Add a unit test for a captured codable object

Author: grynspan

Diff for: Tests/TestingTests/ExitTestTests.swift

@@ -410,6 +410,49 @@ private import _TestingInternals
       }
     }
   }
+
+  class CapturableBaseClass: @unchecked Sendable, Codable {
+    init() {}
+
+    required init(from decoder: any Decoder) throws {}
+    func encode(to encoder: any Encoder) throws {}
+  }
+
+  final class CapturableDerivedClass: CapturableBaseClass, @unchecked Sendable {
+    let x: Int
+
+    init(x: Int) {
+      self.x = x
+      super.init()
+    }
+
+    required init(from decoder: any Decoder) throws {
+      let container = try decoder.singleValueContainer()
+      self.x = try container.decode(Int.self)
+      super.init()
+    }
+
+    override func encode(to encoder: any Encoder) throws {
+      var container = encoder.singleValueContainer()
+      try container.encode(x)
+    }
+  }
+
+  @Test("Capturing an instance of a subclass")
+  func captureSubclass() async {
+    let instance = CapturableDerivedClass(x: 123)
+    await #expect(exitsWith: .success) { [instance = instance as CapturableBaseClass] in
+      #expect((instance as AnyObject) is CapturableBaseClass)
+      // However, because the static type of `instance` is not Derived, we won't
+      // be able to cast it to Derived.
+      #expect(!((instance as AnyObject) is CapturableDerivedClass))
+    }
+    await #expect(exitsWith: .success) { [instance = instance as CapturableDerivedClass] in
+      #expect((instance as AnyObject) is CapturableBaseClass)
+      #expect((instance as AnyObject) is CapturableDerivedClass)
+      #expect(instance.x == 123)
+    }
+  }
 }
 
 // MARK: - Fixtures