Fix warnings in Swift 6.2 about non-sendable types in isolated closures (#1233)

Author: d-ronnqvist

Sources/SwiftDocC/DocumentationService/ExternalReferenceResolverServiceClient.swift

@@ -42,7 +42,7 @@ class ExternalReferenceResolverServiceClient {
         self.convertRequestIdentifier = convertRequestIdentifier
     }
 
-    func sendAndWait(_ request: some Codable) throws -> Data {
+    func sendAndWait(_ request: some Codable & SendableMetatype) throws -> Data {
         let resultGroup = DispatchGroup()
 
         var result: Result<Data?, Error>?

Sources/SwiftDocC/Utility/Collection+ConcurrentPerform.swift

@@ -1,7 +1,7 @@
 /*
  This source file is part of the Swift.org open source project
 
- Copyright (c) 2021 Apple Inc. and the Swift project authors
+ Copyright (c) 2021-2025 Apple Inc. and the Swift project authors
  Licensed under Apache License v2.0 with Runtime Library Exception
 
  See https://swift.org/LICENSE.txt for license information
@@ -20,7 +20,7 @@ private let useConcurrentCollectionExtensions = true
 private let useConcurrentCollectionExtensions = false
 #endif
 
-extension Collection where Index == Int {
+extension Collection where Index == Int, Self: SendableMetatype {
 
     /// Concurrently transforms the elements of a collection.
     /// - Parameters:

Sources/SwiftDocC/Utility/FoundationExtensions/SendableMetatypeShim.swift

@@ -0,0 +1,21 @@
+/*
+ This source file is part of the Swift.org open source project
+
+ Copyright (c) 2025 Apple Inc. and the Swift project authors
+ Licensed under Apache License v2.0 with Runtime Library Exception
+
+ See https://swift.org/LICENSE.txt for license information
+ See https://swift.org/CONTRIBUTORS.txt for Swift project authors
+*/
+
+// In Swift 6.2, metatypes are no longer sendable by default (SE-0470).
+// Instead a type needs to conform to `SendableMetatype` to indicate that its metatype is sendable.
+//
+// However, `SendableMetatype` doesn't exist before Swift 6.1 so we define an internal alias to `Any` here.
+// This means that conformances to `SendableMetatype` has no effect before 6.2 indicates metatype sendability in 6.2 onwards.
+//
+// Note: Adding a protocol requirement to a _public_ API is a breaking change.
+
+#if compiler(<6.2)
+typealias SendableMetatype = Any
+#endif