[FIX] web: display daterange separator in readonly

Before this commit, a forward-ported fix regarding the display of the
arrow separator in daterange fields has been incorrectly adapted to fit
conflicting changes, and the separator does not appear in readonly with
both date values set.

This commit fixes that by restoring the proper conditions.

closes odoo/odoo#128509

Signed-off-by: Géry Debongnie <ged@odoo.com>

addons/web/static/src/views/fields/datetime/datetime_field.js

@@ -213,8 +213,8 @@ export class DateTimeField extends Component {
     shouldShowSeparator() {
         return (
             this.state.range &&
-            this.props.required &&
-            (this.isEmpty(this.startDateField) || this.isEmpty(this.endDateField))
+            (this.props.required ||
+                (!this.isEmpty(this.startDateField) && !this.isEmpty(this.endDateField)))
         );
     }
 

addons/web/static/tests/views/fields/daterange_field_tests.js

@@ -1074,6 +1074,27 @@ QUnit.module("Fields", (hooks) => {
         assert.containsNone(target, ".o_add_date");
     });
 
+    QUnit.test("related start date, required, both start date and end date set", async (assert) => {
+        serverData.models.partner.fields.date_end = { string: "Some Date", type: "date" };
+        const [firstRecord] = serverData.models.partner.records;
+        firstRecord.date_end = firstRecord.date;
+
+        await makeView({
+            type: "form",
+            resModel: "partner",
+            serverData,
+            arch: /* xml */ `
+                <form>
+                    <field name="date" widget="daterange" options="{'start_date_field': 'date_end'}" required="1" />
+                </form>`,
+            resId: 1,
+        });
+
+        assert.strictEqual(getInputs()[0].value, "02/03/2017");
+        assert.containsOnce(target, ".fa-long-arrow-right");
+        assert.strictEqual(getInputs()[1].value, "02/03/2017");
+    });
+
     QUnit.test("list daterange with start date and empty end date", async (assert) => {
         serverData.models.partner.fields.date_end = { string: "Some Date", type: "date" };