[TSCUtility] Use a separate array property wrapper for PolymorphicCodable

Conditional conformance doesn't seem to work that well.

Author: aciidgh

Diff for: Sources/TSCUtility/PolymorphicCodable.swift

@@ -42,21 +42,15 @@ public struct PolymorphicCodable<T: PolymorphicCodableProtocol>: Codable {
     }
 }
 
-extension Array: PolymorphicCodableProtocol where Element: PolymorphicCodableProtocol {
-    public static var implementations: [PolymorphicCodableProtocol.Type] {
-        return [Array<Element>.self]
-    }
+@propertyWrapper
+public struct PolymorphicCodableArray<T: PolymorphicCodableProtocol>: Codable {
+    public let value: [PolymorphicCodable<T>]
 
-    public func encode(to encoder: Encoder) throws {
-        try self.map{ PolymorphicCodable(wrappedValue: $0) }.encode(to: encoder)
+    public init(wrappedValue value: [T]) {
+        self.value = value.map{ PolymorphicCodable(wrappedValue: $0) }
     }
 
-    public init(from decoder: Decoder) throws {
-        var container = try decoder.unkeyedContainer()
-        var items: [PolymorphicCodable<Element>] = []
-        while !container.isAtEnd {
-            items.append(try container.decode(PolymorphicCodable<Element>.self))
-        }
-        self = items.map{ $0.value }
+    public var wrappedValue: [T] {
+        return value.map{ $0.value }
     }
 }

Diff for: Tests/TSCUtilityTests/PolymorphicCodableTests.swift

@@ -34,7 +34,7 @@ struct Animals: Codable {
     @PolymorphicCodable
     var animal2: Animal
 
-    @PolymorphicCodable
+    @PolymorphicCodableArray
     var animals: [Animal]
 }
 
@@ -58,6 +58,7 @@ final class PolymorphicCodableTests: XCTestCase {
 
         XCTAssertEqual(decoded.animals.count, 2)
         XCTAssertEqual(decoded.animals.map{ $0.age }, [5, 3])
+        XCTAssertEqual(decoded.animals.map{ String(reflecting: $0) }, ["TSCUtilityTests.Dog", "TSCUtilityTests.Cat"])
     }
 }