Use independent TypeVar for ColumnElement.cast

Fixed typing issue where :meth:`.ColumnElement.cast` did not allow a
:class:`.TypeEngine` argument independent of the type of the
:class:`.ColumnElement` itself, which is the purpose of
:meth:`.ColumnElement.cast`.

Fixes: #9451
Change-Id: I68119c6a9e8bf896715eea79be2b4f36b1c141de

3 files changed, 19 insertions, 1 deletions

diff --git a/doc/build/changelog/unreleased_20/9451.rst b/doc/build/changelog/unreleased_20/9451.rst
new file mode 100644
index 000000000..5ed6b7e94
--- /dev/null
+++ b/doc/build/changelog/unreleased_20/9451.rst
@@ -0,0 +1,8 @@
+.. change::
+    :tags: bug, typing
+    :tickets: 9451
+
+    Fixed typing issue where :meth:`.ColumnElement.cast` did not allow a
+    :class:`.TypeEngine` argument independent of the type of the
+    :class:`.ColumnElement` itself, which is the purpose of
+    :meth:`.ColumnElement.cast`.
diff --git a/lib/sqlalchemy/sql/elements.py b/lib/sqlalchemy/sql/elements.py
index a416b6ac0..ef4587e18 100644
--- a/lib/sqlalchemy/sql/elements.py
+++ b/lib/sqlalchemy/sql/elements.py
@@ -1635,7 +1635,7 @@ class ColumnElement(
             co._is_clone_of = selectable._is_clone_of.columns.get(key)
         return key, co
 
-    def cast(self, type_: _TypeEngineArgument[_T]) -> Cast[_T]:
+    def cast(self, type_: _TypeEngineArgument[_OPT]) -> Cast[_OPT]:
         """Produce a type cast, i.e. ``CAST(<expression> AS <type>)``.
 
         This is a shortcut to the :func:`_expression.cast` function.
diff --git a/test/ext/mypy/plain_files/sql_operations.py b/test/ext/mypy/plain_files/sql_operations.py
index c55442be9..d658f3d50 100644
--- a/test/ext/mypy/plain_files/sql_operations.py
+++ b/test/ext/mypy/plain_files/sql_operations.py
@@ -2,8 +2,11 @@ import typing
 
 from sqlalchemy import and_
 from sqlalchemy import Boolean
+from sqlalchemy import cast
 from sqlalchemy import column
+from sqlalchemy import DateTime
 from sqlalchemy import false
+from sqlalchemy import Float
 from sqlalchemy import func
 from sqlalchemy import Integer
 from sqlalchemy import or_
@@ -75,6 +78,13 @@ and_(c1.notlike("x"))
 and_(c1.not_ilike("x"))
 and_(c1.notilike("x"))
 
+# issue #9451
+s1 = c1.cast(Integer)
+s2 = c1.cast(Float)
+s3 = c1.op("foobar")("operand").cast(DateTime)
+s4 = cast(c1, Float)
+s5 = cast(c1.op("foobar")("operand"), DateTime)
+
 
 if typing.TYPE_CHECKING: