Ensures billing address is present if it is required

Can be controlled via Spree::Config.billing_address_required config flag
globally or it can be coming from individual payment method, e.g. it can
be disabled by default but required if at least one payment method
requires billing address, and this payment method is gotten from
order available payment methods allowing flexibility to check only
where it is really needed

core/app/models/spree/order.rb

@@ -562,6 +562,18 @@ def ensure_shipping_address
       end
     end
 
+    def ensure_billing_address
+      return unless billing_address_required?
+      return if bill_address&.valid?
+
+      errors.add(:base, I18n.t('spree.bill_address_required'))
+      false
+    end
+
+    def billing_address_required?
+      available_payment_methods.detect {|pm| pm.billing_address_required? }.present?
+    end
+
     def create_proposed_shipments
       if completed?
         raise CannotRebuildShipments.new(I18n.t('spree.cannot_rebuild_shipments_order_completed'))

core/app/models/spree/order/checkout.rb

@@ -82,6 +82,7 @@ def define_state_machine!
 
               after_transition to: :complete, do: :add_payment_sources_to_wallet
               before_transition to: :payment, do: :add_default_payment_from_wallet
+              before_transition to: :payment, do: :ensure_billing_address
 
               before_transition to: :confirm, do: :add_store_credit_payments
 

core/config/locales/en.yml

@@ -1984,6 +1984,7 @@ en:
     ship: ship
     ship_address: Ship Address
     ship_address_required: Valid shipping address required
+    bill_address_required: Valid billing address required
     ship_total: Ship Total
     shipment: Shipment
     shipment_adjustments: Shipment adjustments

core/spec/models/spree/order/checkout_spec.rb

@@ -303,6 +303,15 @@ def assert_state_changed(order, from, to)
           assert_state_changed(order, 'delivery', 'payment')
           expect(order.state).to eq('payment')
         end
+
+        it 'fails if billing address is required and missing' do
+          payment_method = create(:payment_method)
+          allow(payment_method).to receive(:billing_address_required?).and_return(true)
+          allow(order).to receive(:available_payment_methods).and_return([payment_method])
+          order.bill_address = nil
+
+          expect { order.next! }.to raise_error(StateMachines::InvalidTransition, /#{I18n.t('spree.bill_address_required')}/)
+        end
       end
 
       context "without payment required" do