Fix protocol hierarchy to prevent duplicate conformance issue

Refactored CSVIndexedCodable protocol hierarchy to use a shared base
protocol (CSVIndexedBase) that provides the common requirements:
- CSVCodingKeys associatedtype
- csvColumnOrder static property
- _csvColumnOrder marker property

Both CSVIndexedDecodable and CSVIndexedEncodable now refine CSVIndexedBase,
eliminating the "multiple matching properties" error when a type conforms
to both protocols (via CSVIndexedCodable).

Also unified the internal marker protocol to _CSVIndexedMarker for
consistent runtime detection.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: g-cqd

Sources/CSVCoder/CSVDecoder+Streaming.swift

@@ -32,7 +32,7 @@ extension CSVDecoder {
         from url: URL
     ) -> AsyncThrowingStream<T, Error> {
         // Runtime detection of CSVIndexedDecodable conformance
-        let columnOrder = (T.self as? _CSVIndexedDecodableMarker.Type)?._csvColumnOrder
+        let columnOrder = (T.self as? _CSVIndexedMarker.Type)?._csvColumnOrder
         return streamDecode(type, from: url, columnOrder: columnOrder)
     }
 
@@ -51,7 +51,7 @@ extension CSVDecoder {
         from data: Data
     ) -> AsyncThrowingStream<T, Error> {
         // Runtime detection of CSVIndexedDecodable conformance
-        let columnOrder = (T.self as? _CSVIndexedDecodableMarker.Type)?._csvColumnOrder
+        let columnOrder = (T.self as? _CSVIndexedMarker.Type)?._csvColumnOrder
         return streamDecode(type, from: data, columnOrder: columnOrder)
     }
 

Sources/CSVCoder/CSVDecoder.swift

@@ -206,8 +206,8 @@ public final class CSVDecoder: Sendable {
     /// - Returns: An array of decoded values.
     public func decode<T: Decodable>(_ type: [T].Type, from data: Data) throws -> [T] {
         // Runtime detection of CSVIndexedDecodable conformance
-        let columnOrder = (T.self as? _CSVIndexedDecodableMarker.Type)?._csvColumnOrder
-        
+        let columnOrder = (T.self as? _CSVIndexedMarker.Type)?._csvColumnOrder
+
         // Fast path: Zero-copy decoding for UTF-8 data
         return try decodeRowsFromBytes(type, from: data, columnOrder: columnOrder)
     }
@@ -223,7 +223,7 @@ public final class CSVDecoder: Sendable {
     /// - Returns: An array of decoded values.
     public func decode<T: Decodable>(_ type: [T].Type, from string: String) throws -> [T] {
         // Runtime detection of CSVIndexedDecodable conformance
-        let columnOrder = (T.self as? _CSVIndexedDecodableMarker.Type)?._csvColumnOrder
+        let columnOrder = (T.self as? _CSVIndexedMarker.Type)?._csvColumnOrder
         return try decodeRows(type, from: string, columnOrder: columnOrder)
     }
 

Sources/CSVCoder/CSVIndexedCodable.swift

@@ -13,13 +13,31 @@ import Foundation
 /// Internal marker protocol for runtime conformance detection.
 /// Has no associated types, enabling `as?` casting at runtime.
 /// Public to allow protocol refinement but prefixed with underscore to signal internal use.
-public protocol _CSVIndexedDecodableMarker {
+public protocol _CSVIndexedMarker {
     static var _csvColumnOrder: [String] { get }
 }
 
-/// Internal marker for encodable types.
-public protocol _CSVIndexedEncodableMarker {
-    static var _csvColumnOrder: [String] { get }
+// MARK: - Base Protocol
+
+/// Base protocol providing the common requirements for CSV indexed types.
+/// Both CSVIndexedDecodable and CSVIndexedEncodable refine this protocol.
+public protocol CSVIndexedBase: _CSVIndexedMarker {
+    /// The CodingKeys type, which must be CaseIterable to define column order.
+    associatedtype CSVCodingKeys: CodingKey & CaseIterable
+
+    /// Returns the ordered column names derived from CodingKeys.
+    /// Default implementation uses `CSVCodingKeys.allCases`.
+    static var csvColumnOrder: [String] { get }
+}
+
+extension CSVIndexedBase {
+    /// Default implementation: extracts column names from CodingKeys.allCases in order.
+    public static var csvColumnOrder: [String] {
+        CSVCodingKeys.allCases.map { $0.stringValue }
+    }
+
+    /// Internal marker implementation for runtime detection.
+    public static var _csvColumnOrder: [String] { csvColumnOrder }
 }
 
 // MARK: - CSVIndexedDecodable Protocol
@@ -49,47 +67,14 @@ public protocol _CSVIndexedEncodableMarker {
 ///
 /// - Note: The decoder automatically detects conformance at runtime.
 ///   You can use the standard `decode([T].self, from:)` method.
-public protocol CSVIndexedDecodable: Decodable, _CSVIndexedDecodableMarker {
-    /// The CodingKeys type, which must be CaseIterable to define column order.
-    associatedtype CSVCodingKeys: CodingKey & CaseIterable
-
-    /// Returns the ordered column names derived from CodingKeys.
-    /// Default implementation uses `CSVCodingKeys.allCases`.
-    static var csvColumnOrder: [String] { get }
-}
-
-extension CSVIndexedDecodable {
-    /// Default implementation: extracts column names from CodingKeys.allCases in order.
-    public static var csvColumnOrder: [String] {
-        CSVCodingKeys.allCases.map { $0.stringValue }
-    }
-
-    /// Internal marker implementation for runtime detection.
-    static var _csvColumnOrder: [String] { csvColumnOrder }
-}
+public protocol CSVIndexedDecodable: Decodable, CSVIndexedBase {}
 
 // MARK: - CSVIndexedEncodable Protocol
 
 /// A type that can be encoded to CSV with columns in the order of its CodingKeys.
 ///
 /// The encoding order matches the order of cases in your `CodingKeys` enum.
-public protocol CSVIndexedEncodable: Encodable, _CSVIndexedEncodableMarker {
-    /// The CodingKeys type, which must be CaseIterable to define column order.
-    associatedtype CSVCodingKeys: CodingKey & CaseIterable
-
-    /// Returns the ordered column names derived from CodingKeys.
-    static var csvColumnOrder: [String] { get }
-}
-
-extension CSVIndexedEncodable {
-    /// Default implementation: extracts column names from CodingKeys.allCases in order.
-    public static var csvColumnOrder: [String] {
-        CSVCodingKeys.allCases.map { $0.stringValue }
-    }
-
-    /// Internal marker implementation for runtime detection.
-    static var _csvColumnOrder: [String] { csvColumnOrder }
-}
+public protocol CSVIndexedEncodable: Encodable, CSVIndexedBase {}
 
 // MARK: - Combined Protocol