feat: back compat support for method param types

[jorgerangel-msft]

Extends method back-compat to preserve source/binary compatibility when a service method's parameters evolve. For each public/protected last-contract method that changed in a recoverable way, a hidden (EditorBrowsable.Never) overload reproducing the previous signature is generated that delegates to the current method.

Scenarios now supported:

• Value-type parameter nullability removed (T? → T) — adds a hidden overload with the previous nullable signature that forwards via .Value, guarded by Argument.AssertNotNull (also covers extensible enums, matched by name).
• Nullable optional parameter became required — adds a hidden reduced-arity overload that drops the parameter and delegates with its previous default, so omit-callers still compile.

fixes: #11223

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commit: de5576d

[Copilot]

Pull request overview

This PR extends the C# generator’s method back-compat logic to handle additional signature evolution cases (beyond “new optional parameters”), specifically around nullable value-type parameters becoming non-nullable and nullable optional parameters becoming required, while adding targeted unit/golden tests and logging categories.

Changes:

• Add generation of hidden back-compat overloads for T? -> T value-type parameter nullability removal and for nullable parameters that changed from optional to required (via reduced-arity overloads).
• Introduce signature comparison that can optionally include nullability, and use it to avoid duplicate/incorrect overload classification.
• Add/extend tests and expected outputs covering the new back-compat scenarios and cross-pass interactions.

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/Utilities/BackCompatHelperTests.cs	New unit tests for the new back-compat detection helpers.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/TypeProviderTests.cs	New integration/golden-style tests validating emitted overloads for the new scenarios.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/TestData/TypeProviderTests/BuildMethodsForBackCompatibilityUnwrapsRelaxedNullableSharedParameterInOptionalOverload/NullabilityAndOptionalChangeType.cs	Last-contract source stub used by the new cross-pass test.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/TestData/TypeProviderTests/BuildMethodsForBackCompatibilityUnwrapsRelaxedNullableSharedParameterInOptionalOverload.cs	Expected generated output for the cross-pass case (unwrap .Value forwarding).
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/TestData/TypeProviderTests/BuildMethodsForBackCompatibilityAddsOverloadForRelaxedNullableValueTypeParameter/NullabilityChangeType.cs	Last-contract source stub for the T? -> T overload case.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/TestData/TypeProviderTests/BuildMethodsForBackCompatibilityAddsOverloadForRelaxedNullableValueTypeParameter.cs	Expected generated output for the nullability-change overload.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/TestData/TypeProviderTests/BuildMethodsForBackCompatibilityAddsOverloadForNullableParameterThatBecameRequired/OptionalityChangeType.cs	Last-contract source stub for the optional-to-required nullable parameter case.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/TestData/TypeProviderTests/BuildMethodsForBackCompatibilityAddsOverloadForNullableParameterThatBecameRequired.cs	Expected generated output for the reduced-arity optionality-restoration overload.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/src/Utilities/BackCompatHelper.cs	Core implementation: unified overload selection + new detection/build helpers + improved logging.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/src/Providers/TypeProvider.cs	Switch to the new unified back-compat overload entrypoint.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/src/Primitives/MethodSignatureBase.cs	Add a comparer variant that includes nullability in parameter type matching.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/src/Primitives/CSharpType.cs	Add AreNamesEqual(..., checkNullability) helper to support nullability-aware comparisons.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/src/EmitterRpc/Emitter.cs	Map new back-compat categories to readable debug strings.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/src/EmitterRpc/BackCompatibilityChangeCategory.cs	Add new categories for nullability-change and optionality-restoration overload additions.

[Copilot]

Pull request overview

Copilot reviewed 14 out of 14 changed files in this pull request and generated 1 comment.

[Copilot]

Pull request overview

Copilot reviewed 14 out of 14 changed files in this pull request and generated 1 comment.