Tighten MSTEST0065 to fire on argument types, not only the generic T (#8766)

[Evangelink]

Fixes #8766.

Summary

Tighten MSTEST0065 (AvoidAssertAreEqualOnCollectionsAnalyzer) so it fires based on the argument / parameter type at the call site, not only on the generic type argument T. The analyzer previously inspected only targetMethod.TypeArguments[0] and so silently missed real-world pit-of-failure patterns where the caller forces T to a non-collection type.

Now flagged (in addition to today's cases):

csharp int[] arr1 = [1, 2]; int[] arr2 = [1, 2]; Assert.AreEqual<object>(arr1, arr2); // T = object, but arguments are int[] Assert.AreEqual((object)arr1, (object)arr2); // explicit object cast

The analyzer now walks back through implicit conversions on the expected/notExpected and actual arguments via IArgumentOperation.Value.WalkDownConversion() and fires the diagnostic if either argument's un-converted static type implements IEnumerable<T> (and is not string).

Behavior preserved

• Null-literal short-circuit on either expected/notExpected or actual.
• string exclusion.
• The display type in the diagnostic message is the historical generic T when it is itself a collection; otherwise it is the un-converted argument type that triggered the report.
• MSTEST0017 (argument-order analyzer) is untouched.

What is not covered

The acceptance criteria included object x = ...; Assert.AreEqual(x, y) where x is declared as object but assigned a collection. Without dataflow analysis the analyzer cannot see beyond the local's compile-time type object, so that case is intentionally not flagged (an explicit negative test documents this). The mixed case Assert.AreEqual(list, (object)list2) is flagged because at least one argument has a collection static type at the call site.

Risk / migration

This is a tightening of a Warning-severity analyzer and will produce new warnings on existing codebases that hit the newly-covered patterns. That is the desired effect (more pit-of-failure cases caught). Per the issue, this should ship separately from any severity escalation of MSTEST0065.

Tests

Added 7 new tests covering both positive and negative cases:

• Assert.AreEqual<object>(arr1, arr2) — flagged
• Assert.AreNotEqual<object>(arr1, arr2) — flagged
• Assert.AreEqual((object)expected, (object)actual) — flagged
• Assert.AreEqual(list, objectThatIsList) — flagged (one side has collection static type)
• Assert.AreEqual<object>(s1, s2) with string arguments — not flagged
• Assert.AreEqual(objectX, objectY) with both sides declared as object — not flagged (documents dataflow limitation)
• Assert.AreEqual(record1, record2) for value-equal record — not flagged
• Assert.AreEqual<object?>(null, actual) — null-literal short-circuit preserved across the new path

All 31 AvoidAssertAreEqualOnCollectionsAnalyzerTests pass, as do the 123 tests across the related UseProperAssertMethodsAnalyzer / AvoidAssertAreSameWithValueTypesAnalyzer / CollectionAssertToAssertMigrationAnalyzer suites.

[Copilot]

Pull request overview

This PR tightens MSTEST0065 (AvoidAssertAreEqualOnCollectionsAnalyzer) so it can report when callers widen the generic type argument (e.g., T=object) but still pass collection-typed arguments at the call site, and adds unit tests for the newly-covered scenarios. It also updates the test-improver agentic workflow guidance with a repo-specific constraint around analyzer test coverage.

Changes:

• Update MSTEST0065 to fall back to inspecting un-converted argument static types (expected/notExpected + actual) when T itself is not a collection.
• Add new unit tests covering explicit <object> generic arguments, object casts, mixed object/collection static types, and preserved exclusions (string + null-literal short-circuit).
• Document a repository-specific constraint for the test-improver workflow (no VB.NET analyzer tests).

Show a summary per file

File	Description
src/Analyzers/MSTest.Analyzers/AvoidAssertAreEqualOnCollectionsAnalyzer.cs	Adjusts MSTEST0065 reporting logic to consider argument static types (after stripping conversions) when T isn’t itself a collection.
test/UnitTests/MSTest.Analyzers.UnitTests/AvoidAssertAreEqualOnCollectionsAnalyzerTests.cs	Adds coverage for newly-detected call-site widening/casting patterns and key negative cases.
.github/workflows/test-improver.md	Adds repo-specific constraints for the test-improver agent workflow (notably: no VB analyzer tests).

Copilot's findings

• Files reviewed: 3/3 changed files
• Comments generated: 2