Skip C# back compatibility for previously released experimental types

[Copilot]

C# back-compat handling currently preserves experimental APIs as stable contracts, restricting changes even when those types graduate to stable.

• Contract lookup: Exclude released types carrying System.Diagnostics.CodeAnalysis.ExperimentalAttribute from the shared compatibility lookup, covering type-shape preservation and member restoration.
• Compatibility boundaries: Base the decision on the previous release. Newly marking a stable type experimental does not remove its compatibility guarantees; member-level experimental handling remains unchanged.

• Fixes Consider Evaluating Experimental Types as Part of Back Compat Handling #12132

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[pkg-pr-new]

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npm i https://pkg.pr.new/@typespec/http-client-csharp@12159

commit: 8e77a2f

[Copilot]

🟢 Approval recommended

The focused implementation matches the stated compatibility boundary and is comprehensively tested.

0 open findings

What changed in this PR

Prevents C# compatibility logic from preserving APIs on types that were experimental in the previous release.

Changes:

• Filters experimental types from previous-contract lookup.
• Adds coverage for classes and fixed/extensible enums.
• Confirms current experimental status and unrelated attributes do not affect compatibility.

File	Description
SourceInputModel.cs	Excludes previously experimental types.
TypeProviderTests.cs	Tests type compatibility boundaries.
Types.cs	Provides type contract fixtures.
EnumProviderTests.cs	Tests enum compatibility behavior.
Choice.cs (2 files)	Provides fixed and extensible enum fixtures.

🧠 Review effort: Balanced

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[Copilot]

🟢 Approval recommended

The implementation is narrowly scoped and comprehensively tests previous-versus-current experimental status across supported type shapes.

0 open findings

🧠 Review effort: Balanced

[Copilot]

🟡 Changes recommended

The implementation changes member-level compatibility despite the PR description explicitly preserving that boundary.

2 open findings

• Avoid treating experimental members as non-public · New
• Experimental methods incorrectly excluded from compatibility shims

1 resolved since last review

• Protected constructors incorrectly promoted to public API

🧠 Review effort: Balanced

[Copilot]

🔵 Needs a closer look

Member-level behavior contradicts the stated scope, and the documentation overstates fixed-enum handling.

0 open findings

2 resolved since last review

• Avoid treating experimental members as non-public
• Experimental methods incorrectly excluded from compatibility shims

Previously missed (1)

In code that hasn't changed since last review

Fixed enums can resurrect experimental members without filtering

packages/​http-client-csharp/​generator/​docs/​backward-compatibility.md:59

The member-level claim is not true for fixed enums: FixedEnumProvider.cs:129-180 iterates every historical field and can resurrect an experimental enum member without consulting this attribute. Since the PR description also says member handling is unchanged, remove these member-level guarantees; otherwise the fixed-enum path needs matching filtering and coverage.

🧠 Review effort: Balanced

[Copilot]

🔵 Needs a closer look

Member-level filtering contradicts the stated requirement that member-level experimental handling remain unchanged.

0 open findings

🧠 Review effort: Balanced