Type conversion design

Can type conversion be implemented sanely? There are several issues:

• Would converters be declared in Options (pro: specify each converter once, con: runtime errors) or on the fields?
• What does a default converter look like for a boolean? only true and false (exception otherwise)? YAML-like?
• Similarly, is it okay for float or int converters to be lossy or not round-trip? Does that happen in Python, e.g. if a high-precision float is converted to a str?
• How would the logic for a Union look like? Try the converter for each type in the union in order? If so, how is None/Optional handled?
• How would type converters interact with child declarations? Would the order have to be reversed? (First try and resolve a type hint down to one or more XmlDataclass, and if that fails, assume it's an attribute with a converter?)
• Can Mypy be prevented from subtyping converter arguments to object?