assign variant mapping on adapt()

Fixed regression from the 1.4 series where the refactor of the
:meth:`_types.TypeEngine.with_variant` method introduced at
:ref:`change_6980` failed to accommodate for the ``.copy()`` method, which
will lose the variant mappings that are set up. This becomes an issue for
the very specific case of a "schema" type, which includes types such as
:class:`.Enum` and :class:`.ARRAY`, when they are then used in the context
of an ORM Declarative mapping with mixins where copying of types comes into
play.  The variant mapping is now copied as well.

Fixes: #11176
Change-Id: Icf1a2752f60fce863c87ead8b0fe298b0f3d3766

doc/build/changelog/unreleased_20/11176.rst

@@ -0,0 +1,12 @@
+.. change::
+    :tag: bug, sql, regression
+    :tickets: 11176
+
+    Fixed regression from the 1.4 series where the refactor of the
+    :meth:`_types.TypeEngine.with_variant` method introduced at
+    :ref:`change_6980` failed to accommodate for the ``.copy()`` method, which
+    will lose the variant mappings that are set up. This becomes an issue for
+    the very specific case of a "schema" type, which includes types such as
+    :class:`.Enum` and :class:`.ARRAY`, when they are then used in the context
+    of an ORM Declarative mapping with mixins where copying of types comes into
+    play.  The variant mapping is now copied as well.

lib/sqlalchemy/sql/type_api.py

@@ -1005,9 +1005,11 @@ class TypeEngine(Visitable, Generic[_T]):
         types with "implementation" types that are specific to a particular
         dialect.
         """
-        return util.constructor_copy(
+        typ = util.constructor_copy(
             self, cast(Type[TypeEngine[Any]], cls), **kw
         )
+        typ._variant_mapping = self._variant_mapping
+        return typ
 
     def coerce_compared_value(
         self, op: Optional[OperatorType], value: Any

test/sql/test_types.py

@@ -1695,6 +1695,19 @@ class VariantTest(fixtures.TestBase, AssertsCompiledSQL):
         )
         self.composite = self.variant.with_variant(self.UTypeThree(), "mysql")
 
+    def test_copy_doesnt_lose_variants(self):
+        """test #11176"""
+
+        v = self.UTypeOne().with_variant(self.UTypeTwo(), "postgresql")
+
+        v_c = v.copy()
+
+        self.assert_compile(v_c, "UTYPEONE", dialect="default")
+
+        self.assert_compile(
+            v_c, "UTYPETWO", dialect=dialects.postgresql.dialect()
+        )
+
     def test_one_dialect_is_req(self):
         with expect_raises_message(
             exc.ArgumentError, "At least one dialect name is required"