Subtypes and labels that cannot be told apart are reported in play-json-generic

[BrainHorse]

NestedTypeFormat and Enumeration could both build a format that quietly loses information. A subtype declared at the top level was written with an empty type on Scala 2, making all such subtypes identical on the wire. Two subtypes of the same name in different plain objects were given one type on Scala 3, so one read back as the other. A naming strategy mapping two values onto one label left all but one of them unreadable. In each case the format was built successfully and the loss only showed up later as wrong data.
Deriving a format now reports these, so the problem appears where the format is defined rather than in production data:

NestedTypeFormat.of[A]:  Either[String, OFormat[A]]
EnumerationFormat.of[A]: Either[String, Format[A]]

For reviewers:

• Bin compat is preserved. NestedTypeFormat.apply, Enumeration and its members are deprecated but unchanged, so existing code compiles, links and behaves exactly as before. The deprecated Enumeration#format still collapses colliding labels, which its deprecation message states plainly rather than hiding.
• EnumerationFormat is a new object rather than a reshaped Enumeration, because validated labels require the naming strategy at construction, and moving it there broke four MiMa checks.
• No serialized output changes. A package-level plain object keeps the long, package-qualified name Scala 3 has always written for it, pinned by tests. Scala 3 previously derived that name from toString, so an object overriding it got a wrong name or crashed, and in the ordinary parameterless spelling the derivation did not compile at all. The name now comes from the class, which leaves every existing document readable.
• The two Scala versions name subtypes differently and cannot read each other's documents where lexical nesting and sealed nesting diverge. Not a regression, not changed here, and the reason it stays: correcting either side would break stored data. It is pinned by tests in src/test/scala-2 and src/test/scala-3, documented in scaladoc on both, and flagged in the README, since documents crossing between services on different Scala versions are where it bites.

Two pre existing defects are covered but not fixed, each by an ignored test asserting the correct behaviour with the reason and the enabling condition in its body, rather than a passing test that would enshrine the current one:

• FlatTypeFormat names by simple name alone, so it collides on more hierarchies than NestedTypeFormat and has no of to report it. The second subtype writes fine and then fails to read against the first's fields.
• EnumMappings on Scala 3 labels enum values by toString, diverging from Scala 2. Fixing it changes what Scala 3 writes for those enumerations, so it wants its own decision rather than riding along here.

One serialized name does change, on Scala 3. A plain object whose toString was overridden with parentheses and contained a $ was named after the text before that $, so override def toString() = "US$99" was written as US; it is now named after its class, and documents holding the old name no longer read. This is accepted deliberately: the old logic left the ordinary override def toString spelling unable to compile and a paren override without a $ throwing while writing, so the only documents of this shape that can exist are $-containing ones. Details are in the README next to the compatibility table.

Summary by CodeRabbit

• New Features

  • Added safe format creation for nested types and enumerations, with clear errors for naming collisions.
  • Added discriminator metadata for subtype names across Scala 2 and Scala 3.
  • Improved singleton naming consistency during JSON serialization.

• Bug Fixes

  • Prevented overridden toString values from changing serialized singleton names.

• Documentation

  • Documented Scala 2/3 naming differences and compatibility considerations.

• Deprecations

  • Deprecated direct format construction methods in favor of safer alternatives.

[github-actions]

coverage: 63.178% (-14.5%) from 77.634% — amikhailov/generic-discriminator-failfast into master

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

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📒 Files selected for processing (2)

• play-json-generic/src/main/scala/com/evolution/playjson/generic/Enumeration.scala
• play-json-generic/src/main/scala/com/evolution/playjson/generic/NestedTypeFormat.scala

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📝 Walkthrough

Walkthrough

Added cross-version discriminator derivation for nested types, collision-aware NestedTypeFormat and EnumerationFormat factories, singleton naming fixes, deprecations for direct construction, and tests and documentation for naming compatibility.

