Fix snapshots of optional references. (#100)

Co-authored-by: Brandon Williams <[email protected]>

Author: stephencelis

Sources/Sharing/Internal/Reference.swift

@@ -674,7 +674,7 @@ extension _OptionalReference: MutableReference, Equatable where Base: MutableRef
     line: UInt,
     column: UInt
   ) {
-    guard base.snapshot != nil else { return }
+    guard base.wrappedValue != nil else { return }
     base.takeSnapshot(value, fileID: fileID, filePath: filePath, line: line, column: column)
   }
 

Tests/SharingTests/SharedChangeTrackerTests.swift

@@ -103,4 +103,22 @@ import Testing
       $count.withLock { $0 += 1 }
     }
   }
+
+  @Test func unwrappedShared() {
+    let optionalShared = Shared<Int?>(value: 1)
+    let unwrappedShared = Shared(optionalShared)!
+
+    withKnownIssue {
+      do {
+        let tracker = SharedChangeTracker()
+        tracker.track {
+          unwrappedShared.withLock { $0 += 1 }
+        }
+      }
+    } matching: {
+      $0.description == """
+        Issue recorded: Tracked unasserted changes to 'Shared<Int?>(value: Optional(2))': Optional(1) → Optional(2)
+        """
+    }
+  }
 }