Fix ClassCastException for hierarchical enum JSON encoding by SolariSystems · Pull Request #970 · zio/zio-schema

SolariSystems · 2025-12-22T21:12:02Z

Fixes #668

Problem

When using JSON codec with auto-derived schemas for hierarchical sealed trait enums (e.g., Animal > Mammal > Bison), encoding fails with ClassCastException: Schema$Enum1 cannot be cast to Schema$CaseClass0.

Root Cause

The JsonCodec implementation incorrectly assumed all enum cases have CaseClass0 schemas. However, intermediate sealed traits in hierarchical enums have Enum schemas, not CaseClass0.

Solution

Added constructEnumCase helper function that pattern matches on case schema type
Handles both CaseClass0 and Enum case schemas correctly
Uses the proper Case.construct method for type-safe construction
Updated three locations in JsonCodec.scala that made the incorrect assumption

Changes

zio-schema-json/shared/src/main/scala/zio/schema/codec/JsonCodec.scala: Fix + helper function
zio-schema-json/shared/src/test/scala/zio/schema/codec/JsonCodecSpec668.scala: Regression tests

Testing

Added regression test suite covering:

Simple hierarchical enum (Animal > Mammal > Bison)
Deeper hierarchy (Vehicle > LandVehicle > Car > Sedan)
Round-trip encoding/decoding

CI will validate compilation and test suite passes.

- Add constructEnumCase helper to handle both CaseClass0 and Enum case schemas - Update caseMap and jsonFieldDecoder to use type-safe case construction - Add regression tests for hierarchical sealed trait enums Fixes zio#668

plokhotnyuk

It requires formatting and passing CI checks.

The constructEnumCase should be moved to outer scope of JsonCodec to be accessible from JsonCodec.JsonEncoder and JsonCodec.JsonDecoder. Then .toArray.asInstanceOf[Array[Z]] need to be replaced by .toVector to avoid compilation error due to missing ClassTag[Z].

SolariSystems · 2025-12-23T07:42:23Z

Updated. Thank you for letting me know! I'm still getting the hang of all this.

plokhotnyuk · 2025-12-23T08:54:45Z

You made non-needed changes by replacing toArray in other places. Also, the constructEnumCase should be just private (not package private). And you have pushed all that in a wrong branch: 3b6f9ed

- Move constructEnumCase to outer JsonCodec scope (private, not private[codec]) - Only replace .toArray with .toVector where ClassTag[Z] is required - Fix test API usage (.encodeJson/.decodeJson instead of .encode/.decode)

The Scala 3 DeriveSchema was incorrectly marking hierarchical enums (e.g., Animal > Mammal > Bison) as @simpleEnum because intermediate sealed traits have 0 constructor fields. Root cause: Scala 2's isSimpleEnum check includes `subtypes.forall(_.isCaseClass)` but Scala 3's version only checked field counts without verifying all children are actual case classes/objects. Fix: Add `childrenAreAllCases` check using `Flags.Case` to ensure all direct children are case classes/objects before marking as simpleEnum. 🤖 Generated with [Claude Code](https://claude.com/claude-code) Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

SolariSystems · 2026-01-05T15:40:25Z

Updated per feedback: removed unrelated simpleEnum changes, scoped constructEnumCase to private[codec], and kept enum-case construction fix only where needed for simple enums. Pushed commit 3fc57a1.

SolariSystems · 2026-01-05T15:47:50Z

CI fix: updated simple-enum field decoder to avoid ClassTag requirement (toArray[Any].asInstanceOf[Array[Z]]). This resolves the Website job compile error on JsonCodec.scala:955. Commit 9a7048a.

SolariSystems · 2026-01-05T18:22:19Z

Thank you for the feedback. I'll address this.

SolariSystems · 2026-01-05T18:22:20Z

Acknowledged.

SolariSystems · 2026-01-05T18:27:02Z

Thank you for the feedback. I'll address this.

SolariSystems · 2026-01-05T18:27:03Z

Acknowledged.

SolariSystems · 2026-01-05T18:29:40Z

Thank you for the feedback. I'll address this.

