Clear proxy_set cache when creating an annotated column

Fixed an unlikely issue where the "corresponding column" routine for unions
and other :class:`.CompoundSelect` objects could return the wrong column in
some overlapping column situtations, thus potentially impacting some ORM
operations when set operations are in use, if the underlying
:func:`.select` constructs were used previously in other similar kinds of
routines, due to a cached value not being cleared.

Fixes: #4747
Change-Id: I7fb134cac3604f8fe62e220fb24a0945d0a1c56f

3 files changed, 7 insertions, 8 deletions

diff --git a/lib/sqlalchemy/sql/elements.py b/lib/sqlalchemy/sql/elements.py
index cc57e58e5..aa7b7f688 100644
--- a/lib/sqlalchemy/sql/elements.py
+++ b/lib/sqlalchemy/sql/elements.py
@@ -635,6 +635,7 @@ class ColumnElement(
     __visit_name__ = "column_element"
     primary_key = False
     foreign_keys = []
+    _proxies = ()
 
     _label = None
     """The named label that can be used to target
@@ -783,16 +784,13 @@ class ColumnElement(
 
     @util.memoized_property
     def base_columns(self):
-        return util.column_set(
-            c for c in self.proxy_set if not hasattr(c, "_proxies")
-        )
+        return util.column_set(c for c in self.proxy_set if not c._proxies)
 
     @util.memoized_property
     def proxy_set(self):
         s = util.column_set([self])
-        if hasattr(self, "_proxies"):
-            for c in self._proxies:
-                s.update(c.proxy_set)
+        for c in self._proxies:
+            s.update(c.proxy_set)
         return s
 
     def shares_lineage(self, othercolumn):
@@ -4388,6 +4386,8 @@ class AnnotatedColumnElement(Annotated):
     def __init__(self, element, values):
         Annotated.__init__(self, element, values)
         ColumnElement.comparator._reset(self)
+        if self._proxies:
+            ColumnElement.proxy_set._reset(self)
         for attr in ("name", "key", "table"):
             if self.__dict__.get(attr, False) is None:
                 self.__dict__.pop(attr)
diff --git a/lib/sqlalchemy/sql/schema.py b/lib/sqlalchemy/sql/schema.py
index 62ff25a64..d39bc9832 100644
--- a/lib/sqlalchemy/sql/schema.py
+++ b/lib/sqlalchemy/sql/schema.py
@@ -1927,7 +1927,7 @@ class ForeignKey(DialectKWArgs, SchemaItem):
 
         parenttable = self.parent.table
 
-        # assertion, can be commented out.
+        # assertion
         # basically Column._make_proxy() sends the actual
         # target Column to the ForeignKey object, so the
         # string resolution here is never called.
diff --git a/lib/sqlalchemy/sql/selectable.py b/lib/sqlalchemy/sql/selectable.py
index e38de66e0..2c898e835 100644
--- a/lib/sqlalchemy/sql/selectable.py
+++ b/lib/sqlalchemy/sql/selectable.py
@@ -2759,7 +2759,6 @@ class CompoundSelect(GenerativeSelect):
             # to how low in the list of select()s the column occurs, so
             # that the corresponding_column() operation can resolve
             # conflicts
-
             proxy._proxies = [
                 c._annotate({"weight": i + 1}) for (i, c) in enumerate(cols)
             ]