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.)