fix: eliminate MT federal income tax deduction for 2024+

changelog_entry.yaml

@@ -0,0 +1,4 @@
+- bump: patch
+  changes:
+    fixed:
+    - Eliminate Montana federal income tax deduction for 2024+, per Senate Bill 399.

policyengine_us/parameters/gov/states/mt/tax/income/deductions/itemized/federal_income_tax/applies.yaml

@@ -0,0 +1,16 @@
+description: Montana allows a deduction for federal income taxes paid as an itemized deduction, if this is true.
+metadata:
+  label: Montana federal income tax deduction applies
+  period: year
+  unit: bool
+  reference:
+    - title: Montana Tax Reform Update (2024)
+      href: https://mtrevenue.gov/montana-tax-simplification-resource-hub/
+    - title: Montana State Senate Bill 399 (2021)
+      href: https://archive.legmt.gov/bills/2021/sesslaws/ch0503.pdf
+    - title: 2024 Montana Form 2 Instructions (Line 2)
+      href: https://taxsim.nber.org/historical_state_tax_forms/MT/2024/Form_2_2024_Instrux.pdf
+
+values:
+  2021-01-01: true
+  2024-01-01: false

policyengine_us/tests/policy/baseline/gov/states/mt/tax/income/deductions/itemized/mt_federal_income_tax_deduction_unit.yaml

@@ -54,3 +54,24 @@
         state_code: MT
   output:
     mt_federal_income_tax_deduction_unit: 7_000
+
+- name: MT 2024 joint filers should not get federal income tax deduction
+  period: 2024
+  absolute_error_margin: 1
+  input:
+    people:
+      person1:
+        is_tax_unit_head: true
+      person2:
+        is_tax_unit_spouse: true
+    tax_units:
+      tax_unit:
+        members: [person1, person2]
+        filing_status: JOINT
+        income_tax_before_refundable_credits: 10_000
+    households:
+      household:
+        members: [person1, person2]
+        state_code: MT
+  output:
+    mt_federal_income_tax_deduction_unit: 0

policyengine_us/variables/gov/states/mt/tax/income/deductions/itemized/federal_tax/mt_federal_income_tax_deduction_unit.py

@@ -14,10 +14,13 @@ class mt_federal_income_tax_deduction_unit(Variable):
     defined_for = StateCode.MT
 
     def formula(tax_unit, period, parameters):
-        filing_status = tax_unit("filing_status", period)
         p = parameters(
             period
         ).gov.states.mt.tax.income.deductions.itemized.federal_income_tax
+        # Federal income tax deduction eliminated in 2024 under SB 399
+        if not p.applies:
+            return 0
+        filing_status = tax_unit("filing_status", period)
         federal_income_tax_limit = p.cap[filing_status]
 
         federal_income_tax = tax_unit(