[FIX] account: check exchange rate

An issue occurs in the following situation:
- Define a currency exchange rate at day 1 and day 2
- Create an invoice at day 1, and calculate the taxes. Do not set an
  invoice date!
- Validate the invoice at day 2

The exchange rate for taxes is the rate at day 1, while the exchange
rate for other amounts is the rate at day 2.

There is actually no way to know what was the rate applied for the
taxes at invoice validation. There are two solutions:
- recompute the taxes at validation
- force the user to set an invoice date and recompute manually the
  taxes

The first solution might have unexpected effects, therefore the second
solution is applied.

Fixes #13473
opw-688517

addons/account/account_invoice.py

@@ -807,11 +807,20 @@ class account_invoice(models.Model):
 
             ctx = dict(self._context, lang=inv.partner_id.lang)
 
+            company_currency = inv.company_id.currency_id
             if not inv.date_invoice:
+                # FORWARD-PORT UP TO SAAS-6
+                if inv.currency_id != company_currency and inv.tax_line:
+                    raise except_orm(
+                        _('Warning!'),
+                        _('No invoice date!'
+                            '\nThe invoice currency is not the same than the company currency.'
+                            ' An invoice date is required to determine the exchange rate to apply. Do not forget to update the taxes!'
+                        )
+                    )
                 inv.with_context(ctx).write({'date_invoice': fields.Date.context_today(self)})
             date_invoice = inv.date_invoice
 
-            company_currency = inv.company_id.currency_id
             # create the analytical lines, one move line per invoice line
             iml = inv._get_analytic_lines()
             # check if taxes are all computed