check for TypeDecorator when handling getitem

Fixed issue where :class:`.TypeDecorator` would not correctly proxy the
``__getitem__()`` operator when decorating the :class:`.ARRAY` datatype,
without explicit workarounds.

Fixes: #7249
Change-Id: I3273572b4757e41fb5952639cb867314227d368a

2 files changed, 27 insertions, 2 deletions

diff --git a/lib/sqlalchemy/sql/default_comparator.py b/lib/sqlalchemy/sql/default_comparator.py
index 512fca8d0..619be2cd1 100644
--- a/lib/sqlalchemy/sql/default_comparator.py
+++ b/lib/sqlalchemy/sql/default_comparator.py
@@ -232,7 +232,11 @@ def _in_impl(
 def _getitem_impl(
     expr: ColumnElement[Any], op: OperatorType, other: Any, **kw: Any
 ) -> ColumnElement[Any]:
-    if isinstance(expr.type, type_api.INDEXABLE):
+    if (
+        isinstance(expr.type, type_api.INDEXABLE)
+        or isinstance(expr.type, type_api.TypeDecorator)
+        and isinstance(expr.type.impl_instance, type_api.INDEXABLE)
+    ):
         other = coercions.expect(
             roles.BinaryElementRole, other, expr=expr, operator=op
         )
diff --git a/lib/sqlalchemy/sql/type_api.py b/lib/sqlalchemy/sql/type_api.py
index efaf5d2a7..6c1a99daa 100644
--- a/lib/sqlalchemy/sql/type_api.py
+++ b/lib/sqlalchemy/sql/type_api.py
@@ -1551,8 +1551,11 @@ class TypeDecorator(SchemaEventTarget, ExternalType, TypeEngine[_T]):
        the default rules of :meth:`.TypeEngine.coerce_compared_value` should
        be used in order to deal with operators like index operations::
 
+            from sqlalchemy import JSON
+            from sqlalchemy import TypeDecorator
+
             class MyJsonType(TypeDecorator):
-                impl = postgresql.JSON
+                impl = JSON
 
                 cache_ok = True
 
@@ -1562,6 +1565,24 @@ class TypeDecorator(SchemaEventTarget, ExternalType, TypeEngine[_T]):
        Without the above step, index operations such as ``mycol['foo']``
        will cause the index value ``'foo'`` to be JSON encoded.
 
+       Similarly, when working with the :class:`.ARRAY` datatype, the
+       type coercion for index operations (e.g. ``mycol[5]``) is also
+       handled by :meth:`.TypeDecorator.coerce_compared_value`, where
+       again a simple override is sufficient unless special rules are needed
+       for particular operators::
+
+            from sqlalchemy import ARRAY
+            from sqlalchemy import TypeDecorator
+
+            class MyArrayType(TypeDecorator):
+                impl = ARRAY
+
+                cache_ok = True
+
+                def coerce_compared_value(self, op, value):
+                    return self.impl.coerce_compared_value(op, value)
+
+
     """
 
     __visit_name__ = "type_decorator"