[5.1.x] Fixed #35469 -- Removed deferred SQL to create index removed by AlterField operation.

Backport of 99f23eaabd8da653f046dc1d19f5008c030a4f79 from main.

4 files changed, 70 insertions, 9 deletions

diff --git a/django/db/backends/base/schema.py b/django/db/backends/base/schema.py
index 38136e7213..e5f28d9c6a 100644
--- a/django/db/backends/base/schema.py
+++ b/django/db/backends/base/schema.py
@@ -1582,12 +1582,23 @@ class BaseDatabaseSchemaEditor:
         )
 
     def _delete_index_sql(self, model, name, sql=None):
-        return Statement(
+        statement = Statement(
             sql or self.sql_delete_index,
             table=Table(model._meta.db_table, self.quote_name),
             name=self.quote_name(name),
         )
 
+        # Remove all deferred statements referencing the deleted index.
+        table_name = statement.parts["table"].table
+        index_name = statement.parts["name"]
+        for sql in list(self.deferred_sql):
+            if isinstance(sql, Statement) and sql.references_index(
+                table_name, index_name
+            ):
+                self.deferred_sql.remove(sql)
+
+        return statement
+
     def _rename_index_sql(self, model, old_name, new_name):
         return Statement(
             self.sql_rename_index,
diff --git a/django/db/backends/ddl_references.py b/django/db/backends/ddl_references.py
index 75787ef8ab..cb8d2defd2 100644
--- a/django/db/backends/ddl_references.py
+++ b/django/db/backends/ddl_references.py
@@ -21,6 +21,12 @@ class Reference:
         """
         return False
 
+    def references_index(self, table, index):
+        """
+        Return whether or not this instance references the specified index.
+        """
+        return False
+
     def rename_table_references(self, old_table, new_table):
         """
         Rename all references to the old_name to the new_table.
@@ -52,6 +58,9 @@ class Table(Reference):
     def references_table(self, table):
         return self.table == table
 
+    def references_index(self, table, index):
+        return self.references_table(table) and str(self) == index
+
     def rename_table_references(self, old_table, new_table):
         if self.table == old_table:
             self.table = new_table
@@ -207,6 +216,12 @@ class Statement(Reference):
             for part in self.parts.values()
         )
 
+    def references_index(self, table, index):
+        return any(
+            hasattr(part, "references_index") and part.references_index(table, index)
+            for part in self.parts.values()
+        )
+
     def rename_table_references(self, old_table, new_table):
         for part in self.parts.values():
             if hasattr(part, "rename_table_references"):
diff --git a/tests/backends/test_ddl_references.py b/tests/backends/test_ddl_references.py
index 86984ed3e8..8975b97124 100644
--- a/tests/backends/test_ddl_references.py
+++ b/tests/backends/test_ddl_references.py
@@ -166,10 +166,13 @@ class ForeignKeyNameTests(IndexNameTests):
 
 
 class MockReference:
-    def __init__(self, representation, referenced_tables, referenced_columns):
+    def __init__(
+        self, representation, referenced_tables, referenced_columns, referenced_indexes
+    ):
         self.representation = representation
         self.referenced_tables = referenced_tables
         self.referenced_columns = referenced_columns
+        self.referenced_indexes = referenced_indexes
 
     def references_table(self, table):
         return table in self.referenced_tables
@@ -177,6 +180,9 @@ class MockReference:
     def references_column(self, table, column):
         return (table, column) in self.referenced_columns
 
