SK2StoreProduct: simplify currencyCode extraction (#2485)

I noticed this new property thanks to [this
tweet](https://twitter.com/ariskox/status/1658067009609904129?s=61&t=Zz639sMJdwHISm4AKYXoCw).

This changes the "future" implementation of
`SK2StoreProduct.currencyCode` to use [the new (backdeployed)
property](https://developer.apple.com/documentation/storekit/product/4044347-priceformatstyle).
We still need the existing implementation because `@_backDeploy` won't
work if compiling with `Xcode 13.x`.

Sources/Purchasing/StoreKitAbstractions/SK1StoreProduct.swift

@@ -45,7 +45,7 @@ internal struct SK1StoreProduct: StoreProductType {
     var price: Decimal { return underlyingSK1Product.price as Decimal }
 
     var localizedPriceString: String {
-        return priceFormatter?.string(from: underlyingSK1Product.price) ?? ""
+        return self.priceFormatter?.string(from: underlyingSK1Product.price) ?? ""
     }
 
     var productIdentifier: String { return underlyingSK1Product.productIdentifier }
@@ -59,7 +59,7 @@ internal struct SK1StoreProduct: StoreProductType {
     var subscriptionGroupIdentifier: String? { underlyingSK1Product.subscriptionGroupIdentifier }
 
     var priceFormatter: NumberFormatter? {
-        priceFormatterProvider.priceFormatterForSK1(with: underlyingSK1Product.priceLocale)
+        return self.priceFormatterProvider.priceFormatterForSK1(with: self.underlyingSK1Product.priceLocale)
     }
 
     @available(iOS 11.2, macOS 10.13.2, tvOS 11.2, watchOS 6.2, *)

Sources/Purchasing/StoreKitAbstractions/SK2StoreProduct.swift

@@ -46,13 +46,7 @@ internal struct SK2StoreProduct: StoreProductType {
 
     var localizedDescription: String { underlyingSK2Product.description }
 
-    var currencyCode: String? {
-        // note: if we ever need more information from the jsonRepresentation object, we
-        // should use Codable or another decoding method to clean up this code.
-        let attributes = jsonDict["attributes"] as? [String: Any]
-        let offers = attributes?["offers"] as? [[String: Any]]
-        return offers?.first?["currencyCode"] as? String
-    }
+    var currencyCode: String? { self._currencyCode }
 
     var price: Decimal { underlyingSK2Product.price }
 
@@ -69,18 +63,14 @@ internal struct SK2StoreProduct: StoreProductType {
             Logger.appleError("Can't initialize priceFormatter for SK2 product! Could not find the currency code")
             return nil
         }
-        return priceFormatterProvider.priceFormatterForSK2(withCurrencyCode: currencyCode)
+
+        return self.priceFormatterProvider.priceFormatterForSK2(withCurrencyCode: currencyCode)
     }
 
     var subscriptionGroupIdentifier: String? {
         underlyingSK2Product.subscription?.subscriptionGroupID
     }
 
-    private var jsonDict: [String: Any] {
-        let decoded = try? JSONSerialization.jsonObject(with: self.underlyingSK2Product.jsonRepresentation, options: [])
-        return decoded as? [String: Any] ?? [:]
-    }
-
     var subscriptionPeriod: SubscriptionPeriod? {
         guard let skSubscriptionPeriod = underlyingSK2Product.subscription?.subscriptionPeriod else {
             return nil
@@ -100,6 +90,39 @@ internal struct SK2StoreProduct: StoreProductType {
 
 }
 
+@available(iOS 15.0, tvOS 15.0, watchOS 8.0, macOS 12.0, *)
+private extension SK2StoreProduct {
+
+    var _currencyCode: String? {
+        #if swift(>=5.7)
+        if #available(iOS 16.0, macOS 13.0, tvOS 16.0, watchOS 9.0, *) {
+            return self.currencyCodeFromPriceFormat
+        }
+        #endif
+
+        // note: if we ever need more information from the jsonRepresentation object, we
+        // should use Codable or another decoding method to clean up this code.
+        let attributes = jsonDict["attributes"] as? [String: Any]
+        let offers = attributes?["offers"] as? [[String: Any]]
+        return offers?.first?["currencyCode"] as? String
+    }
+
+    private var jsonDict: [String: Any] {
+        let decoded = try? JSONSerialization.jsonObject(with: self.underlyingSK2Product.jsonRepresentation, options: [])
+        return decoded as? [String: Any] ?? [:]
+    }
+
+    // This is marked as `@_backDeploy`, but it's only visible when compiling with Xcode 14.x
+    // and for some reason only returning a non-empty string on iOS 16+.
+    #if swift(>=5.7)
+    @available(iOS 16.0, macOS 13.0, tvOS 16.0, watchOS 9.0, *)
+    private var currencyCodeFromPriceFormat: String {
+        return self.underlyingSK2Product.priceFormatStyle.currencyCode
+    }
+    #endif
+
+}
+
 @available(iOS 15.0, tvOS 15.0, watchOS 8.0, macOS 12.0, *)
 extension SK2StoreProduct: Hashable {