Don't report MSTEST0065 on collection types that declare their own equality

[Evangelink]

Fixes #9971

Problem

MSTEST0065 (AvoidAssertAreEqualOnCollections) fires purely because the compared type implements IEnumerable<T>. It doesn't check whether the type also defines its own equality. For a type that is both a collection and meaningfully IEquatable<T> (or overrides object.Equals), Assert.AreEqual is the correct call — but the rule flagged it anyway, producing false positives that forced people to disable the whole rule.

Approach

The analyzer fundamentally can't read intent: Assert.AreEqual(a, b) on a collection that is also IEquatable<T> is genuinely ambiguous (type equality vs. element/content equality). So the question isn't "how do we detect intent" but "which way do we err when we can't tell?".

For a default-on Warning, false positives are more expensive than false negatives (a noisy warning gets suppressed wholesale). When a type implements IEquatable<self> or overrides object.Equals, the author deliberately opted into a custom, non-reference equality that Assert.AreEqual honors via EqualityComparer<T>.Default — the strongest static signal we're not in accidental-reference-equality territory. So we suppress there.

Changes

AvoidAssertAreEqualOnCollectionsAnalyzer no longer reports when the compared type declares its own equality. Precise semantics (mirroring (comparer ?? EqualityComparer<T>.Default).Equals(...)):

• Opt-out keys off the selected generic type argument T, not the argument's runtime collection type — widening (Assert.AreEqual<object>(c, c)) still reports.
• Opt-out is skipped only for a genuinely custom comparer. A null/default/default(IEqualityComparer<T>) comparer or an explicit EqualityComparer<T>.Default argument is equivalent to the parameterless overload (Assert falls back to the default comparer — Assert.AreEqual.cs:146), so those keep the opt-out.
• IEquatable<T> detection requires the exact self type argument. IEquatable<T> is invariant, so a collection implementing only IEquatable<Base> (used as a derived type) still uses reference equality via EqualityComparer<Derived>.Default and is correctly reported; explicit-interface implementations count.
• object.Equals detection follows the override chain to its root and requires it to be System.Object.Equals (a derived override of a base new virtual bool Equals(object) does not count), and stops before System.ValueType/System.Enum so struct collections with no equality of their own still report.
• Type-parameter constraints: where T : IEquatable<T> (self) or a class constraint overriding object.Equals suppress; a bare where T : IEnumerable<...>, or where T : ISelf where ISelf : IEquatable<ISelf> (a concrete T need not implement IEquatable<T>), still report. Transitive type-parameter constraints (where T : U where U : ...) are followed, keeping T as the equality target.

Known, accepted slippage

Types like ImmutableArray<T> implement IEquatable<T> but compare by the underlying array reference, so they're no longer flagged even though element-wise is usually what you'd want. There's no static way to tell "IEquatable meaning content" from "IEquatable meaning identity", and chasing it just reintroduces intent-guessing.

Verification

• Analyzer + test project build clean.
• Full AvoidAssertAreEqualOnCollectionsAnalyzerTests: 49/49 pass (covers widened-to-object, custom/null/default/default(...)/explicit-default comparer, IEquatable<self> incl. explicit-interface and IEquatable<Base>-only, object.Equals override, new virtual Equals base, struct with/without own equality, and the various direct and transitive type-parameter constraint forms — for both AreEqual and AreNotEqual).

[Copilot]

Pull request overview

Suppresses MSTEST0065 for collection types with custom equality semantics, addressing #9971.

Changes:

• Detects self-typed IEquatable<T> implementations and object.Equals overrides.
• Adds the IEquatable<T> well-known type name.
• Adds analyzer tests for equality scenarios.

Show a summary per file

File	Description
AvoidAssertAreEqualOnCollectionsAnalyzer.cs	Adds custom-equality detection.
WellKnownTypeNames.cs	Registers IEquatable<T>.
AvoidAssertAreEqualOnCollectionsAnalyzerTests.cs	Tests diagnostic suppression and reporting.

Review details

• Files reviewed: 3/3 changed files
• Comments generated: 5
• Review effort level: Medium

[github-actions]

Note

🤖 Automated review by GitHub Copilot. Generated by the Expert Code Review workflow. To request a follow-up action, reply by tagging @copilot directly.

Review Summary

This PR correctly suppresses MSTEST0045 for collection types that declare their own equality semantics (via IEquatable<self> or object.Equals override), fixing issue #9971. The implementation is clean and well-tested.

#	Dimension	Verdict
1	Algorithmic Correctness	✅ Clean
2	Threading & Concurrency	N/A
3	Security & IPC Contract Safety	N/A
4	Public API & Binary Compatibility	✅ Clean — no public API change
5	Performance & Allocations	⚠️ Minor (see inline)
6	Cross-TFM Compatibility	✅ Handled — equatableSymbol is nullable for old TFMs
7	Error Handling	N/A
8	Diagnostics & Observability	N/A
9	Naming & Coding Style	✅ Clean
10	Documentation & Comments	✅ Good inline comments
11	Test Quality	✅ Four test cases covering key scenarios
12	Localization	N/A
13	Configuration & Options	N/A
14	Resource Management	N/A
15	Backward Compatibility	✅ Fewer false positives only
16	Code Organization	✅ Clean
17	Extensibility	N/A
18	Dependency Management	N/A
19	Build Infrastructure	N/A
20	CLI Options	N/A
21	Agentic Workflows	N/A
22	PowerShell Hygiene	N/A

Overall: Looks good. One minor performance note below.

[Copilot]

Review details

• Files reviewed: 3/3 changed files
• Comments generated: 2
• Review effort level: Medium

[Copilot]

Review details

• Files reviewed: 3/3 changed files
• Comments generated: 4
• Review effort level: Medium

[Copilot]

Review details

• Files reviewed: 3/3 changed files
• Comments generated: 1
• Review effort level: Medium

[Copilot]

Review details

• Files reviewed: 3/3 changed files
• Comments generated: 3
• Review effort level: Low

[Copilot]

Review details

• Files reviewed: 3/3 changed files
• Comments generated: 1
• Review effort level: Medium

[Copilot]

Review details

• Files reviewed: 3/3 changed files
• Comments generated: 1
• Review effort level: Medium

[Copilot]

Review details

• Files reviewed: 3/3 changed files
• Comments generated: 0 new
• Review effort level: Medium

[github-actions]

🧪 Test quality grade — PR #9978

Grade	Test	Notes
A (90–100)	new AvoidAssertAreEqualOnCollectionsAnalyzerTests. WhenUsingAssertAreEqualOnCollectionWithBaseTypeDefaultComparer_ ReportDiagnostic	Well-crafted edge-case test; explanatory comment on contravariance; assertion verifies exact diagnostic type and type argument.

_{This advisory comment was generated automatically. Grades are heuristic
and informational — they do not block merging. Re-run with
/grade-tests.}

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🤖 Automated content by GitHub Copilot. Generated by the Grade Tests on PR (on open / sync) workflow. · 35.8 AIC · ⌖ 6.38 AIC · ⊞ 8.9K · [◷]( · ◷)

[Evangelink]

/backport to rel/4.3

[github-actions]

Started backporting to rel/4.3: https://github.com/microsoft/testfx/actions/runs/29490461211