WPA Pension Credit property test checks only the later Guarantee Credit amount #2652

policyengine_uk/tests/test_pension_credit_widowed_parents_allowance_properties.py, in test_guarantee_credit_falls_pound_for_pound_with_widowed_parents_allowance, checks that Guarantee Credit falls pound for pound with extra Pension Credit income wherever Guarantee Credit is paid. But its mask, paid = gc[..., 1:] > 0, looks only at the later amount.

Where Guarantee Credit starts from zero, it rises by less than the income falls. The independent review of #2645 (round 2, note N4) gives this case, calculated by hand rather than run:

• a single carer in England with £10,500 of IIDB;
• WPA going from £0 to £1,250, which displaces Carer's Allowance;
• Pension Credit income going from £14,995.40 to £14,475.40, against a minimum guarantee of £14,879.80;
• so Guarantee Credit goes from £0 to £404.40, while the assertion expects a £520 rise.

A Hypothesis draw like this would fail the test, although the model is right.

The fix is to require Guarantee Credit to be paid at both amounts, and to add the case as an explicit example.