make implicit mysql index check working with any kind of index/constr…

…aint

django/db/backends/mysql/schema.py

@@ -172,7 +172,14 @@ def _create_missing_fk_index(
         first_field_name = None
         if fields:
             first_field_name = fields[0]
-        elif expressions:
+        elif (
+            expressions
+            and self.connection.features.supports_expression_indexes
+            # expression is a simple F-object
+            and isinstance(expressions[0], F)
+            # expression doesn't contain custom lookup (field__lookup)
+            and LOOKUP_SEP not in expressions[0].name
+        ):
             first_field_name = expressions[0].name
 
         if not first_field_name:

tests/migrations/test_operations.py

@@ -5416,6 +5416,96 @@ def assert_index_created(operations):
             ]
         )
 
+    def test_composed_func_indexes_dont_fail_implicit_fk_index_check(self):
+        """
+        Test that implicit FK index check works with any kind of index/constraint,
+        that shouldn't create any additional index.
+        See the tests above to find types of indexed that should do that.
+        """
+
+        app_label = "test_cfidcfkid"
+
+        create_author = migrations.CreateModel(
+            "Author",
+            [("id", models.BigAutoField(primary_key=True))],
+            options={},
+        )
+        create_book = migrations.CreateModel(
+            "Book",
+            [
+                ("id", models.BigAutoField(primary_key=True)),
+                ("author", models.ForeignKey(to="Author", on_delete=models.CASCADE)),
+                ("title", models.CharField(max_length=100)),
+            ],
+            options={},
+        )
+
+        project_state = self.apply_operations(
+            app_label,
+            ProjectState(),
+            [
+                create_author,
+                create_book,
+            ],
+        )
+
+        def get_author_field_indexes():
+            Book = project_state.apps.get_model(app_label, "Book")
+            columns = ["author_id"]
+            with connection.schema_editor() as editor:
+                return editor._constraint_names(Book, index=True, column_names=columns)
+
+        def assert_index_not_created(operations):
+            initial_indexes = get_author_field_indexes()
+            self.apply_operations(app_label, project_state, operations)
+            actual_indexes = get_author_field_indexes()
+            self.assertEqual(set(actual_indexes), set(initial_indexes))
+
+        desc_expression_index = models.Index(
+            models.F("author").desc(),
+            models.F("title"),
+            name="author_title_index",
+        )
+        assert_index_not_created(
+            [
+                migrations.AddIndex("Book", desc_expression_index),
+                migrations.RemoveIndex("Book", desc_expression_index.name),
+            ]
+        )
+
+        lookup_index = models.Index(models.F("title__lower"), name="title_index")
+        with register_lookup(models.CharField, Lower):
+            assert_index_not_created(
+                [
+                    migrations.AddIndex("Book", lookup_index),
+                    migrations.RemoveIndex("Book", lookup_index.name),
+                ]
+            )
+
+        if connection.features.supports_table_check_constraints:
+            check_constraint = models.CheckConstraint(
+                check=models.Q(title__contains="a"),
+                name="title_check",
+            )
+            assert_index_not_created(
+                [
+                    migrations.AddConstraint("Book", check_constraint),
+                    migrations.RemoveConstraint("Book", check_constraint.name),
+                ]
+            )
+
+        if connection.features.supports_index_column_ordering:
+            negative_index = models.Index(
+                fields=["-author", "title"],
+                name="author_title_index",
+            )
+            assert_index_not_created(
+                [
+                    migrations.AddIndex("Book", negative_index),
+                    migrations.RemoveIndex("Book", negative_index.name),
+                ]
+            )
+
     def test_composed_indexes_dont_create_dup_implicit_fk_index_when_deleting(self):
         """
         When we delete a composed index with a FK included, we shouldn't create the