Fix DefaultIfEmptyIterator.TryGetElementAt returning default value for out-of-bounds indices

[Copilot]

Fixes a regression introduced in .NET 9 where ElementAtOrDefault on DefaultIfEmpty iterators was incorrectly returning the custom default value instead of default(TSource) for out-of-bounds indices.

Problem:

int result = Array.Empty<int>().DefaultIfEmpty(1).ElementAtOrDefault(1);
Console.WriteLine(result); // Prints 1, should print 0

The issue was in DefaultIfEmptyIterator<TSource>.TryGetElementAt which always returned _default (the custom default value) regardless of whether the element was found, instead of returning default(TSource) when found is false.

Root Cause:
The method was setting found = true only for index 0 but always returning _default, even for out-of-bounds indices where it should return default(TSource).

Fix:

• Modified TryGetElementAt to return _default only when index == 0 and found = true
• Return default(TSource) for all other out-of-bounds indices where found remains false
• Added comprehensive regression test covering both value and reference types

Behavior After Fix:

• Array.Empty<int>().DefaultIfEmpty(999).ElementAtOrDefault(0) → Returns 999 (unchanged)
• Array.Empty<int>().DefaultIfEmpty(999).ElementAtOrDefault(1) → Returns 0 (was 999)
• Array.Empty<string>().DefaultIfEmpty("default").ElementAtOrDefault(1) → Returns null (was "default")

The fix ensures ElementAtOrDefault behaves consistently with the documented contract of returning default(TSource) for out-of-bounds access.

Fixes #119834.

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[Copilot]

Pull Request Overview

Fixes a regression in .NET 9 where ElementAtOrDefault on DefaultIfEmpty iterators incorrectly returned the custom default value instead of default(TSource) for out-of-bounds indices.

• Modified TryGetElementAt to return the custom default only for index 0, and default(TSource) for out-of-bounds indices
• Added comprehensive regression tests covering both value and reference types with various out-of-bounds scenarios

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/libraries/System.Linq/src/System/Linq/DefaultIfEmpty.SpeedOpt.cs	Fixed TryGetElementAt logic to return correct defaults for out-of-bounds access
src/libraries/System.Linq/tests/DefaultIfEmptyTests.cs	Added regression test verifying ElementAtOrDefault behavior for out-of-bounds indices

[eiriktsarpalis]

/backport to release/10.0-staging

[github-actions]

Started backporting to release/10.0-staging: https://github.com/dotnet/runtime/actions/runs/17829880616

[eiriktsarpalis]

/backport to release/9.0-staging

[github-actions]

Started backporting to release/9.0-staging: https://github.com/dotnet/runtime/actions/runs/17829888883

[github-actions]

@eiriktsarpalis an error occurred while backporting to "release/10.0-staging", please check the run log for details!

Error: The specified backport target branch "release/10.0-staging" wasn't found in the repo.

[eiriktsarpalis]

/backport to release/10.0

[github-actions]

Started backporting to release/10.0: https://github.com/dotnet/runtime/actions/runs/17830033506

[jeffhandley]

/backport to release/10.0-rc2

[github-actions]

Started backporting to release/10.0-rc2: https://github.com/dotnet/runtime/actions/runs/17952465675