Use ConfigureAwait(false) when awaiting UnionWithAsync.

Ix.NET/Source/System.Linq.Async/System/Linq/Operators/Distinct.cs

@@ -121,7 +121,7 @@ namespace System.Linq
             {
                 var s = new Set<TSource>(_comparer);
 
-                await s.UnionWithAsync(_source, cancellationToken);
+                await s.UnionWithAsync(_source, cancellationToken).ConfigureAwait(false);
 
                 return s;
             }

Ix.NET/Source/System.Linq.Async/System/Linq/Operators/Union.cs

@@ -169,7 +169,7 @@ namespace System.Linq
                         return set;
                     }
 
-                    await set.UnionWithAsync(enumerable, cancellationToken);
+                    await set.UnionWithAsync(enumerable, cancellationToken).ConfigureAwait(false);
                 }
             }