From 958457cc323a91a8cf04a486586fd7fa13b2ae74 Mon Sep 17 00:00:00 2001
From: Jonathan Grynspan <jgrynspan@apple.com>
Date: Tue, 17 Dec 2024 17:13:57 -0500
Subject: [PATCH] Move collection diffing type-erasure to an init

---
 .../Expectations/ExpectationContext.swift     | 24 ++-----------------
 .../CollectionDifferenceAdditions.swift       | 22 +++++++++++++++++
 2 files changed, 24 insertions(+), 22 deletions(-)

diff --git a/Sources/Testing/Expectations/ExpectationContext.swift b/Sources/Testing/Expectations/ExpectationContext.swift
index ac136cb6d..2bde68612 100644
--- a/Sources/Testing/Expectations/ExpectationContext.swift
+++ b/Sources/Testing/Expectations/ExpectationContext.swift
@@ -222,26 +222,6 @@ extension __ExpectationContext {
 // MARK: - Collection comparison and diffing
 
 extension __ExpectationContext {
-  /// Convert an instance of `CollectionDifference` to one that is type-erased
-  /// over elements of type `Any`.
-  ///
-  /// - Parameters:
-  ///   - difference: The difference to convert.
-  ///
-  /// - Returns: A type-erased copy of `difference`.
-  private static func _typeEraseCollectionDifference(_ difference: CollectionDifference<some Any>) -> CollectionDifference<Any> {
-    CollectionDifference<Any>(
-      difference.lazy.map { change in
-        switch change {
-        case let .insert(offset, element, associatedWith):
-          return .insert(offset: offset, element: element as Any, associatedWith: associatedWith)
-        case let .remove(offset, element, associatedWith):
-          return .remove(offset: offset, element: element as Any, associatedWith: associatedWith)
-        }
-      }
-    )!
-  }
-
   /// Generate a description of a previously-computed collection difference.
   ///
   /// - Parameters:
@@ -317,7 +297,7 @@ extension __ExpectationContext {
 
     if !result {
       differences[opID] = { [lhs, rhs] in
-        Self._typeEraseCollectionDifference(lhs.difference(from: rhs))
+        CollectionDifference<Any>(lhs.difference(from: rhs))
       }
     }
 
@@ -380,7 +360,7 @@ extension __ExpectationContext {
           return nil
         }
 
-        return Self._typeEraseCollectionDifference(diff)
+        return CollectionDifference<Any>(diff)
       }
     }
 
diff --git a/Sources/Testing/Support/Additions/CollectionDifferenceAdditions.swift b/Sources/Testing/Support/Additions/CollectionDifferenceAdditions.swift
index 94a09b14c..df942fbca 100644
--- a/Sources/Testing/Support/Additions/CollectionDifferenceAdditions.swift
+++ b/Sources/Testing/Support/Additions/CollectionDifferenceAdditions.swift
@@ -8,6 +8,28 @@
 // See https://swift.org/CONTRIBUTORS.txt for Swift project authors
 //
 
+extension CollectionDifference<Any> {
+  /// Convert an instance of `CollectionDifference` to one that is type-erased
+  /// over elements of type `Any`.
+  ///
+  /// - Parameters:
+  ///   - difference: The difference to convert.
+  init(_ difference: CollectionDifference<some Any>) {
+    self.init(
+      difference.lazy.map { change in
+        switch change {
+        case let .insert(offset, element, associatedWith):
+          return .insert(offset: offset, element: element as Any, associatedWith: associatedWith)
+        case let .remove(offset, element, associatedWith):
+          return .remove(offset: offset, element: element as Any, associatedWith: associatedWith)
+        }
+      }
+    )!
+  }
+}
+
+// MARK: -
+
 extension CollectionDifference.Change {
   /// The element that was changed.
   var element: ChangeElement {