render type.impl when type is a TypeDecorator instance

Fixes: sqlalchemy#1386

alembic/autogenerate/render.py

@@ -803,6 +803,9 @@ def _repr_type(
     if rendered is not False:
         return rendered
 
+    if isinstance(type_, sqltypes.TypeDecorator):
+        type_ = type_.impl
+
     if hasattr(autogen_context.migration_context, "impl"):
         impl_rt = autogen_context.migration_context.impl.render_type(
             type_, autogen_context

tests/test_autogen_render.py

@@ -33,6 +33,7 @@
 from sqlalchemy.sql import literal_column
 from sqlalchemy.sql import table
 from sqlalchemy.types import TIMESTAMP
+from sqlalchemy.types import TypeDecorator
 from sqlalchemy.types import UserDefinedType
 
 from alembic import autogenerate
@@ -1078,6 +1079,21 @@ def test_render_add_column(self):
             "server_default='5', nullable=True))",
         )
 
+    def test_render_add_column_type_decorator(self):
+        self.autogen_context.opts["user_module_prefix"] = None
+
+        class MyType(TypeDecorator):
+            impl = Integer
+
+        op_obj = ops.AddColumnOp(
+            "foo", Column("x", MyType, server_default="5")
+        )
+        eq_ignore_whitespace(
+            autogenerate.render_op_text(self.autogen_context, op_obj),
+            "op.add_column('foo', sa.Column('x', sa.Integer(), "
+            "server_default='5', nullable=True))",
+        )
+
     @testing.emits_warning("Can't validate argument ")
     def test_render_add_column_custom_kwarg(self):
         col = Column(