Changes

Discriminator and format derivation

Layer / File(s)	Summary
Discriminator metadata derivation play-json-generic/src/main/scala-2/..., play-json-generic/src/main/scala-3/...	Added Discriminators derivation for Scala 2 coproducts and Scala 3 mirrors. Added runtime and hierarchical subtype names. Updated Scala 2 reads and writes to use the shared discriminator helper.
Collision-aware format factories play-json-generic/src/main/scala/com/evolution/playjson/generic/..., play-json-generic/src/main/scala-2/..., README.md	Added NestedTypeFormat.of and EnumerationFormat.of. Both detect duplicate wire names. Deprecated direct apply construction and retained construction through unsafe. Documented Scala 2 and Scala 3 naming differences.
Naming behavior validation play-json-generic/src/test/...	Added fixtures and tests for nested names, singleton objects, overridden toString, duplicate names, naming collisions, and enumeration formats. Updated test helpers to handle Either-based format construction.

Estimated code review effort: 3 (Moderate) | ~25 minutes

Merge Risk: 🔵 Low · up to 1b9e3

The validated factory may still accept a custom writer that emits the same discriminator for multiple subtypes, producing data that cannot be read reliably; the change is otherwise bounded, but the owner should confirm the contract or align validation before merging.

Sequence Diagram(s)

sequenceDiagram
  participant Caller
  participant NestedTypeFormat
  participant Discriminators
  participant PlayJsonFormat
  Caller->>NestedTypeFormat: call of[A]
  NestedTypeFormat->>Discriminators: derive subtype metadata
  Discriminators-->>NestedTypeFormat: return List[Discriminator]
  NestedTypeFormat->>NestedTypeFormat: detect duplicate wire names
  NestedTypeFormat->>PlayJsonFormat: build format when names are unique
  PlayJsonFormat-->>Caller: return Either[String, OFormat[A]]

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🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly summarizes the main change: reporting indistinguishable subtype names and enum labels in play-json-generic.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0 files. (2 skipped: 2 unsupported.)
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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• Create PR with unit tests
• Commit unit tests in branch amikhailov/generic-discriminator-failfast

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[coderabbitai]

Actionable comments posted: 1

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Inline comments:
In
`@play-json-generic/src/main/scala/com/evolution/playjson/generic/NestedTypeFormat.scala`:
- Around line 31-41: Update NestedTypeFormat.of so duplicate-discriminator
validation reflects the actual type names emitted by the supplied
NestedTypeWrites, rather than relying only on discriminators.all. Ensure a
writer that maps multiple subtypes to the same name is rejected with the
existing validation result, either by deriving both metadata sources from one
implementation or by requiring a writer that exposes its emitted discriminators.

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Reviewing files that changed from the base of the PR and between b36590b and 98f0c43.

📒 Files selected for processing (19)

• README.md
• play-json-generic/src/main/scala-2/com/evolution/playjson/generic/Discriminators.scala
• play-json-generic/src/main/scala-2/com/evolution/playjson/generic/NestedTypeReads.scala
• play-json-generic/src/main/scala-2/com/evolution/playjson/generic/NestedTypeWrites.scala
• play-json-generic/src/main/scala-2/com/evolution/playjson/generic/Util.scala
• play-json-generic/src/main/scala-3/com/evolution/playjson/generic/Discriminators.scala
• play-json-generic/src/main/scala-3/com/evolution/playjson/generic/NestedTypeWrites.scala
• play-json-generic/src/main/scala-3/com/evolution/playjson/generic/Util.scala
• play-json-generic/src/main/scala/com/evolution/playjson/generic/Enumeration.scala
• play-json-generic/src/main/scala/com/evolution/playjson/generic/EnumerationFormat.scala
• play-json-generic/src/main/scala/com/evolution/playjson/generic/FlatTypeFormat.scala
• play-json-generic/src/main/scala/com/evolution/playjson/generic/NestedTypeFormat.scala
• play-json-generic/src/test/scala-2/com/evolution/playjson/generic/Scala2DiscriminatorSpec.scala
• play-json-generic/src/test/scala-3/com/evolution/playjson/generic/Scala3DiscriminatorSpec.scala
• play-json-generic/src/test/scala/com/evolution/playjson/generic/DiscriminatorFixtures.scala
• play-json-generic/src/test/scala/com/evolution/playjson/generic/DiscriminatorSpec.scala
• play-json-generic/src/test/scala/com/evolution/playjson/generic/EnumerationDerivalSpec.scala
• play-json-generic/src/test/scala/com/evolution/playjson/generic/JsonFormatSpec.scala
• play-json-generic/src/test/scala/com/evolution/playjson/generic/NestedTypeFormatSpec.scala

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[Z1kkurat]

@mr-git i have no objections to merging this, so unless you want to give it another round we could merge it, i think