Fix decoding bugs

Sources/TOMLKit/Decoder/UnkeyedDecodingContainer.swift

@@ -168,8 +168,9 @@ extension InternalTOMLDecoder.UDC {
 				strictDecoding: self.strictDecoding,
 				notDecodedKeys: self.notDecodedKeys
 			)
+			let decodable = try T(from: decoder)
 			self.currentIndex += 1
-			return try T(from: decoder)
+			return decodable
 		}
 	}
 
@@ -187,7 +188,7 @@ extension InternalTOMLDecoder.UDC {
 			)
 		}
 
-		return KeyedDecodingContainer<NestedKey>(
+		let container = KeyedDecodingContainer<NestedKey>(
 			InternalTOMLDecoder.KDC(
 				table: table,
 				codingPath: self.codingPath + TOMLCodingKey(index: self.currentIndex),
@@ -197,6 +198,8 @@ extension InternalTOMLDecoder.UDC {
 				notDecodedKeys: self.notDecodedKeys
 			)
 		)
+		self.currentIndex += 1
+		return container
 	}
 
 	func nestedUnkeyedContainer() throws -> UnkeyedDecodingContainer {
@@ -211,14 +214,16 @@ extension InternalTOMLDecoder.UDC {
 			)
 		}
 
-		return InternalTOMLDecoder.UDC(
+		let container = InternalTOMLDecoder.UDC(
 			nestedArray,
 			codingPath: self.codingPath + TOMLCodingKey(index: self.currentIndex),
 			userInfo: self.userInfo,
 			dataDecoder: self.dataDecoder,
 			strictDecoding: self.strictDecoding,
 			notDecodedKeys: self.notDecodedKeys
 		)
+		self.currentIndex += 1
+		return container
 	}
 
 	func superDecoder() throws -> Decoder {

Tests/TOMLKitTests/TOMLKitTests.swift

@@ -447,4 +447,58 @@ final class TOMLKitTests: XCTestCase {
 			XCTFail("DecodingError did not occur.")
 		}
 	}
+
+	func testFailingToDecodingDateFromUDCShouldNotIncreaseIndex() throws {
+		let udc = InternalTOMLDecoder.UDC(
+			["Not a date"],
+			codingPath: [],
+			userInfo: [:],
+			dataDecoder: { $0.string != nil ? Data(base64Encoded: $0.string!) : nil },
+			strictDecoding: false,
+			notDecodedKeys: InternalTOMLDecoder.NotDecodedKeys()
+		)
+
+		XCTAssertThrowsError(try udc.decode(Date.self))
+		XCTAssertEqual(udc.currentIndex, 0)
+	}
+
+	func testDecodingObjectFromUDCShouldIncreaseIndex() throws {
+		struct StringCodingKey: CodingKey, Equatable {
+			var stringValue: String
+
+			init(stringValue: String) {
+				self.stringValue = stringValue
+			}
+
+			var intValue: Int? { nil }
+			init?(intValue: Int) { nil }
+		}
+
+
+		let udc = InternalTOMLDecoder.UDC(
+			[TOMLTable(["key": "value"], inline: false)],
+			codingPath: [],
+			userInfo: [:],
+			dataDecoder: { $0.string != nil ? Data(base64Encoded: $0.string!) : nil },
+			strictDecoding: false,
+			notDecodedKeys: InternalTOMLDecoder.NotDecodedKeys()
+		)
+
+		let _ = try udc.nestedContainer(keyedBy: StringCodingKey.self)
+		XCTAssertEqual(udc.currentIndex, 1)
+	}
+
+	func testDecodingArrayFromUDCShouldIncreaseIndex() throws {
+		let udc = InternalTOMLDecoder.UDC(
+			[["value"]],
+			codingPath: [],
+			userInfo: [:],
+			dataDecoder: { $0.string != nil ? Data(base64Encoded: $0.string!) : nil },
+			strictDecoding: false,
+			notDecodedKeys: InternalTOMLDecoder.NotDecodedKeys()
+		)
+
+		let _ = try udc.nestedUnkeyedContainer()
+		XCTAssertEqual(udc.currentIndex, 1)
+	}
 }