Fix adaption in AnnotatedLabel; repair needless expense in coercion

Fixed regression involving clause adaption of labeled ORM compound
elements, such as single-table inheritance discriminator expressions with
conditionals or CASE expressions, which could cause aliased expressions
such as those used in ORM join / joinedload operations to not be adapted
correctly, such as referring to the wrong table in the ON clause in a join.

This change also improves a performance bump that was located within the
process of invoking :meth:`_sql.Select.join` given an ORM attribute
as a target.

Fixes: #6550
Change-Id: I98906476f0cce6f41ea00b77c789baa818e9d167

1 files changed, 28 insertions, 0 deletions

diff --git a/test/sql/test_external_traversal.py b/test/sql/test_external_traversal.py
index 9e829baea..3469dcb37 100644
--- a/test/sql/test_external_traversal.py
+++ b/test/sql/test_external_traversal.py
@@ -458,6 +458,34 @@ class ClauseTest(fixtures.TestBase, AssertsCompiledSQL):
             select(f), "SELECT t1_1.col1 * :col1_1 AS anon_1 FROM t1 AS t1_1"
         )
 
+    @testing.combinations(
+        (lambda t1: t1.c.col1, "t1_1.col1"),
+        (lambda t1: t1.c.col1 == "foo", "t1_1.col1 = :col1_1"),
+        (
+            lambda t1: case((t1.c.col1 == "foo", "bar"), else_=t1.c.col1),
+            "CASE WHEN (t1_1.col1 = :col1_1) THEN :param_1 ELSE t1_1.col1 END",
+        ),
+        argnames="case, expected",
+    )
+    @testing.combinations(False, True, argnames="label_")
+    @testing.combinations(False, True, argnames="annotate")
+    def test_annotated_label_cases(self, case, expected, label_, annotate):
+        """test #6550"""
+
+        t1 = table("t1", column("col1"))
+        a1 = t1.alias()
+
+        expr = case(t1=t1)
+
+        if label_:
+            expr = expr.label(None)
+        if annotate:
+            expr = expr._annotate({"foo": "bar"})
+
+        adapted = sql_util.ClauseAdapter(a1).traverse(expr)
+
+        self.assert_compile(adapted, expected)
+
     @testing.combinations((null(),), (true(),))
     def test_dont_adapt_singleton_elements(self, elem):
         """test :ticket:`6259`"""