Added failing test to document behavior with observation/identified a…

…rray (#3346)

* Added failing test to document behavior with observation and identified array.

* Update Tests/ComposableArchitectureTests/ObservableTests.swift

---------

Co-authored-by: Stephen Celis <stephen@stephencelis.com>

Tests/ComposableArchitectureTests/ObservableTests.swift

@@ -18,6 +18,30 @@ final class ObservableTests: BaseTCATestCase {
  XCTAssertEqual(state.count, 1)
  }
 
+ func testCopyMutation() async {
+ XCTTODO("""
+ Ideally this test would pass but it does not because making a copy of a child state, mutating
+ it, and assigning it does not change the identified array's IDs, and therefore the fast-path
+ of _$isIdentityEqual prevents observation.
+ """)
+
+ var state = ParentState(children: [ChildState(count: 42)])
+ let countDidChange = self.expectation(description: "count.didChange")
+ var copy = state.children[0]
+ copy.count += 1
+
+ withPerceptionTracking {
+ _ = state.children[0].count
+ } onChange: {
+ countDidChange.fulfill()
+ }
+
+ state.children[0] = copy
+
+ await self.fulfillment(of: [countDidChange], timeout: 0.1)
+ XCTAssertEqual(state.children[0].count, 43)
+ }
+
  func testReplace() async {
  XCTTODO("Ideally this would pass but we cannot detect this kind of mutation currently.")