Adds a @LossyOptional property wrapper

Sources/BetterCodable/LossyOptional.swift

@@ -0,0 +1,9 @@
+public struct DefaultNilStrategy<T: Codable>: DefaultCodableStrategy {
+    public static var defaultValue: T? { nil }
+}
+
+/// Decodes optional types, defaulting to `nil` instead of throwing an error, if applicable.
+///
+/// `@LossyOptional` decodes optionals, and defaults to `nil` in cases where the decoder fails (e.g. decoding a non-url string to the `URL` type)
+public typealias LossyOptional<T> = DefaultCodable<DefaultNilStrategy<T>> where T: Codable
+

Tests/BetterCodableTests/LossyOptionalTests.swift

@@ -0,0 +1,68 @@
+import XCTest
+import BetterCodable
+
+class DefaultNilTests: XCTestCase {
+    /// This test demonstrates the problem that `@LossyOptional` solves. When decoding
+    /// optional types, it often the case that we end up with an error instead of
+    /// defaulting back to `nil`.
+    func testDecodingBadUrlAsOptionalWithoutDefaultNil() {
+        struct Fixture: Codable {
+            var a: URL?
+        }
+
+        let jsonData = #"{"a":"https://website .test"}"#.data(using: .utf8)!
+
+        XCTAssertThrowsError(try JSONDecoder().decode(Fixture.self, from: jsonData))
+    }
+
+    func testDecodingWithUrlConversions() throws {
+        struct Fixture: Codable {
+            @LossyOptional var a: URL?
+            @LossyOptional var b: URL?
+        }
+
+        let badUrlString = "https://website .test"
+        let goodUrlString = "https://website.test"
+
+        let jsonData = #"{"a":"\#(badUrlString)", "b":"\#(goodUrlString)"}"#.data(using: .utf8)!
+        let fixture = try JSONDecoder().decode(Fixture.self, from: jsonData)
+
+        XCTAssertNil(fixture.a)
+        XCTAssertEqual(fixture.b, URL(string: goodUrlString))
+    }
+
+    func testDecodingWithIntegerConversions() throws {
+        struct Fixture: Codable {
+            @LossyOptional var a: Int?
+            @LossyOptional var b: Int?
+        }
+
+        let jsonData = #"{ "a": 3.14, "b": 3 }"#.data(using: .utf8)!
+        let fixture = try JSONDecoder().decode(Fixture.self, from: jsonData)
+
+        XCTAssertNil(fixture.a)
+        XCTAssertEqual(fixture.b, 3)
+    }
+
+    func testDecodingWithNullValue() throws {
+        struct Fixture: Codable {
+            @LossyOptional var a: String?
+        }
+
+        let jsonData = #"{"a":null}"#.data(using: .utf8)!
+        let fixture = try JSONDecoder().decode(Fixture.self, from: jsonData)
+
+        XCTAssertNil(fixture.a)
+    }
+
+    func testDecodingWithMissingKey() throws {
+        struct Fixture: Codable {
+            @LossyOptional var a: String?
+        }
+
+        let jsonData = "{}".data(using: .utf8)!
+        let fixture = try JSONDecoder().decode(Fixture.self, from: jsonData)
+
+        XCTAssertNil(fixture.a)
+    }
+}