[System.Text.Json] Use root-level patterns for generated unions

[eiriktsarpalis]

Uses root-level type patterns for source-generated union deconstructors. Candidate for .NET 11 backport.

Note

This pull request was created with GitHub Copilot.

[azure-pipelines]

Azure Pipelines:
Successfully started running 4 pipeline(s).
12 pipeline(s) were filtered out due to trigger conditions.
There may be pipelines that require an authorized user to comment /azp run to run.

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-system-text-json
See info in area-owners.md if you want to be subscribed.

[Copilot]

🟡 Changes recommended

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

This PR updates the System.Text.Json source generator’s emitted union deconstructor switch to use root-level type patterns (instead of matching through the union’s Value property), and refreshes the associated source-generator baselines and test commentary to reflect the new output.

Changes:

• Update JsonSourceGenerator.Emitter to emit root-level case patterns for UnionDeconstructor switch arms.
• Update source-generator output baselines for recursive and subclass-case union scenarios across netcoreapp and net462.
• Adjust unit-test comments to reflect the new “root-level pattern” approach.

File summaries

File	Description
src/libraries/System.Text.Json/gen/JsonSourceGenerator.Emitter.cs	Changes union deconstructor codegen to use root-level type patterns.
src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.Unit.Tests/JsonSourceGeneratorOutputTests.cs	Updates commentary for union baseline verification cases.
src/libraries/System.Text.Json/tests/Common/UnionTests.cs	Updates explanatory comments around union pattern behavior expectations.
src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.Unit.Tests/Baselines/SubclassCaseUnionType/netcoreapp/MyContext.Shape.g.cs.txt	Baseline update reflecting new deconstructor switch patterns.
src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.Unit.Tests/Baselines/SubclassCaseUnionType/net462/MyContext.Shape.g.cs.txt	Baseline update reflecting new deconstructor switch patterns.
src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.Unit.Tests/Baselines/RecursiveUnionType/netcoreapp/MyContext.Nat.g.cs.txt	Baseline update reflecting new deconstructor switch patterns.
src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.Unit.Tests/Baselines/RecursiveUnionType/net462/MyContext.Nat.g.cs.txt	Baseline update reflecting new deconstructor switch patterns.
src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.Unit.Tests/Baselines/RecursiveNullableUnionType/netcoreapp/MyContext.Nat.g.cs.txt	Baseline update reflecting new deconstructor switch patterns.
src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.Unit.Tests/Baselines/RecursiveNullableUnionType/net462/MyContext.Nat.g.cs.txt	Baseline update reflecting new deconstructor switch patterns.

Review details

• Files reviewed: 9/9 changed files
• Comments generated: 1
• Review effort level: Lite

[Copilot]

🟢 Approval recommended

Review details

• Files reviewed: 9/9 changed files
• Comments generated: 0 new
• Review effort level: Lite

[eiriktsarpalis]

/ba-g test failures are unrelated to STJ

[eiriktsarpalis]

/backport to release/11.0

[github-actions]

Started backporting to release/11.0 (link to workflow run)