fix(cli): preserve variant-inherited keys in discriminated union example validation

[patrickthornton]

Closes FER-10932

Summary

• fern check was emitting spurious Example is missing required property "<key>" errors for discriminated-union examples when the union had inferred base-properties AND the variants also inherited those same property keys via their extends chain.
• When the union validator constructed the variant-level example, it stripped every key that matched the union's base-properties — but the variant still required those keys via its own extends, so the variant validator then flagged them as missing.
• Fix: keep keys the variant itself declares (own properties or inherited via extends) when building the example handed to variant-level validation. Keys owned exclusively by the union are still filtered as before.

Example

Surfaced on an Auth0 spec where ConnectionResponseContent is a 55-variant discriminated union with infer-discriminated-union-base-properties: true. The inferred base-properties (id, name, display_name, …) overlap with what each variant inherits via extends: ConnectionResponseCommon → CreateConnectionCommon → ConnectionCommon. Before the fix, fern check emitted 9 false errors; after, it's clean:

# before
[error] Example is missing required property "response.body.id"
[error] Example is missing required property "response.body.name"
[error] Example is missing required property "response.body.connections.0.id"
...
Found 9 errors and 19 warnings.

# after
Found 0 errors and 19 warnings.

The minimal repro is captured as a fixture in this PR. Variants Dog/Cat both extends: AnimalBase (which declares id, name), and the union Animal declares the same two keys under base-properties:

AnimalBase:
  properties:
    id: string
    name: string
Dog:
  extends: AnimalBase
  properties:
    breed: string
Animal:
  discriminant: kind
  base-properties:
    id: string
    name: string
  union:
    dog: Dog
  examples:
    - value: { kind: dog, id: rex-1, name: Rex, breed: corgi }

Pre-fix, the example above produced false Example is missing required property "id" / "name" violations; post-fix it validates cleanly. A deliberately-broken example (missing id) still surfaces the violation, confirming the validator isn't going lax.

Test plan

• New regression fixture valid-example-type/fixtures/union-with-extends-overlap/ + test case asserts only the deliberately-invalid example produces a violation
• Reverting the fix locally makes the new test fail; restoring it makes it pass
• All 70 existing @fern-api/fern-definition-validator tests still pass
• fern check against the original auth0 reproduction goes from 9 errors to 0

🤖 Generated with Claude Code

---

[claude]

Claude Code Review

This repository is configured for manual code reviews. Comment @claude review to trigger a review and subscribe this PR to future pushes, or @claude review once for a one-time review.

_{Tip: disable this comment in your organization's Code Review settings.}

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 3 additional findings.

[github-actions]

Docs Generation Benchmark Results

Comparing PR branch against median of 5 nightly run(s) on main (latest: 2026-06-01T05:35:53Z).

Fixture	main	PR	Delta
docs	230.6s (n=5)	221.4s (35 versions)	-9.2s (-4.0%)

Docs generation runs fern generate --docs --preview end-to-end against the benchmark fixture with 35 API versions (each version: markdown processing + OpenAPI-to-IR + FDR upload).
Delta is computed against the nightly baseline on main.
Baseline from nightly run(s) on main (latest: 2026-06-01T05:35:53Z). Trigger benchmark-baseline to refresh.
Last updated: 2026-06-01 20:40 UTC

[github-actions]

SDK Generation Benchmark Results

Comparing PR branch against median of 5 nightly run(s) on main (latest: 2026-06-01T05:35:53Z).

Full benchmark table (click to expand)

Generator	Spec	main (generator)	main (E2E)	PR (generator)	Delta
csharp-sdk	square	73s (n=5)	110s (n=5)	63s	-10s (-13.7%)
go-sdk	square	136s (n=5)	286s (n=5)	130s	-6s (-4.4%)
java-sdk	square	225s (n=5)	273s (n=5)	233s	+8s (+3.6%)
php-sdk	square	59s (n=5)	81s (n=5)	50s	-9s (-15.3%)
python-sdk	square	137s (n=5)	236s (n=5)	130s	-7s (-5.1%)
ruby-sdk-v2	square	90s (n=5)	125s (n=5)	88s	-2s (-2.2%)
rust-sdk	square	167s (n=5)	170s (n=5)	166s	-1s (-0.6%)
swift-sdk	square	55s (n=5)	745s (n=5)	53s	-2s (-3.6%)
ts-sdk	square	229s (n=5)	234s (n=5)	223s	-6s (-2.6%)

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: 2026-06-01T05:35:53Z). Trigger benchmark-baseline to refresh.
Last updated: 2026-06-01 20:42 UTC