Fix CancellationTokenMultiplexer return to pool

Fixes a operator precedence bug and adds tests.

If the PooledCancellationTokenSource had inlined tokens but no extraTokens, the Count would evaluate to 0 which would prevent the CancellationTokenRegistrations from being disposed.

The PooledCancellationTokenSource would then be returned to the pool but still be cancelable by the tokens which were preivously attached.

Author: timothycoleman

src/DotNext.Tests/Threading/CancellationTokenMultiplexerTests.cs

@@ -40,6 +40,23 @@ public static void CheckPooling()
         }
     }
 
+    [Fact]
+    public async Task CheckPoolingNonInterference()
+    {
+        var multiplexer = new CancellationTokenMultiplexer();
+
+        using var cts = new CancellationTokenSource();
+
+        await multiplexer.Combine([cts.Token, cts.Token, cts.Token]).DisposeAsync();
+
+        // same source is reused from pool, but should now not be associated with cts.
+        await using var combined = multiplexer.Combine([new(), new(), new()]);
+
+        cts.Cancel();
+
+        False(combined.Token.IsCancellationRequested);
+    }
+
     [Fact]
     public static void ExtraListOverflow()
     {

src/DotNext.Threading/Threading/CancellationTokenMultiplexer.CTS.cs

@@ -37,7 +37,7 @@ private void Add(CancellationTokenRegistration registration)
         private ref CancellationTokenRegistration FirstInlinedRegistration
             => ref Unsafe.As<InlinedTokenList, CancellationTokenRegistration>(ref inlinedList);
 
-        public int Count => inlinedTokenCount + extraTokens?.Count ?? 0;
+        public int Count => inlinedTokenCount + (extraTokens?.Count ?? 0);
 
         public ref CancellationTokenRegistration this[int index]
         {