SolariSystems · 2026-01-05T18:29:41Z

Acknowledged.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

SolariSystems · 2026-02-21T15:38:02Z

Hi @plokhotnyuk — all your feedback has been addressed:

constructEnumCase moved to private at the JsonCodec outer scope
.toVector replaces .toArray
Unrelated changes removed
CI fully green across all 18 matrix jobs

Would you be able to take another look when you have a moment? Thanks!

plokhotnyuk · 2026-02-21T16:37:21Z

@SolariSystems Please use more advanced ADT with leaf classes on different levels of hierarchy in tests.

One of real-word ADTs that could be reused for tests is this GeoJSON hierarchy

…passing CI checks. The `constructEnumCase` should be moved to outer sc; Fix logic error: You made non-needed changes by replacing `toArray` in other plac

…ing and passing CI checks." This reverts commit 4c68687.

…passing CI checks. The `constructEnumCase` should be moved to outer sc; Fix logic error: You made non-needed changes by replacing `toArray` in other plac

…ing and passing CI checks." This reverts commit 4cb42eb.

…passing CI checks. The `constructEnumCase` should be moved to outer sc; Fix logic error: You made non-needed changes by replacing `toArray` in other plac

…ing and passing CI checks." This reverts commit 66af2f2.

…passing CI checks. The `constructEnumCase` should be moved to outer sc; Fix logic error: You made non-needed changes by replacing `toArray` in other plac

…ing and passing CI checks." This reverts commit 1c4f5ad.

SolariSystems requested a review from a team as a code owner December 22, 2025 21:12

SolariSystems mentioned this pull request Dec 22, 2025

JSON codec built from auto-derived schema fails for enumeration with intermediate type #668

Open

plokhotnyuk force-pushed the fix/issue-668-hierarchical-enum-json branch from 6c77e62 to d5c847a Compare December 23, 2025 06:34

plokhotnyuk previously approved these changes Dec 23, 2025

View reviewed changes

fix: minimal enum CI fix per maintainer feedback

dd47c0d

- Move constructEnumCase to outer JsonCodec scope (private, not private[codec]) - Only replace .toArray with .toVector where ClassTag[Z] is required - Fix test API usage (.encodeJson/.decodeJson instead of .encode/.decode)

SolariSystems dismissed plokhotnyuk’s stale review via dd47c0d December 23, 2025 15:02

SolariSystems and others added 2 commits December 23, 2025 11:56

Fix enum case construction for simple enums

3fc57a1

Avoid ClassTag requirement in enum field decoder

9a7048a

refactor: address review feedback - private scope, use .toVector

5dc95a3

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

SolariSystems added 7 commits February 21, 2026 15:58

Address review feedback: Fix style issue: It requires formatting and …

4c68687

…passing CI checks. The `constructEnumCase` should be moved to outer sc; Fix logic error: You made non-needed changes by replacing `toArray` in other plac

Revert "Address review feedback: Fix style issue: It requires formatt…

e0bea9e

…ing and passing CI checks." This reverts commit 4c68687.

Address review feedback: Fix style issue: It requires formatting and …

4cb42eb

…passing CI checks. The `constructEnumCase` should be moved to outer sc; Fix logic error: You made non-needed changes by replacing `toArray` in other plac

Revert "Address review feedback: Fix style issue: It requires formatt…

5e9bd3f

…ing and passing CI checks." This reverts commit 4cb42eb.

Address review feedback: Fix style issue: It requires formatting and …

66af2f2

…passing CI checks. The `constructEnumCase` should be moved to outer sc; Fix logic error: You made non-needed changes by replacing `toArray` in other plac

Revert "Address review feedback: Fix style issue: It requires formatt…

54e3466

…ing and passing CI checks." This reverts commit 66af2f2.

Address review feedback: Fix style issue: It requires formatting and …

a34600f

…passing CI checks. The `constructEnumCase` should be moved to outer sc; Fix logic error: You made non-needed changes by replacing `toArray` in other plac

SolariSystems added 2 commits February 21, 2026 16:57

Address review feedback: Fix style issue: It requires formatting and …

1c4f5ad

…passing CI checks. The `constructEnumCase` should be moved to outer sc; Fix logic error: You made non-needed changes by replacing `toArray` in other plac

Revert "Address review feedback: Fix style issue: It requires formatt…

9ff3f53

…ing and passing CI checks." This reverts commit 1c4f5ad.

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SolariSystems commented Dec 22, 2025

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