[http-client-csharp] Preserve last-contract model base types

[Wei Hu (live1206)]

Summary

• preserve a model's previously shipped CLR base when the current base hierarchy no longer contains it
• retain a newer immediate base when it already derives from the last-contract base
• resolve generated, referenced, nested, and constructed-generic preserved bases against the current build using complete CLR metadata identity
• continue resolution past unusable cached placeholders to find a usable current-compilation base
• keep properties shipped on the previous GA base inherited without duplicating them on the derived model
• keep an explicit custom base authoritative when restoring the GA base would conflict across partial declarations
• skip restoration when constructor chaining, direct property collisions, or the current discriminator hierarchy would make the generated C# invalid

This intentionally does not materialize properties introduced only by a displaced, newer TypeSpec base. That broader reconciliation can be added when an SDK use case requires it.

Fixes #11816

Validation

• Generated, referenced, multi-level nested, and constructed-generic base regressions verify the previous CLR inheritance relationship is restored and the correct metadata identity is selected.
• The promoted-base regression verifies a current base that already derives from the previous base is retained.
• The inherited-property regression verifies GA base properties remain available and are not duplicated on the derived model.
• Custom-base and constructor regressions verify MTG retains the current base and emits a diagnostic when restoring the previous base would produce invalid C#.
• Generated and symbol-backed property-collision regressions verify final back-compatible CLR names and compile the resulting model hierarchies.
• The discriminator regression verifies restoration does not detach a model from its current polymorphic hierarchy and compiles the generated deserializer.

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/@typespec/http-client-csharp@11818

commit: 8c08b6e

[github-actions]

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[Wei Hu (live1206)]

Regen: Azure/azure-sdk-for-net#62636
The diff of model factory is from #11703

[Copilot]

🔵 Needs a closer look

It changes core model inheritance resolution and constructor-chaining constraints in the generator, which is high-impact even with good regression coverage.

Pull request overview

This PR extends the http-client-csharp generator’s last-contract back-compat logic to preserve a model’s previously shipped CLR base type when the current TypeSpec base hierarchy no longer contains it, while guarding against cases that would produce invalid C# (e.g., conflicting partial bases or inaccessible constructor chaining).

Changes:

• Update ModelProvider base-type selection to restore last-contract bases when safe, retain promoted bases when already derived, and resolve preserved bases against the current build.
• Add constructor-accessibility checks for symbol-backed base types and emit a new diagnostic for incompatible custom-base conflicts.
• Add targeted regression tests and test data for generated, referenced, and framework base-type preservation scenarios.

File summaries

File	Description
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/ModelProviders/TestData/ModelProviderTests/BackCompat_ReferencedLastContractBaseWithInternalParameterlessConstructorIsNotRestored(LastContract)/Models.cs	Adds last-contract baseline source for an external base with an internal parameterless ctor (should not be restored).
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/ModelProviders/TestData/ModelProviderTests/BackCompat_NonGeneratedLastContractBaseWithoutParameterlessConstructorIsNotRestored(LastContract)/Models.cs	Adds last-contract baseline source for a non-generated base lacking a parameterless ctor (should not be restored).
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/ModelProviders/TestData/ModelProviderTests/BackCompat_NonGeneratedLastContractBaseWithoutParameterlessConstructorIsNotRestored(Current)/ExternalBase.cs	Adds current-build external base type used to validate constructor-chaining constraints.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/ModelProviders/TestData/ModelProviderTests/BackCompat_LastContractBaseRestoresInheritedProperties/Models.cs	Adds baseline types to validate inherited properties remain inherited and are not duplicated.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/ModelProviders/TestData/ModelProviderTests/BackCompat_CustomBaseTakesPrecedenceOverDifferentLastContractBase(LastContract)/Models.cs	Adds last-contract baseline for a previous base used in the custom-base precedence scenario.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/ModelProviders/TestData/ModelProviderTests/BackCompat_CustomBaseTakesPrecedenceOverDifferentLastContractBase(Current)/Models.cs	Adds current-build custom base partial to validate partial-base conflict handling.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/ModelProviders/TestData/ModelProviderTests/BackCompat_BaseTypeChangePreservesNonGeneratedLastContractBaseType/DerivedModel.cs	Adds baseline derived model inheriting from a framework type (e.g., System.Exception).
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/ModelProviders/TestData/ModelProviderTests/BackCompat_BaseTypeChangePreservesLastContractBaseType/Models.cs	Adds baseline types to validate restoring a previously shipped generated base.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/ModelProviders/ModelProviderTests.cs	Adds regression tests covering base restoration, promoted bases, inherited properties, custom-base conflicts, and ctor-accessibility constraints.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/src/Utilities/DiagnosticCodes.cs	Introduces incompatible-backcompat-base-type diagnostic code.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/src/Providers/NamedTypeSymbolProvider.cs	Adds HasAccessibleParameterlessConstructor to validate ctor accessibility across assembly boundaries.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/src/Providers/ModelProvider.cs	Implements last-contract base-type preservation logic and resolves preserved bases against the current compilation/providers.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/src/EmitterRpc/Emitter.cs	Adds display label for the new back-compat change category.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/src/EmitterRpc/BackCompatibilityChangeCategory.cs	Adds ModelBaseTypePreserved category for back-compat reporting.

