[release/10.0] Fix DefaultIfEmptyIterator.TryGetElementAt returning default value for out-of-bounds indices (#119845)

* Initial plan

* Fix DefaultIfEmptyIterator.TryGetElementAt to return default when element not found

Co-authored-by: eiriktsarpalis <[email protected]>

* Add regression test and finalize fix for DefaultIfEmptyIterator.TryGetElementAt

Co-authored-by: eiriktsarpalis <[email protected]>

---------

Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: eiriktsarpalis <[email protected]>

Author: 3 people

src/libraries/System.Linq/src/System/Linq/DefaultIfEmpty.SpeedOpt.cs

@@ -1,6 +1,7 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System;
 using System.Collections;
 using System.Collections.Generic;
 
@@ -77,9 +78,10 @@ public override int GetCount(bool onlyIfCheap)
                 if (index == 0)
                 {
                     found = true;
+                    return _default;
                 }
 
-                return _default;
+                return default;
             }
 
             public override bool Contains(TSource value)

src/libraries/System.Linq/tests/DefaultIfEmptyTests.cs

@@ -124,5 +124,36 @@ public void First_Last_ElementAt()
             Assert.Equal(42, empty.DefaultIfEmpty(42).ElementAt(0));
             Assert.Throws<ArgumentOutOfRangeException>(() => empty.DefaultIfEmpty(42).ElementAt(1));
         }
+
+        [Fact]
+        public void ElementAtOrDefault_OutOfBounds_ReturnsTypeDefault()
+        {
+            // Regression test for https://github.com/dotnet/runtime/issues/119834
+            // ElementAtOrDefault should return default(T) for out-of-bounds indices,
+            // not the DefaultIfEmpty default value
+
+            // Test with empty source
+            int[] empty = [];
+            var defaultIfEmpty = empty.DefaultIfEmpty(999);
+            
+            // Index 0 should return the DefaultIfEmpty value (999)
+            Assert.Equal(999, defaultIfEmpty.ElementAtOrDefault(0));
+            
+            // Out-of-bounds indices should return default(int) which is 0, not 999
+            Assert.Equal(0, defaultIfEmpty.ElementAtOrDefault(1));
+            Assert.Equal(0, defaultIfEmpty.ElementAtOrDefault(2));
+            Assert.Equal(0, defaultIfEmpty.ElementAtOrDefault(-1));
+            
+            // Test with different type (string)
+            string[] emptyStrings = [];
+            var defaultIfEmptyString = emptyStrings.DefaultIfEmpty("default");
+            
+            // Index 0 should return the DefaultIfEmpty value
+            Assert.Equal("default", defaultIfEmptyString.ElementAtOrDefault(0));
+            
+            // Out-of-bounds indices should return default(string) which is null
+            Assert.Null(defaultIfEmptyString.ElementAtOrDefault(1));
+            Assert.Null(defaultIfEmptyString.ElementAtOrDefault(2));
+        }
     }
 }