Convert line quantity into product default unit to compare with forecast

The forecast quantity on product is always in the default unit. So to make the
same test as on check stock quantity, we need to convert the line quantity
into the default product unit.

Closes #11958

modules/sale_stock_quantity/sale.py

@@ -211,6 +211,7 @@ class Line(metaclass=PoolMeta):
         Lang = pool.get('ir.lang')
         Move = pool.get('stock.move')
         Product = pool.get('product.product')
+        UoM = pool.get('product.uom')
         lang = Lang.get()
         if (self.sale_state == 'draft'
                 and self.sale
@@ -226,16 +227,18 @@ class Line(metaclass=PoolMeta):
             locations = [self.sale.warehouse.id]
             with Transaction().set_context(
                     locations=locations,
-                    stock_date_end=sale_date,
-                    uom=self.unit):
+                    stock_date_end=sale_date):
                 product = Product(self.product.id)
-                if product.forecast_quantity < self.quantity:
+                quantity = UoM.compute_qty(
+                    self.unit, self.quantity,
+                    product.default_uom, round=False)
+                if product.forecast_quantity < quantity:
                     yield ('warning', gettext(
                             'sale_stock_quantity'
                             '.msg_product_forecast_quantity_lower',
                             forecast_quantity=lang.format_number_symbol(
-                                product.forecast_quantity, self.unit,
-                                self.unit.digits),
+                                product.forecast_quantity, product.default_uom,
+                                product.default_uom.digits),
                             product=self.product.rec_name,
                             quantity=lang.format_number_symbol(
                                 self.quantity, self.unit,