[3.0.x] Fixed #30800 -- Fixed migrations crash when altering a field …

…with custom db_type().

Regression in 1378d66.
Backport of 580e644 from master

django/db/backends/postgresql/schema.py

@@ -42,7 +42,10 @@ def _field_indexes_sql(self, model, field):
     def _field_data_type(self, field):
         if field.is_relation:
             return field.rel_db_type(self.connection)
-        return self.connection.data_types[field.get_internal_type()]
+        return self.connection.data_types.get(
+            field.get_internal_type(),
+            field.db_type(self.connection),
+        )
 
     def _create_like_index_sql(self, model, field):
         """

tests/schema/tests.py

@@ -857,6 +857,26 @@ def test_alter_char_field_decrease_length(self):
             with self.assertRaisesMessage(DataError, msg):
                 editor.alter_field(Author, old_field, new_field, strict=True)
 
+    @unittest.skipUnless(connection.vendor == 'postgresql', 'PostgreSQL specific')
+    def test_alter_field_with_custom_db_type(self):
+        from django.contrib.postgres.fields import ArrayField
+
+        class Foo(Model):
+            field = ArrayField(CharField(max_length=255))
+
+            class Meta:
+                app_label = 'schema'
+
+        with connection.schema_editor() as editor:
+            editor.create_model(Foo)
+        self.isolated_local_models = [Foo]
+        old_field = Foo._meta.get_field('field')
+        new_field = ArrayField(CharField(max_length=16))
+        new_field.set_attributes_from_name('field')
+        new_field.model = Foo
+        with connection.schema_editor() as editor:
+            editor.alter_field(Foo, old_field, new_field, strict=True)
+
     def test_alter_textfield_to_null(self):
         """
         #24307 - Should skip an alter statement on databases with