Fix C# reference-map accessibility for hidden union types

[live1206]

Summary

• restore the provider-graph reference map after its main-branch rollback in Revert "Use provider C# reference map with pre-write internalization" #11282, preserving the performance improvement over Roslyn-based generated-source analysis
• keep all follow-up compatibility fixes already developed in this PR:
  • preserve concrete and nested discriminator variants
  • distinguish metadata-only union items erased to BinaryData from semantic public and implementation dependencies
  • retain generated types that were public in the configured last GA contract, including effectively public nested types
  • preserve the owning model's accessibility when serialization partial providers have different default modifiers
  • derive collection-header helper dependencies from the selected HttpRequestApi, avoiding unused PipelineRequestHeadersExtensions output
• reconcile Preserve internal C# helper types during trimming #11285 with the restored provider graph:
  • retain its shared InternalHelperProvider modifiers and lazy helper construction
  • preserve its intentional non-root registration so generator-owned internal helpers remain emitted without becoming accessibility roots
• add positive and negative coverage for accessibility, retention, GA contracts, nested effective accessibility, serialization ownership, metadata-only references, and request-helper dependencies

Fixes #11291.

Main-branch reconciliation

This branch merges current main and explicitly reverts squash commit 05406488b from #11282. The existing #11288 commits remain intact as follow-up commits rather than being rebased or squashed away.

Validation

• passed 1,712 core generator tests and 1,504 ClientModel tests
• passed package build, full C# test-library regeneration, formatting, Cop, and type-aware oxlint
• regenerated all 46 Azure and unbranded data-plane libraries with RegenPreview.ps1: 46 passed, 0 failed, with no untracked files or unexpected request helpers
• repeated the full SDK regeneration from latest Azure SDK main; the resulting 72-file expected patch was byte-for-byte identical
• reproduced the service-wide Purview generation command with the local generator; it completed successfully and produced only the expected DataMap helper removal
• regenerated App Configuration and Projects.Agents after restoring Preserve internal C# helper types during trimming #11285 helper retention; representative internal helper files remain byte-for-byte unchanged while the valid reference-map cleanup remains

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/@typespec/http-client-csharp@11288

commit: 15face8

[github-actions]

No changes needing a change description found.

[live1206]

Expected Azure SDK regeneration diff

The combined RegenPreview.ps1 -Azure -Unbranded run completed with 46 passed, 0 failed. The resulting SDK diff is 72 files, +11/-3,661. There are no untracked files.

Service/package	Diff	Expected reason
Azure.AI.Agents.Persistent	1 modified, -2	Remove unused imports
Azure.AI.Extensions.OpenAI	1 modified, +1	Restore an experimental-warning pragma
Azure.AI.Projects	2 modified, +3/-3	Remove an unused import and correct MRW warning pragmas
Azure.AI.Projects.Agents	38 files, +4/-2,834	Remove 35 duplicate internal tool-choice files and update related MRW registrations, docs, and imports
Azure.AI.AnomalyDetector	1 deleted, -60	Remove unused URI helper
Azure.Data.AppConfiguration	1 modified, -2	Remove stale ResponseError MRW registration/import
Azure.AI.Language.Conversations	1 deleted, -60	Remove unused URI helper
Azure.AI.Language.QuestionAnswering.Inference	1 deleted, -60	Remove unused URI helper
Azure.AI.Language.Text	1 deleted, -60	Remove unused URI helper
Azure.Communication.JobRouter	1 modified, -2	Remove stale ResponseError MRW registration/import
Azure.Security.CodeTransparency	1 deleted, -60	Remove unused URI helper
Azure.AI.ContentSafety	2 modified, -3	Remove stale MRW registration and unused imports
Azure.Messaging.EventGrid.Namespaces	1 deleted, -60	Remove unused URI helper
Azure.Messaging.EventGrid.SystemEvents	1 deleted, -60	Remove unused URI helper
Azure.Monitor.Ingestion	1 deleted, -60	Remove unused URI helper
Azure.Monitor.OpenTelemetry.Exporter	1 deleted, -60	Remove unused URI helper
Azure.Monitor.Query.Logs	2 files, -65	Remove unused URI helper and model-factory imports
Azure.Analytics.OnlineExperimentation	1 modified, -2	Remove stale MRW registration/import
Azure.Analytics.Purview.DataMap	1 deleted, -60	Remove unused URI helper
Azure.Data.SchemaRegistry	1 modified, -2	Remove stale MRW registration/import
Azure.Template	1 modified, -2	Remove unused model-factory imports
Azure.AI.Speech.Transcription	1 modified, -1	Remove unused model-factory import
Azure.AI.Translation.Document	2 modified, -3	Remove stale MRW registration and unused imports
Azure.AI.Translation.Text	1 deleted, -60	Remove unused URI helper
Azure.AI.Vision.ImageAnalysis	2 modified, -4	Remove stale MRW registration and unused imports
Azure.AI.VoiceLive	5 files, +3/-76	Remove unused URI helper; internalize two preview-only models and remove their factory method

There are no GA public API changes. The only accessibility change is the VoiceLive preview-only surface that was not present in GA 1.1.0.

