Reordering overloads of Union.

Ix.NET/Source/System.Interactive.Async/Union.cs

@@ -2,35 +2,32 @@
 // The .NET Foundation licenses this file to you under the Apache 2.0 License.
 // See the LICENSE file in the project root for more information. 
 
-using System;
 using System.Collections.Generic;
-using System.Linq;
-using System.Threading.Tasks;
 
 namespace System.Linq
 {
     public static partial class AsyncEnumerable
     {
-        public static IAsyncEnumerable<TSource> Union<TSource>(this IAsyncEnumerable<TSource> first, IAsyncEnumerable<TSource> second, IEqualityComparer<TSource> comparer)
+        public static IAsyncEnumerable<TSource> Union<TSource>(this IAsyncEnumerable<TSource> first, IAsyncEnumerable<TSource> second)
         {
             if (first == null)
                 throw new ArgumentNullException(nameof(first));
             if (second == null)
                 throw new ArgumentNullException(nameof(second));
-            if (comparer == null)
-                throw new ArgumentNullException(nameof(comparer));
 
-            return first.Concat(second).Distinct(comparer);
+            return first.Union(second, EqualityComparer<TSource>.Default);
         }
 
-        public static IAsyncEnumerable<TSource> Union<TSource>(this IAsyncEnumerable<TSource> first, IAsyncEnumerable<TSource> second)
+        public static IAsyncEnumerable<TSource> Union<TSource>(this IAsyncEnumerable<TSource> first, IAsyncEnumerable<TSource> second, IEqualityComparer<TSource> comparer)
         {
             if (first == null)
                 throw new ArgumentNullException(nameof(first));
             if (second == null)
                 throw new ArgumentNullException(nameof(second));
+            if (comparer == null)
+                throw new ArgumentNullException(nameof(comparer));
 
-            return first.Union(second, EqualityComparer<TSource>.Default);
+            return first.Concat(second).Distinct(comparer);
         }
     }
 }