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Add support for @discriminated unions - #6050
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📦 Package size report1 package changed size, +3.21 KB (+0.0%) packed overall.
13 package(s) with no notable change
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Go emitter diffBaseline Diff summary: 39 file(s), +2967 / -3 Rendered diff: inline on the run summary, or the emitter-diff-go-html artifact. Informational check (core/eng/emitter-diff); does not block the PR. |
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🟡 Changes recommended
Union collection responses can fail generation or decoding due to missing union naming and discriminator-aware handling, while string-encoded numeric union examples produce uncompilable Go samples.
2 open issues
What changed in this PR
Adds Go emitter support for @discriminated unions using polymorphic interfaces and variant models. Unions without a base model reject unknown discriminator values rather than falling back to a base type.
Changes:
- Supports inline and object-envelope variants, custom property names, and example generation.
- Updates polymorphic code generation and fake-server handling.
- Adds scenario baselines, adaptation tests, and Spector coverage.
| File | Description |
|---|---|
| packages/typespec-go/test/unittest/type-adaptation.test.ts | Tests union interface and variant name collisions. |
| packages/typespec-go/test/unittest/scenarios/discriminated-unions.md | Adds generated baselines for unions, examples, and hierarchies. |
| packages/typespec-go/test/unittest/scenario-suites/discriminated-unions.test.ts | Registers the union scenario suite. |
| packages/typespec-go/test/unittest/scenario-runner.ts | Enables union inheritance in test fixtures. |
| packages/typespec-go/test/unittest/example-adaptation.test.ts | Tests nested example values and invalid discriminators. |
| packages/typespec-go/test/http-specs/type/union/disriminatedgroup/discriminatednoenvelopedefault_client_test.go | Tests default inline union requests and responses. |
| packages/typespec-go/test/http-specs/type/union/disriminatedgroup/discriminatednoenvelopecustomdiscriminator_client_test.go | Tests inline unions with a custom discriminator. |
| packages/typespec-go/test/http-specs/type/union/disriminatedgroup/discriminatedenvelopeobjectdefault_client_test.go | Tests default object-envelope unions. |
| packages/typespec-go/test/http-specs/type/union/disriminatedgroup/discriminatedenvelopeobjectcustomproperties_client_test.go | Tests custom envelope property names. |
| packages/typespec-go/test/http-specs/type/union/disriminatedgroup/discriminated_client_test.go | Tests invalid tags, null responses, serialization, and fake round trips. |
| packages/typespec-go/src/tcgcadapter/types.ts | Adapts unions into interfaces and concrete variants. |
| packages/typespec-go/src/tcgcadapter/clients.ts | Adapts union responses and raw example values. |
| packages/typespec-go/src/codemodel/type.ts | Makes polymorphic interface base models optional. |
| packages/typespec-go/src/codegen/fake/servers.ts | Preserves intermediate clients needed for nested server dispatch. |
| packages/typespec-go/src/codegen/core/polymorphics.ts | Emits decoding errors for unions without a fallback model. |
| packages/typespec-go/src/codegen/core/models.ts | Adds marker methods for union variants. |
| packages/typespec-go/src/codegen/core/interfaces.ts | Generates union interfaces without base-model accessors. |
| packages/typespec-go/src/codegen/core/helpers.ts | Handles interfaces without base models during serialization traversal. |
| packages/typespec-go/src/codegen/core/example.ts | Selects a concrete variant for generated placeholder examples. |
| packages/typespec-go/spector.config.http.yaml | Enables discriminated-union Spector scenarios. |
| .chronus/changes/main-2026-10-6-13-52-58.md | Documents the new Go emitter capability. |
Review effort: Balanced
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You can try these changes here
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Joel Hendrix (jhendrixMSFT)
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🟡 Changes recommended
Helper discovery skips AnyResult response types, potentially leaving generated packages uncompilable, while discriminator decoding still fails for nested collections, fake request bodies, additional properties, and whitespace-padded null.
5 open issues
2 resolved since last review
Review effort: Balanced
| if (interfaceType.rootType) { | ||
| text += `${indent.get()}b = &${prefix}${interfaceType.rootType.name}{}\n`; | ||
| } else { | ||
| text += `${indent.get()}return nil, fmt.Errorf("unrecognized discriminator value %v for ${interfaceType.name}", m["${interfaceType.discriminatorField}"])\n`; |
| return unmarshallerText; | ||
| } | ||
| if (format === "JSON" || format === "XML") { | ||
| if (format === "JSON" && (go.isSlice(type, "interface") || go.isMap(type, "interface"))) { |
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| if (type.discriminatedOptions && !this.getSseMetadata(type)) { | ||
| return this.getDiscriminatedUnion(type); |
| modelType.fields.push( | ||
| new go.ModelField("AdditionalProperties", addlPropsType, "", annotations), | ||
| ); | ||
| fields.push(new go.ModelField("AdditionalProperties", addlPropsType, "", annotations)); |
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The generated code is polymorphic like for @Discriminator types, the only difference is there's no fallback base type.
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The generated code is polymorphic like for Discriminator types, the only difference is there's no fallback base type.