https://forums.swift.org/t/any-info-on-the-typeeraser-attribute/37826/2

typeEraser+dynamicReplacement.swift

extension Swift.Never: MyData {
    var body: Never {
        fatalError()
    }
}

struct AnyData: MyData {
    typealias Body = Never
    private let __data: any MyData
    
    init<V>(erasing data: V) where V : MyData {
        self.__data = data
    }
    
    var body: Never {
        fatalError()
    }
}

@_typeEraser(AnyData) protocol MyData {
    associatedtype Body : MyData
    var body: Self.Body { get }
}

struct HelloData: MyData {
    dynamic var body: some MyData {
        fatalError("Never Called") // Never이어도 _dynamicReplacement에 의해 HelloData로 바뀌지만, _typeEraser으로 인해 AnyData라는 Type으로 고정
    }
}

extension HelloData {
    @_dynamicReplacement(for: body)
    var _body: some MyData {
        HelloData()
    }
}

print(HelloData().body)