[release/6.0-staging] Fix Item4 is missing in some ValueTuples' IStructuralEquatable.Equals

[github-actions]

Backport of #91461 to release/6.0-staging

/cc @jeffhandley @hamarb123 @skyoxZ

Customer Impact

When ValueTuple has 5 or more items, the bool IStructuralEquatable.Equals(object? other, IEqualityComparer comparer) implementation skips over Item4. This leads to instances returning as equal when they are not.

using System.Collections;

var a = ValueTuple.Create(1, 2, 3, 4, 5);
var b = ValueTuple.Create(1, 2, 3, 0, 5);

// Any comparer will result in the buggy behavior
var comparer = StructuralComparisons.StructuralEqualityComparer;
var equal = ((IStructuralEquatable)a).Equals(b, comparer);

This was reported by a customer in #91457 and fixed in .NET 8 with #91461. This latent behavioral bug could be silently affecting customer applications with potential data loss and data corruption issues. The bug only surfaces when explicitly using IStructuralEquatable.Equals with a comparer provided. That usage pattern would be most prevalent within framework and library usage where it could be hard for customers to diagnose or work around the issue if encountered.

A GitHub code search revealed signal of such usage patterns being used. Here are a couple examples: 1, 2

Testing

The ValueTuple implementation was scanned for other errors like this and they were all fixed together. Unit tests were added to ensure each item of the tuple affects equality.

Risk

Medium. This is a behavioral correction to a latent bug. It's plausible that data has been persisted based on the incorrect behavior; in such a case, the behavioral correction might surface for customers.

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Issue Details

---

Backport of #91461 to release/6.0-staging

/cc @jeffhandley @hamarb123

Customer Impact

Testing

Risk

IMPORTANT: If this backport is for a servicing release, please verify that:

• The PR target branch is release/X.0-staging, not release/X.0.

• If the change touches code that ships in a NuGet package, you have added the necessary package authoring and gotten it explicitly reviewed.

Author:	github-actions[bot]
Assignees:	jeffhandley
Labels:	area-System.Runtime
Milestone:	-

[carlossanlop]

Same request as in the 7.0 one: #91471 (comment)

[jeffhandley]

This has my support for servicing into 6.0. It's a latent functional bug that could lead to notable customer impact.

Adding @artl93 for review/approval.

[artl93]

M2 approved.

[carlossanlop]

From the email conversation, we will wait for this to cook a bit in 8.0, then we will service this to 7.0 and 6.0 in the next servicing release.