swift-mirror/userdocs/diagnostics/property-wrapper-requirements.md

Property Wrapper Implementation Requirements

If a type is marked with the @propertyWrapper attribute, it must meet certain requirements to be a valid property wrapper.

First, all property wrapper types must have a property named wrappedValue. This property cannot be static and must have the same access level as the property wrapper type. If the property wrapper provides a projectedValue property, it is subject to the same requirements.

Second, none of a property wrapper's initializers may be failable. Additionally, if a property wrapper initializer has a wrappedValue parameter, the type of that parameter must either be the same as the type of the wrappedValue property or an @autoclosure of that type.