE as the eligible tax bands for transferring Marriage
Allowance. Replaces that with a list parameter
gov.hmrc.income_tax.allowances.marriage_allowance.eligible_bands so the
set is declarative, reform-friendly, and documents itself against ITA
2007 s.55B.

Also adds NONE to the default list. Someone whose taxable income sits
below the personal allowance is band NONE, which the old check
rejected — but ITA 2007 s.55B(1)(a) only excludes transferors who are
liable at the higher rate or above, so a zero-tax spouse is eligible
to transfer their unused allowance (this is a common motivation for
electing MA in the first place).

Adds regression YAML cases for NONE, STARTER, and INTERMEDIATE bands.
The existing BASIC / HIGHER / ADDITIONAL assertions are unchanged.

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

* Route CPS expanded-MA reform through the eligible_bands parameter

The CPS expanded-Marriage-Allowance reform was overriding
meets_marriage_allowance_income_conditions with its own hard-coded
BASIC | STARTER | INTERMEDIATE check (plus the HIGHER/ADDITIONAL
expansion), so the baseline parametrisation and the NONE fix from
291774a did not reach simulations running under that reform.

Replace the hard-coded base with the same np.isin(band.decode_to_str(),
eligible_bands) lookup used in the baseline variable, then OR the
existing HIGHER/ADDITIONAL expansion on top. Both code paths now read
the parameter, and the NONE-band eligibility flows through to reform
runs for free.

Also adjust the changelog fragment to mention both call sites.

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

---------

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

Author: vahid-ahmadi

changelog.d/395.md

@@ -0,0 +1 @@
+- Parametrise the Marriage Allowance transferor-eligible tax bands as `gov.hmrc.income_tax.allowances.marriage_allowance.eligible_bands` and include `NONE` (below-personal-allowance earners) in the default list, matching ITA 2007 s.55B — a person with no taxable income is still eligible to transfer their unused allowance and was previously rejected by the hard-coded check. The CPS expanded-Marriage-Allowance reform now also consumes the parameter, so both code paths agree on the baseline eligible-band set.

policyengine_uk/parameters/gov/hmrc/income_tax/allowances/marriage_allowance/eligible_bands.yaml

@@ -0,0 +1,14 @@
+description: Tax bands whose members may transfer personal allowance under the Marriage Allowance election (ITA 2007 s.55B). A transferor is eligible if they are not liable at the higher rate or above; the Scottish intermediate rate is treated as within the basic-rate envelope for this purpose.
+values:
+  0000-01-01:
+    - NONE
+    - STARTER
+    - BASIC
+    - INTERMEDIATE
+metadata:
+  unit: list
+  label: Marriage Allowance eligible tax bands
+  period: year
+  reference:
+    - title: "Income Tax Act 2007, s. 55B"
+      href: https://www.legislation.gov.uk/ukpga/2007/3/section/55B

policyengine_uk/reforms/cps/marriage_tax_reforms.py

@@ -34,18 +34,18 @@ class meets_marriage_allowance_income_conditions(Variable):
         value_type = bool
         reference = "https://www.legislation.gov.uk/ukpga/2007/3/section/55B"
 
-        def formula(person, period):
+        def formula(person, period, parameters):
             band = person("tax_band", period)
+            eligible_bands = parameters(
+                period
+            ).gov.hmrc.income_tax.allowances.marriage_allowance.eligible_bands
+            base_eligible = np.isin(band.decode_to_str(), eligible_bands)
+            # Expand to higher bands if the reform's expansion conditions are met.
             bands = band.possible_values
-            return (
-                (band == bands.BASIC)
-                | (band == bands.STARTER)
-                | (band == bands.INTERMEDIATE)
-                | (  # Expand to higher bands if reform conditions are met.
-                    person("meets_expanded_ma_conditions", period)
-                    & ((band == bands.HIGHER) | (band == bands.ADDITIONAL))
-                )
+            expansion = person("meets_expanded_ma_conditions", period) & (
+                (band == bands.HIGHER) | (band == bands.ADDITIONAL)
             )
+            return base_eligible | expansion
 
     class marriage_allowance(Variable):
         value_type = float

policyengine_uk/tests/policy/baseline/gov/hmrc/income_tax/allowances/meets_marriage_allowance_income_conditions.yaml

@@ -18,3 +18,24 @@
     tax_band: ADDITIONAL
   output:
     meets_marriage_allowance_income_conditions: false
+
+- name: Below-PA earner (band NONE) is eligible for Marriage Allowance (regression for #395)
+  period: 2021
+  input:
+    tax_band: NONE
+  output:
+    meets_marriage_allowance_income_conditions: true
+
+- name: Scottish starter rate band is eligible for Marriage Allowance
+  period: 2021
+  input:
+    tax_band: STARTER
+  output:
+    meets_marriage_allowance_income_conditions: true
+
+- name: Scottish intermediate rate band is eligible for Marriage Allowance
+  period: 2021
+  input:
+    tax_band: INTERMEDIATE
+  output:
+    meets_marriage_allowance_income_conditions: true

policyengine_uk/variables/gov/hmrc/income_tax/allowances/meets_marriage_allowance_income_conditions.py

@@ -1,5 +1,5 @@
 from policyengine_uk.model_api import *
-from numpy import ceil
+import numpy as np
 
 
 class meets_marriage_allowance_income_conditions(Variable):
@@ -10,11 +10,9 @@ class meets_marriage_allowance_income_conditions(Variable):
     value_type = bool
     reference = "https://www.legislation.gov.uk/ukpga/2007/3/section/55B"
 
-    def formula(person, period):
+    def formula(person, period, parameters):
         band = person("tax_band", period)
-        bands = band.possible_values
-        return (
-            (band == bands.BASIC)
-            | (band == bands.STARTER)
-            | (band == bands.INTERMEDIATE)
-        )
+        eligible_bands = parameters(
+            period
+        ).gov.hmrc.income_tax.allowances.marriage_allowance.eligible_bands
+        return np.isin(band.decode_to_str(), eligible_bands)