[Copilot]

Pull request overview

This PR updates the C# generator’s provider reference-map analysis to preserve established public API surface (including last GA contract/public nested types) while preventing BinaryData-erased unions from incorrectly “publicizing” internal/abstract union members, and it fixes request-header helper retention by deriving collection-header helper dependencies from the selected HttpRequestApi.

Changes:

• Extend reference-map graph construction to distinguish union item references that affect public surface vs metadata-only union items (erased to BinaryData) and to root types that were public in the last contract.
• Improve discriminator-derived reachability handling for “known variant” cases (including intermediate bases and nested scenarios).
• Make collection-header helper retention conditional on HttpRequestApi capabilities, avoiding generation/retention of unused PipelineRequestHeadersExtensions for non-pipeline request APIs; add positive/negative test coverage.

Reviewed changes

Copilot reviewed 14 out of 14 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/ReferenceMap/ProviderReferenceMapAnalyzerTests.cs	Adds coverage for BinaryData/metadata-only unions, discriminator/nested accessibility, and last-contract rooting.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/ModelProviders/ModelProviderTests.cs	Verifies unverifiable additional-properties types are treated as metadata-only unions.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/src/ReferenceMap/ProviderReferenceMapAnalyzer.Helpers.cs	Adds last-contract public rooting helpers and threads union-item exclusions into custom-code rooting.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/src/ReferenceMap/ProviderReferenceMapAnalyzer.cs	Integrates union-item exclusions into public graph analysis and roots last-contract public types.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/src/ReferenceMap/ProviderReferenceMapAnalyzer.Candidates.cs	Roots last-contract public types during removal; refines discriminator reachability propagation.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/src/ReferenceMap/ProviderReferenceMapAnalyzer.BodyReferences.cs	Adds union-item exclusion handling and respects UnionItemTypeReferenceKind during reference traversal.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/src/Providers/ModelProvider.cs	Marks unverifiable types as metadata-only unions (to avoid retaining them as real dependencies).
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/src/Primitives/CSharpType.cs	Introduces UnionItemTypeReferenceKind and plumbs it through union construction/cloning.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/test/TestHelpers/MockHelpers.cs	Allows injecting a custom HttpRequestApi in ClientModel tests.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/test/ReferenceMap/ClientBodyDependencyReferenceMapTests.cs	Adds negative test ensuring scalar headers don’t retain PipelineRequestHeadersExtensions.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/test/Providers/RestClientProviders/RestClientProviderTests.cs	Adds coverage for request-API-dependent collection-header helper retention.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/src/Providers/RestClientProvider.cs	Uses HttpRequestApi capability to decide which collection-header helper type (if any) to retain.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/src/Providers/PipelineRequestProvider.cs	Implements the request-API helper capability for the default pipeline request API.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/src/Providers/Abstractions/HttpRequestApi.cs	Adds GetCollectionHeaderHelperType() capability hook (default: none).

[live1206]

Revalidated the combined Azure/unbranded regeneration from a fresh worktree at latest azure-sdk-for-net main commit d75bb4711893c84e7e3c4ec3062404f33ad693b2: 46 passed, 0 failed. The same 72 files changed with the same +11/-3,661 totals, and the complete SDK patch is byte-for-byte identical to the previously reviewed result.

[live1206]

latest regen: Azure/azure-sdk-for-net#61111

[jorgerangel-msft]

It looks like there is at least 1 change in the regen preview that doesn't compile: details

[live1206]

Follow-up for the Azure SDK full-regeneration build failure in Azure/azure-sdk-for-net#61111:

• Built all 26 affected SDK packages: 24 passed; Projects.Agents failed on unresolved OpenAI.InternalToolChoiceParam XML documentation, and Projects failed only transitively.
• Fixed the issue generically in b7bb20a: :C formatting now renders providers removed by the reference map as <c> text instead of invalid cref links, including removed generic arguments.
• Regenerated Projects.Agents with the local emitter; the stale reference is now <c>global::OpenAI.InternalToolChoiceParam</c>.
• Projects.Agents and Projects both build successfully across net8.0, net10.0, and netstandard2.0.
• Full validation passed: 1,712 core tests, 1,504 ClientModel tests, complete generated-library regeneration, package build/format, Cop, and type-aware oxlint.

[live1206]

Restored #11285’s intentional generic InternalHelperProvider non-root retention in 3a9384605. Helpers remain internal and do not become accessibility roots, but they are no longer removed solely because provider-graph reachability finds no call site. Focused App Configuration and Projects.Agents regeneration keeps representative helper files byte-for-byte unchanged while preserving the valid reference-map cleanup.

[live1206]

latest regen: Azure/azure-sdk-for-net#61150

[JoshLove-msft]

Reference-map correctness findings.

--generated by Copilot

[live1206]

latest regen: Azure/azure-sdk-for-net#61203