Review details

• Files reviewed: 14/14 changed files
• Comments generated: 0
• Review effort level: Lite

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[Copilot]

🔵 Needs a closer look

It changes core back-compat inheritance selection logic with broad API-surface implications that warrants final human verification beyond the added regressions.

Review details

• Files reviewed: 14/14 changed files
• Comments generated: 0 new
• Review effort level: Lite

[Copilot]

🔵 Needs a closer look

The base-restoration property-collision guard currently treats private base properties as collisions, which can incorrectly block restoring a last-contract base type.

Review details

Suppressed comments (1)

Previously missed (1) — in code that hasn't changed since the last review.

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/src/Providers/ModelProvider.cs:391

• HasDirectPropertyNameCollision treats private properties on a symbol-backed/base type as collisions. In C#, private base members are not inherited, so they cannot cause the CS0108 hiding warning/error and shouldn’t block restoring the last-contract base; this can incorrectly skip base restoration and break assignability compatibility.

• Files reviewed: 21/21 changed files
• Comments generated: 0 new
• Review effort level: Lite

[Copilot]

🔵 Needs a closer look

The change alters core model inheritance/back-compat behavior with many edge-case guards (resolution, constructors, collisions, discriminator hierarchies) that warrants final human review despite strong regression coverage.

Review details

• Files reviewed: 21/21 changed files
• Comments generated: 0 new
• Review effort level: Lite

[Wei Hu (live1206)]

latest regen: Azure/azure-sdk-for-net#62687

[Copilot]

🟡 Changes recommended

There is a C# compile issue in ScmModelProvider where an override reduces method accessibility, which must be corrected before merge.

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

Review details

• Files reviewed: 21/21 changed files
• Comments generated: 1
• Review effort level: Lite

[Copilot]

🔵 Needs a closer look

It introduces non-trivial base-type reconciliation logic that can affect broad model/serialization behaviors, so it warrants final human review despite strong regression coverage.

Review details

• Files reviewed: 21/21 changed files
• Comments generated: 0 new
• Review effort level: Lite

[Copilot]

🟢 Approval recommended

The back-compat base restoration logic is guarded for known invalid-code cases and is covered by targeted regressions (including compilation checks) plus documentation updates.

Review details

• Files reviewed: 22/22 changed files
• Comments generated: 0 new
• Review effort level: Lite

[Copilot]

🔵 Needs a closer look

It changes core model-type construction/back-compat behavior in the generator (including symbol/metadata resolution and inheritance/collision rules) and warrants a final human review despite strong regression coverage.

Review details

• Files reviewed: 23/23 changed files
• Comments generated: 0 new
• Review effort level: Lite