Add union extends discriminated union Spector scenarios

[Chenjie Shi (tadelesh)]

Summary

Add 18 GET/PUT Spector contracts for discriminated unions with explicit union extends model constraints and nominally inherited alternatives.

• Cover declared and injected discriminators, custom discriminator names and wire tags, enum discriminator types, direct/transitive inheritance, and recursive model/array/dictionary/nullable references.
• Preserve existing unconstrained discriminated-union scenarios and regenerate the scenario summary.
• Make Spector's scenario and mock validators honor per-scenario project feature flags, including union-extends, while preserving existing validation imports and emitter/linter behavior. Surface configuration diagnostics and add virtual-filesystem regression tests.

These scenarios define wire contracts, not language-specific SDK representations. Envelope scenarios are deferred.

Related: Azure/typespec-azure#5703.

Validation

• Relevant package/dependency build: pnpm -r --filter "@typespec/http-specs..." build.
• Spector regression suite: 40 tests passed.
• Scenario and mock API validation passed for the complete HTTP-specs corpus.
• All 18 remaining contracts passed against the Spector mock server.
• Spelling checks, repository-wide pnpm format, pnpm lint, documentation regeneration, and git diff --check passed.

The full pnpm build is blocked by an unrelated typespec-vs NuGet restore failure (NU1301, TLS handshake failure contacting api.nuget.org).

[pkg-pr-new]

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commit: e914166

[Copilot]

🟢 Approval recommended

The contracts, mocks, and configuration handling are consistent, with focused regression coverage and no blocking issues identified.

0 open findings

What changed in this PR

Adds 22 Spector wire contracts for constrained discriminated unions and enables configuration-aware validation.

Changes:

• Covers inherited alternatives, discriminator variations, envelopes, and recursive containers.
• Honors scenario feature flags and reports configuration diagnostics while preserving validation behavior.
• Adds regression tests, documentation, and package changelogs.

File	Description
packages/​spector/​test/​compile-scenario.test.ts	Tests feature flags and validation configuration.
packages/​spector/​src/​spec-utils/​compile-scenario.ts	Centralizes configuration-aware compilation.
packages/​spector/​src/​scenarios-resolver.ts	Uses the shared compilation helper.
packages/​spector/​src/​actions/​validate-mock-apis.ts	Applies configuration-aware mock validation.
packages/​spector/​README.md	Documents scenario feature flags.
packages/​http-specs/​specs/​type/​union/​discriminated/​extends/​tspconfig.yaml	Enables union-extends.
packages/​http-specs/​specs/​type/​union/​discriminated/​extends/​mockapi.ts	Defines mocks for all 22 contracts.
packages/​http-specs/​specs/​type/​union/​discriminated/​extends/​main.tsp	Defines constrained-union scenarios.
packages/​http-specs/​spec-summary.md	Documents the new endpoints and payloads.
.chronus/​changes/​spector-scenario-project-features.md	Records the Spector validation fix.
.chronus/​changes/​discriminated-union-extends-scenarios.md	Records the new HTTP scenarios.

🧠 Review effort: Balanced

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[github-actions]

All changed packages have been documented.

• ✅ @typespec/http-specs
• ✅ @typespec/spector

Show changes

@typespec/http-specs - feature ✏️

Add Spector scenarios for discriminated unions with model constraints, covering explicit and injected discriminators, custom wire tags, enum discriminators, transitive inheritance, and recursive containers.

@typespec/spector - fix ✏️

Honor per-scenario project feature flags when compiling scenarios and validating mock APIs, and report configuration diagnostics.

[azure-sdk-automation]

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[Copilot]

🟢 Approval recommended

No blocking issues remain; the retained changelog documentation suggestion is minor.

1 open finding

• Add required discriminated-union example to feature changelog · New

🧠 Review effort: Balanced

[Weidong Xu (weidongxu-microsoft)]

Could we separate recursive-union coverage from explicit-null preservation?

nullableFriend?: Pet | null permits omission, but Recursive.put and its @scenarioDoc require "nullableFriend": null exactly. With the fixed Java emitter, that optional null field is omitted, so the mock rejects the request with HTTP 400.

Recursive subtype dispatch for properties, arrays and dictionaries, plus a non-null nullable-union round trip, passed in both sync and async checks. Consider using a non-null Pet for nullableFriend in this fixture and updating @scenarioDoc, with explicit-null preservation covered separately.