chore(deps): update dependency polyfill to v9

[thomhurst]

This PR contains the following updates:

Package	Type	Update	Change
Polyfill	nuget	major	8.9.1 -> 9.0.0

---

Configuration

📅 Schedule: Branch creation - At any time (no schedule defined), Automerge - At any time (no schedule defined).

🚦 Automerge: Enabled.

♻ Rebasing: Whenever PR is behind base branch, or you tick the rebase/retry checkbox.

🔕 Ignore: Close this PR and you won't be reminded about this update again.

---

• If you want to rebase/retry this PR, check this box

---

This PR has been generated by Renovate Bot.

[claude]

Claude finished @thomhurst's task —— View job

---

Code Review Complete

Reviewing the Polyfill dependency update from v8.9.1 to v9.0.0:

Todo List

• Read the changed file to understand the update
• Check repository guidelines in CLAUDE.md
• Analyze potential breaking changes from major version bump
• Provide comprehensive review feedback

---

Review Feedback

✅ Code Quality and Best Practices

The change is minimal and well-structured:

• File: TUnit.Core/TUnit.Core.targets:11 - Updates _TUnitPolyfillVersion from 8.9.1 to 9.0.0
• Scope: Only affects projects targeting netstandard2.0, netstandard2.1, or .NETFramework
• Implementation: Properly uses MSBuild properties and conditional logic
• Integration: Handles both CPM (Central Package Management) and non-CPM scenarios correctly

⚠️ Potential Issues & Considerations

Major Version Bump Concerns:

• This is a major version upgrade (8.x → 9.x), which typically indicates breaking changes
• However, since Polyfill is a compile-time only dependency (<PrivateAssets>all</PrivateAssets>), runtime impact is minimal
• The package only affects compilation for older target frameworks that need modern C# features

Recommended Actions:

1. Test compilation with projects targeting the affected frameworks (netstandard2.0/2.1, .NET Framework)
2. Verify source generator tests pass since this could affect generated code for older targets
3. Check CI/CD pipeline to ensure all target frameworks build successfully

🔒 Security Concerns

• Low risk: Polyfill is a well-maintained Microsoft-ecosystem package by Simon Cropp
• Compile-time only: No runtime dependencies are introduced
• Scope limited: Only affects legacy target frameworks

⚡ Performance Considerations

• Positive impact: Newer versions often include performance improvements
• No runtime overhead: Package assets are excluded from runtime (<ExcludeAssets>runtime; native; contentfiles; build; buildtransitive</ExcludeAssets>)
• Build time: May improve compilation performance for older target frameworks

🧪 Test Coverage

Recommendations:

• Run the full test suite with dotnet test to ensure no compilation issues
• Pay special attention to projects using older target frameworks
• Consider testing with dotnet build across all TFMs

TUnit-Specific Testing:

• Source generator tests should pass: dotnet test TUnit.Core.SourceGenerator.Tests
• No snapshot updates should be needed since this is a build-time dependency change

Summary

This is a low-risk dependency update that should be safe to merge. The Polyfill package is well-maintained and only affects compilation for legacy target frameworks. The change follows TUnit's MSBuild conventions properly.

Recommendation: ✅ Approve - but verify CI passes and consider manual testing on older target frameworks.

---