Fix empty struct union candidate selection

[gcanti]

Summary by CodeRabbit

• Bug Fixes

  • Corrected schema handling for empty object structures in unions.
  • Empty object schemas now correctly accept all non-nullish values while continuing to reject undefined.

• Tests

  • Expanded coverage across primitive values, objects, arrays, functions, symbols, bigints, and null.

[changeset-bot]

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Latest commit: b27cb01

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[coderabbitai]

No actionable comments were generated in the recent review. 🎉

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Run ID: f6f3e48e-6848-43fd-92bd-5538c29c5022

📥 Commits

Reviewing files that changed from the base of the PR and between 30effdb and b27cb01.

📒 Files selected for processing (2)

• packages/effect/src/SchemaAST.ts
• packages/effect/test/schema/Schema.test.ts

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📝 Walkthrough

Walkthrough

Changes

Empty Struct Union Semantics

Layer / File(s)	Summary
Broaden empty object candidates and validate unions packages/effect/src/SchemaAST.ts, packages/effect/test/schema/Schema.test.ts	Empty object AST nodes now include primitive and function candidate types, with tests covering successful decoding of non-undefined values and rejection of undefined.

Estimated code review effort: 2 (Simple) | ~10 minutes

🚥 Pre-merge checks | ✅ 2

✅ Passed checks (2 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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[github-actions]

Bundle Size Analysis

File Name	Current Size	Previous Size	Difference
basic.ts	6.64 KB	6.64 KB	0.00 KB (0.00%)
batching.ts	9.42 KB	9.42 KB	0.00 KB (0.00%)
brand.ts	6.25 KB	6.25 KB	0.00 KB (0.00%)
cache.ts	10.13 KB	10.13 KB	0.00 KB (0.00%)
config.ts	19.08 KB	19.08 KB	+0.00 KB (+0.01%)
differ.ts	18.38 KB	18.38 KB	-0.00 KB (-0.01%)
http-client.ts	20.73 KB	20.73 KB	0.00 KB (0.00%)
logger.ts	10.30 KB	10.30 KB	0.00 KB (0.00%)
metric.ts	8.56 KB	8.56 KB	0.00 KB (0.00%)
optic.ts	7.41 KB	7.41 KB	0.00 KB (0.00%)
pubsub.ts	14.17 KB	14.17 KB	0.00 KB (0.00%)
queue.ts	11.10 KB	11.10 KB	0.00 KB (0.00%)
schedule.ts	10.28 KB	10.28 KB	0.00 KB (0.00%)
schema-class.ts	18.16 KB	18.16 KB	0.00 KB (0.00%)
schema-fromJsonSchemaDocument.ts	27.86 KB	27.86 KB	0.00 KB (0.00%)
schema-representation-roundtrip.ts	24.14 KB	24.14 KB	+0.00 KB (+0.01%)
schema-string-transformation.ts	12.68 KB	12.68 KB	0.00 KB (0.00%)
schema-string.ts	10.36 KB	10.36 KB	0.00 KB (0.00%)
schema-template-literal.ts	14.38 KB	14.38 KB	0.00 KB (0.00%)
schema-toArbitraryLazy.ts	20.93 KB	20.93 KB	-0.00 KB (-0.01%)
schema-toCodeDocument.ts	23.34 KB	23.34 KB	-0.00 KB (-0.01%)
schema-toCodecJson.ts	18.27 KB	18.27 KB	-0.00 KB (-0.01%)
schema-toEquivalence.ts	18.01 KB	18.01 KB	-0.00 KB (-0.01%)
schema-toFormatter.ts	17.86 KB	17.86 KB	-0.00 KB (-0.01%)
schema-toJsonSchemaDocument.ts	21.47 KB	21.47 KB	-0.00 KB (-0.00%)
schema-toRepresentation.ts	18.55 KB	18.55 KB	-0.00 KB (-0.01%)
schema.ts	17.43 KB	17.43 KB	+0.00 KB (+0.01%)
stm.ts	12.07 KB	12.07 KB	0.00 KB (0.00%)
stream.ts	9.36 KB	9.36 KB	0.00 KB (0.00%)

[nikelborm]

Would I need to update #6434 after this lands?