Prevent __init__ from being called for Alias, subclasses

The :class:`.Alias` class and related subclasses :class:`.CTE`,
:class:`.Lateral` and :class:`.TableSample` have been reworked so that it is
not possible for a user to construct the objects directly.  These constructs
require that the standalone construction function or selectable-bound method
be used to instantiate new objects.

Fixes: #4509
Change-Id: I74ae4786cb3ae625dab33b00bfd6bdc4e1219139

1 files changed, 13 insertions, 0 deletions

diff --git a/test/sql/test_selectable.py b/test/sql/test_selectable.py
index 04c0e6102..a4d3e1b40 100644
--- a/test/sql/test_selectable.py
+++ b/test/sql/test_selectable.py
@@ -28,6 +28,7 @@ from sqlalchemy import type_coerce
 from sqlalchemy import TypeDecorator
 from sqlalchemy import union
 from sqlalchemy import util
+from sqlalchemy.sql import Alias
 from sqlalchemy.sql import column
 from sqlalchemy.sql import elements
 from sqlalchemy.sql import expression
@@ -871,6 +872,18 @@ class SelectableTest(
         Table("t1", MetaData(), c1)
         eq_(c1._label, "t1_c1")
 
+    def test_no_alias_construct(self):
+        a = table("a", column("x"))
+
+        assert_raises_message(
+            NotImplementedError,
+            "The Alias class is not intended to be constructed directly.  "
+            r"Please use the alias\(\) standalone function",
+            Alias,
+            a,
+            "foo",
+        )
+
 
 class RefreshForNewColTest(fixtures.TestBase):
     def test_join_uninit(self):