replace Variant with direct feature inside of TypeEngine

The :meth:`_sqltypes.TypeEngine.with_variant` method now returns a copy of
the original :class:`_sqltypes.TypeEngine` object, rather than wrapping it
inside the ``Variant`` class, which is effectively removed (the import
symbol remains for backwards compatibility with code that may be testing
for this symbol). While the previous approach maintained in-Python
behaviors, maintaining the original type allows for clearer type checking
and debugging.

Fixes: #6980
Change-Id: I158c7e56306b886b5b82b040205c428a5c4a242c

1 files changed, 4 insertions, 0 deletions

diff --git a/lib/sqlalchemy/testing/assertions.py b/lib/sqlalchemy/testing/assertions.py
index 2acf15195..795b53804 100644
--- a/lib/sqlalchemy/testing/assertions.py
+++ b/lib/sqlalchemy/testing/assertions.py
@@ -515,6 +515,10 @@ class AssertsCompiledSQL:
                     if hasattr(test_statement, "_return_defaults"):
                         self._return_defaults = test_statement._return_defaults
 
+            @property
+            def _variant_mapping(self):
+                return self.test_statement._variant_mapping
+
             def _default_dialect(self):
                 return self.test_statement._default_dialect()