Use equality operators when stringify in domain clause with singleton

Closes #13557

sao/src/common.js

@@ -1298,10 +1298,18 @@
                         .replace('not', '!').trim();
                 }
                 if (operator.endsWith('in')) {
-                    if (operator == 'not in') {
-                        operator = '!';
+                    if (value instanceof Array && value.length == 1) {
+                        if (operator == 'not in') {
+                            operator = '!=';
+                        } else {
+                            operator = '=';
+                        }
                     } else {
-                        operator = '';
+                        if (operator == 'not in') {
+                            operator = '!';
+                        } else {
+                            operator = '';
+                        }
                     }
                 }
                 var formatted_value = this.format_value(field, value, target);

sao/tests/sao.js

@@ -2318,7 +2318,9 @@
         [[['name', 'ilike', 'Doe\\%']], 'Name: =Doe%'],
         [[['name', 'not ilike', '%Doe%']], 'Name: !Doe'],
         [[['name', 'in', ['John', 'Jane']]], 'Name: John;Jane'],
+        [[['name', 'in', ['John']]], 'Name: =John'],
         [[['name', 'not in', ['John', 'Jane']]], 'Name: !John;Jane'],
+        [[['name', 'not in', ['John']]], 'Name: !=John'],
         [[
             ['name', 'ilike', '%Doe%'],
             ['name', 'ilike', '%Jane%']

tryton/tryton/common/domain_parser.py

@@ -647,10 +647,16 @@ class DomainParser(object):
                 operator = operator.rstrip(def_operator
                     ).replace('not', '!').strip()
             if operator.endswith('in'):
-                if operator == 'not in':
-                    operator = '!'
+                if isinstance(value, (list, tuple)) and len(value) == 1:
+                    if operator == 'not in':
+                        operator = '!='
+                    else:
+                        operator = '='
                 else:
-                    operator = ''
+                    if operator == 'not in':
+                        operator = '!'
+                    else:
+                        operator = ''
             formatted_value = format_value(field, value, target, self.context)
             if (operator in OPERATORS
                     and field['type'] in ('char', 'text', 'selection')

tryton/tryton/tests/test_common_domain_parser.py

@@ -681,9 +681,13 @@ class DomainParserTestCase(TestCase):
             dom.string([('name', 'not ilike', '%Doe%')]), 'Name: !Doe')
         self.assertEqual(
             dom.string([('name', 'in', ['John', 'Jane'])]), 'Name: John;Jane')
+        self.assertEqual(
+            dom.string([('name', 'in', ['John'])]), 'Name: =John')
         self.assertEqual(
             dom.string([('name', 'not in', ['John', 'Jane'])]),
             'Name: !John;Jane')
+        self.assertEqual(
+            dom.string([('name', 'not in', ['John'])]), 'Name: !=John')
         self.assertEqual(
             dom.string([
                     ('name', 'ilike', '%Doe%'),