fix(go): dedupe union base properties that collide with extended properties

[iamnamananand996]

Description

Second in the one-by-one series to clear seed/go-sdk/seed.yml allowedFailures. Fixes the property-access fixture.

A discriminated union that both extends an object and declares base-properties with an overlapping name produced duplicate Go fields. In the fixture, UserOrAdminDiscriminated does:

UserOrAdminDiscriminated:
  base-properties:
    normal: string   # collides with Foo.normal below
    foo: Foo
  extends: Foo        # Foo has: normal, read, write
  union: { user: User, admin: Admin, empty: {} }

Both the extended Foo.normal and the base-property normal mapped to a Normal field, so the v1 model generator emitted Normal twice → types.go:488: Normal redeclared, duplicate GetNormal, and duplicate struct-literal keys. The generated SDK did not compile.

Changes Made

• generators/go/internal/generator/model.go: added unionExtendedPropertyNames() helper that collects (transitively) the exported field names contributed by a union's extends. In VisitUnion (struct def, UnmarshalJSON, MarshalJSON) and getTypeFieldsForUnion (getters), base properties whose exported name is already contributed by an extended object are skipped — the extended property already covers them.
• seed/go-sdk/seed.yml: removed property-access from allowedFailures.
• Regenerated seed/go-sdk/property-access/ (types.go, types_test.go): single Normal field/getter now.
• Added changelog generators/go/sdk/changes/unreleased/fix-union-base-property-collision.yml (type: fix).
• Updated README.md generator (if applicable) — n/a

Testing

• pnpm seed test --generator go-sdk --fixture property-access --local now passes (was a build failure: Normal redeclared).
• Full local go-sdk seed suite: only property-access output changed; no other fixture's generated code changed (verified via git status). Remaining failures are the still-listed allowedFailures plus known flaky WireMock runtime tests that generate byte-identical code to main.
• v1 generator go build ./..., go vet, go test ./internal/generator/..., and gofmt all clean.

Link to Devin session: https://app.devin.ai/sessions/6ba63e4405694aa8ab8875f0dbfd9afc
Requested by: @iamnamananand996

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✅ Devin Review: No Issues Found

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SDK Generation Benchmark Results

Comparing PR branch against median of 5 nightly run(s) on main (latest: 2026-06-05T05:30:09Z).

Full benchmark table (click to expand)

Generator	Spec	main (generator)	main (E2E)	PR (generator)	Delta
go-sdk	square	139s (n=5)	283s (n=5)	128s	-11s (-7.9%)

main (generator): generator-only time via --skip-scripts (includes Docker image build, container startup, IR parsing, and code generation — this is the same Docker-based flow customers use via fern generate). main (E2E): full customer-observable time including build/test scripts (nightly baseline, informational). Delta is computed against generator-only baseline.
⚠️ = generation exited with a non-zero exit code (timing may not reflect a successful run).
Baseline from nightly runs on main (latest: 2026-06-05T05:30:09Z). Trigger benchmark-baseline to refresh.
Last updated: 2026-06-05 20:26 UTC