Fixed missing Dispose call on Union on empty enumerable (#40384)

Author: bbartels

src/System.Linq/src/System/Linq/Union.cs

@@ -99,10 +99,11 @@ public sealed override bool MoveNext()
                     for (IEnumerable<TSource> enumerable = GetEnumerable(0); enumerable != null; enumerable = GetEnumerable(_state - 1))
                     {
                         IEnumerator<TSource> enumerator = enumerable.GetEnumerator();
+                        SetEnumerator(enumerator);
+
                         ++_state;
                         if (enumerator.MoveNext())
                         {
-                            SetEnumerator(enumerator);
                             StoreFirst();
                             return true;
                         }

src/System.Linq/tests/EnumerableTests.cs

@@ -345,5 +345,25 @@ public DelegateIterator(
 
             void IDisposable.Dispose() => _dispose();
         }
+
+        protected sealed class DisposeTrackingEnumerable<T> : IEnumerable<T>
+        {
+            public bool EnumeratorDisposed { get; private set; }
+
+
+            public IEnumerator<T> GetEnumerator() => new Enumerator(this);
+            IEnumerator IEnumerable.GetEnumerator() => new Enumerator(this);
+
+            private sealed class Enumerator : IEnumerator<T>
+            {
+                private readonly DisposeTrackingEnumerable<T> _enumerable;
+                public Enumerator(DisposeTrackingEnumerable<T> enumerable) => _enumerable = enumerable;
+                public void Dispose() => _enumerable.EnumeratorDisposed = true;
+                public T Current => default;
+                object IEnumerator.Current => default;
+                public bool MoveNext() => false;
+                public void Reset() { }
+            }
+        }
     }
 }

src/System.Linq/tests/UnionTests.cs

@@ -4,6 +4,7 @@
 
 using System;
 using System.Collections.Generic;
+using System.Collections;
 using Xunit;
 
 namespace System.Linq.Tests
@@ -413,5 +414,19 @@ public void HashSetWithBuiltInComparer_HashSetContainsNotUsed()
             Assert.Equal(new[] { "A", "a" }, input2.Union(input1, EqualityComparer<string>.Default));
             Assert.Equal(new[] { "A" }, input2.Union(input1, StringComparer.OrdinalIgnoreCase));
         }
+
+        [Fact]
+        public void UnionFollowedBySelectOnEmptyEnumerableInvokesDispose()
+        {
+            var enum1 = new DisposeTrackingEnumerable<int>();
+            var enum2 = new DisposeTrackingEnumerable<int>();
+            Assert.False(enum1.EnumeratorDisposed);
+            Assert.False(enum2.EnumeratorDisposed);
+
+            foreach (int value in enum1.Union(enum2)) { }
+
+            Assert.True(enum1.EnumeratorDisposed);
+            Assert.True(enum2.EnumeratorDisposed);
+        }
     }
 }