Trying to created nested property wrappers in Swift by conforming the property wrappers themselves to StringProtocol.

NestedPropertyWrappers.swift

@propertyWrapper
struct B {
  var wrappedValue: Int
}

@propertyWrapper
struct Clamp {
  var wrappedValue: Int
}

struct C {
  @Uppercased @Lowercased var foo = "HeelOLllo"
}

//func g() {
//    var c = C()
//    c.foo = 3
//}



@propertyWrapper
struct Lowercased<S: StringProtocol> {
    private var text: S
    var wrappedValue: S {
        get { S(string: text.lowercased()) }
        set { text = newValue }
    }
    init<AnyString: StringProtocol>(_ wrappedValue: AnyString)  {
        self.text = S(string: wrappedValue)
    }
}

extension Lowercased: StringProtocol {
    typealias UTF8View = S.UTF8View
    typealias UTF16View = S.UTF16View
    typealias UnicodeScalarView = S.UnicodeScalarView

    var utf8: S.UTF8View { wrappedValue.utf8 }
    var utf16: S.UTF16View { wrappedValue.utf16 }
    var unicodeScalars: S.UnicodeScalarView { wrappedValue.unicodeScalars }

    func lowercased() -> String { wrappedValue.lowercased() }
    func uppercased() -> String { wrappedValue.uppercased() }

    init(stringLiteral: String) {
        self.init(S(string: stringLiteral))
    }

    init<C, Encoding>(decoding codeUnits: C, as sourceEncoding: Encoding.Type) where C : Collection, Encoding : _UnicodeEncoding, C.Element == Encoding.CodeUnit {
        self.init(String(decoding: codeUnits, as: sourceEncoding))
    }

    init(cString nullTerminatedUTF8: UnsafePointer<CChar>) {
        self.init(String(cString: nullTerminatedUTF8))
    }

    init<Encoding>(decodingCString nullTerminatedCodeUnits: UnsafePointer<Encoding.CodeUnit>, as sourceEncoding: Encoding.Type) where Encoding : _UnicodeEncoding {
        self.init(String(decodingCString: nullTerminatedCodeUnits, as: sourceEncoding))
    }

    func withCString<Result>(_ body: (UnsafePointer<CChar>) throws -> Result) rethrows -> Result { try wrappedValue.withCString(body) }
    func withCString<Result, Encoding>(encodedAs targetEncoding: Encoding.Type, _ body: (UnsafePointer<Encoding.CodeUnit>) throws -> Result) rethrows -> Result where Encoding : _UnicodeEncoding {
        try wrappedValue.withCString(encodedAs: targetEncoding, body)
    }

    func index(before i: String.Index) -> String.Index { wrappedValue.index(before: i) }
    func index(after i: String.Index) -> String.Index { wrappedValue.index(after: i) }
    subscript(position: String.Index) -> Character { wrappedValue[position] }
    var startIndex: String.Index { wrappedValue.startIndex }
    var endIndex: String.Index { wrappedValue.endIndex }
    var description: String { wrappedValue.description }
    mutating func write(_ string: String) { text = S(string: string) }
    func write<Target>(to target: inout Target) where Target : TextOutputStream {
        wrappedValue.write(to: &target)
    }
}

@propertyWrapper
struct Uppercased<S: StringProtocol> {
    private var text: S
    var wrappedValue: S {
        get { S(string: text.uppercased()) }
        set { text = newValue }
    }
    init<AnyString: StringProtocol>(_ wrappedValue: AnyString)  {
        self.text = S(string: wrappedValue)
    }
}

extension Uppercased: StringProtocol {
    typealias UTF8View = S.UTF8View
    typealias UTF16View = S.UTF16View
    typealias UnicodeScalarView = S.UnicodeScalarView

    var utf8: S.UTF8View { wrappedValue.utf8 }
    var utf16: S.UTF16View { wrappedValue.utf16 }
    var unicodeScalars: S.UnicodeScalarView { wrappedValue.unicodeScalars }

    func lowercased() -> String { wrappedValue.lowercased() }
    func uppercased() -> String { wrappedValue.uppercased() }

    init(stringLiteral: String) {
        self.init(S(string: stringLiteral))
    }

    init<C, Encoding>(decoding codeUnits: C, as sourceEncoding: Encoding.Type) where C : Collection, Encoding : _UnicodeEncoding, C.Element == Encoding.CodeUnit {
        self.init(String(decoding: codeUnits, as: sourceEncoding))
    }

    init(cString nullTerminatedUTF8: UnsafePointer<CChar>) {
        self.init(String(cString: nullTerminatedUTF8))
    }

    init<Encoding>(decodingCString nullTerminatedCodeUnits: UnsafePointer<Encoding.CodeUnit>, as sourceEncoding: Encoding.Type) where Encoding : _UnicodeEncoding {
        self.init(String(decodingCString: nullTerminatedCodeUnits, as: sourceEncoding))
    }

    func withCString<Result>(_ body: (UnsafePointer<CChar>) throws -> Result) rethrows -> Result { try wrappedValue.withCString(body) }
    func withCString<Result, Encoding>(encodedAs targetEncoding: Encoding.Type, _ body: (UnsafePointer<Encoding.CodeUnit>) throws -> Result) rethrows -> Result where Encoding : _UnicodeEncoding {
        try wrappedValue.withCString(encodedAs: targetEncoding, body)
    }

    func index(before i: String.Index) -> String.Index { wrappedValue.index(before: i) }
    func index(after i: String.Index) -> String.Index { wrappedValue.index(after: i) }
    subscript(position: String.Index) -> Character { wrappedValue[position] }
    var startIndex: String.Index { wrappedValue.startIndex }
    var endIndex: String.Index { wrappedValue.endIndex }
    var description: String { wrappedValue.description }
    mutating func write(_ string: String) { text = S(string: string) }
    func write<Target>(to target: inout Target) where Target : TextOutputStream {
        wrappedValue.write(to: &target)
    }
}

extension StringProtocol {

    init<AnyString: StringProtocol>(string: AnyString) {
        var me = Self(stringLiteral: "")
        string.withCString { me = Self.init(cString: $0) }
        self = me
    }
}