[5.1.x] Fixed #35732 -- Wrapped ConcatPair expression in parentheses to ensure operator precedence.

When ConcatPair was updated to use || this lost the implicit wrapping from CONCAT(...).
This broke the WHERE clauses when used in combination with PostgreSQL trigram similarity.

Regression in 6364b6ee1071381eb3a23ba6b821fc0d6f0fce75.

Backport of c3ca6075cc0ad425bcf905fe14062f38eb9fbcbf from main.

Co-authored-by: Emiliano Cuenca <106986074+emicuencac@users.noreply.github.com>

3 files changed, 22 insertions, 2 deletions

diff --git a/django/db/models/functions/text.py b/django/db/models/functions/text.py
index df826ffdb5..9c48659bf9 100644
--- a/django/db/models/functions/text.py
+++ b/django/db/models/functions/text.py
@@ -78,7 +78,7 @@ class ConcatPair(Func):
         return super(ConcatPair, coalesced).as_sql(
             compiler,
             connection,
-            template="%(expressions)s",
+            template="(%(expressions)s)",
             arg_joiner=" || ",
             **extra_context,
         )
diff --git a/docs/releases/5.1.2.txt b/docs/releases/5.1.2.txt
index 234c9e0bd5..1cf3f9df09 100644
--- a/docs/releases/5.1.2.txt
+++ b/docs/releases/5.1.2.txt
@@ -9,4 +9,6 @@ Django 5.1.2 fixes several bugs in 5.1.1.
 Bugfixes
 ========
 
-* ...
+* Fixed a regression in Django 5.1 that caused a crash when using the
+  PostgreSQL lookup :lookup:`trigram_similar` on output fields from ``Concat``
+  (:ticket:`35732`).
diff --git a/tests/postgres_tests/test_trigram.py b/tests/postgres_tests/test_trigram.py
index 812403a324..b6c88c38a6 100644
--- a/tests/postgres_tests/test_trigram.py
+++ b/tests/postgres_tests/test_trigram.py
@@ -1,3 +1,6 @@
+from django.db.models import F, Value
+from django.db.models.functions import Concat
+
 from . import PostgreSQLTestCase
 from .models import CharFieldModel, TextFieldModel
 
@@ -149,6 +152,21 @@ class TrigramTest(PostgreSQLTestCase):
             ],
         )
 
+    def test_trigram_concat_precedence(self):
+        search_term = "im matthew"
+        self.assertSequenceEqual(
+            self.Model.objects.annotate(
+                concat_result=Concat(
+                    Value("I'm "),
+                    F("field"),
+                    output_field=self.Model._meta.get_field("field"),
+                ),
+            )
+            .filter(concat_result__trigram_similar=search_term)
+            .values("field"),
+            [{"field": "Matthew"}],
+        )
+
 
 class TrigramTextFieldTest(TrigramTest):
     """