+    def references_index(self, table, index):
+        return (table, index) in self.referenced_indexes
+
     def rename_table_references(self, old_table, new_table):
         if old_table in self.referenced_tables:
             self.referenced_tables.remove(old_table)
@@ -195,32 +201,43 @@ class MockReference:
 class StatementTests(SimpleTestCase):
     def test_references_table(self):
         statement = Statement(
-            "", reference=MockReference("", {"table"}, {}), non_reference=""
+            "", reference=MockReference("", {"table"}, {}, {}), non_reference=""
         )
         self.assertIs(statement.references_table("table"), True)
         self.assertIs(statement.references_table("other"), False)
 
     def test_references_column(self):
         statement = Statement(
-            "", reference=MockReference("", {}, {("table", "column")}), non_reference=""
+            "",
+            reference=MockReference("", {}, {("table", "column")}, {}),
+            non_reference="",
         )
         self.assertIs(statement.references_column("table", "column"), True)
         self.assertIs(statement.references_column("other", "column"), False)
 
+    def test_references_index(self):
+        statement = Statement(
+            "",
+            reference=MockReference("", {}, {}, {("table", "index")}),
+            non_reference="",
+        )
+        self.assertIs(statement.references_index("table", "index"), True)
+        self.assertIs(statement.references_index("other", "index"), False)
+
     def test_rename_table_references(self):
-        reference = MockReference("", {"table"}, {})
+        reference = MockReference("", {"table"}, {}, {})
         statement = Statement("", reference=reference, non_reference="")
         statement.rename_table_references("table", "other")
         self.assertEqual(reference.referenced_tables, {"other"})
 
     def test_rename_column_references(self):
-        reference = MockReference("", {}, {("table", "column")})
+        reference = MockReference("", {}, {("table", "column")}, {})
         statement = Statement("", reference=reference, non_reference="")
         statement.rename_column_references("table", "column", "other")
         self.assertEqual(reference.referenced_columns, {("table", "other")})
 
     def test_repr(self):
-        reference = MockReference("reference", {}, {})
+        reference = MockReference("reference", {}, {}, {})
         statement = Statement(
             "%(reference)s - %(non_reference)s",
             reference=reference,
@@ -229,7 +246,7 @@ class StatementTests(SimpleTestCase):
         self.assertEqual(repr(statement), "<Statement 'reference - non_reference'>")
 
     def test_str(self):
-        reference = MockReference("reference", {}, {})
+        reference = MockReference("reference", {}, {}, {})
         statement = Statement(
             "%(reference)s - %(non_reference)s",
             reference=reference,
diff --git a/tests/indexes/tests.py b/tests/indexes/tests.py
index 107703c39a..0c4158a886 100644
--- a/tests/indexes/tests.py
+++ b/tests/indexes/tests.py
@@ -3,7 +3,7 @@ from unittest import skipUnless
 
 from django.conf import settings
 from django.db import connection
-from django.db.models import CASCADE, ForeignKey, Index, Q
+from django.db.models import CASCADE, CharField, ForeignKey, Index, Q
 from django.db.models.functions import Lower
 from django.test import (
     TestCase,
@@ -87,6 +87,24 @@ class SchemaIndexesTests(TestCase):
             str(index.create_sql(Article, editor)),
         )
 
+    @skipUnlessDBFeature("can_create_inline_fk", "can_rollback_ddl")
+    def test_alter_field_unique_false_removes_deferred_sql(self):
+        field_added = CharField(max_length=127, unique=True)
+        field_added.set_attributes_from_name("charfield_added")
+
+        field_to_alter = CharField(max_length=127, unique=True)
+        field_to_alter.set_attributes_from_name("charfield_altered")
+        altered_field = CharField(max_length=127, unique=False)
+        altered_field.set_attributes_from_name("charfield_altered")
+
+        with connection.schema_editor() as editor:
+            editor.add_field(ArticleTranslation, field_added)
+            editor.add_field(ArticleTranslation, field_to_alter)
+            self.assertEqual(len(editor.deferred_sql), 2)
+            editor.alter_field(ArticleTranslation, field_to_alter, altered_field)
+            self.assertEqual(len(editor.deferred_sql), 1)
+            self.assertIn("charfield_added", str(editor.deferred_sql[0].parts["name"]))
+
 
 class SchemaIndexesNotPostgreSQLTests(TransactionTestCase):
     available_apps = ["indexes"]