[1.8.x] Fixed #25517 -- Made Concat function idempotent on SQLite.

Backport of 6c95b134e9 from master

tests/db_functions/tests.py

@@ -1,8 +1,11 @@
 from __future__ import unicode_literals
 
+from unittest import skipUnless
+
+from django.db import connection
 from django.db.models import CharField, TextField, Value as V
 from django.db.models.functions import (
-    Coalesce, Concat, Length, Lower, Substr, Upper,
+    Coalesce, Concat, ConcatPair, Length, Lower, Substr, Upper,
 )
 from django.test import TestCase
 from django.utils import six, timezone
@@ -154,6 +157,19 @@ class FunctionTests(TestCase):
         expected = article.title + ' - ' + article.text
         self.assertEqual(expected.upper(), article.title_text)
 
+    @skipUnless(connection.vendor == 'sqlite', "sqlite specific implementation detail.")
+    def test_concat_coalesce_idempotent(self):
+        pair = ConcatPair(V('a'), V('b'))
+        # Check nodes counts
+        self.assertEqual(len(list(pair.flatten())), 3)
+        self.assertEqual(len(list(pair.coalesce().flatten())), 7)  # + 2 Coalesce + 2 Value()
+        self.assertEqual(len(list(pair.flatten())), 3)
+
+    def test_concat_sql_generation_idempotency(self):
+        qs = Article.objects.annotate(description=Concat('title', V(': '), 'summary'))
+        # Multiple compilations should not alter the generated query.
+        self.assertEqual(str(qs.query), str(qs.all().query))
+
     def test_lower(self):
         Author.objects.create(name='John Smith', alias='smithj')
         Author.objects.create(name='Rhonda')