Currently, the process_bind_param method on TypeDecorator[_T] has signature:

Unfortunately, it looks like this is incorrect: I believe it can return anything that the underlying impl can accept. For instance, in SQLAlchemy's tests there's type decorators that return ints:

class MyNewIntType(types.TypeDecorator):
    impl = Integer

    def process_bind_param(self, value, dialect):
        return value * 10

    def process_result_value(self, value, dialect):
        return value * 10

    def copy(self):
        return MyNewIntType()

The process_bind_param return value should probably be loosened to match the Optional[Any] of its inverse operation process_result_value.

(This probably applies to process_literal_param too.)