[scala-sttp4-jsoniter] extend Primitive type with UUID, LocalDate and OffsetDateTime

[reynaldjoabet]

Extend the Primitive union type in helpers.mustache with UUID,
LocalDate, and OffsetDateTime so that path, query, header, cookie, and
form parameters typed with these formats serialize via .toString() on the
fast primitive path. Without this, fields generated from format: uuid,
format: date, and format: date-time fall through to the Mirror-based
object path and fail at compile time because they are not products or sums.

Add an asString extension method on Primitive.A bare .toString is
correct for UUID and the numeric/boolean/string primitives, but produces the
wrong representation for Java's default LocalDate/
OffsetDateTime as toString on these does not match the expected wire format. asString
formats OffsetDateTime with DateTimeFormatter.ISO_OFFSET_DATE_TIME and LocalDate
with ISO_LOCAL_DATE, falling back to .toString for everything else, so date
and date-time parameters are emitted as ISO-8601 strings.

@lbialy @flsh86

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---

Summary by cubic

Extended Primitive in scala-sttp4-jsoniter to include UUID, LocalDate, and OffsetDateTime, and added ISO string formatting via a new asString fast path across all parameter serializers. This removes the Mirror fallback and fixes compile errors in generated clients.

• Bug Fixes
  • Path, query, header, cookie, and form params with these formats now serialize as primitives using ISO/RFC 3339 strings and compile.
  • Updated petstore sample to reflect the new Primitive union and formatting.

^{Written for commit 03a7f9b. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 2 files

_{Re-trigger cubic}

[wing328]

thanks for the PR

cc @clasnake (2017/07), @shijinkui (2018/01), @ramzimaalej (2018/03), @chameleon82 (2020/03), @Bouillie (2020/04) @Fish86 (2023/06)

[wing328]

@reynaldjoabet are you using this change in your production environment to confirm it works for your use cases?

[wing328]

Is there a minimal spec to easily reproduce the issue and confirm the fix?

That will help speed up the review and get it merged sooner

[reynaldjoabet]

Is there a minimal spec to easily reproduce the issue and confirm the fix?

Yes. Three single-parameter snippets, each one independently reproduces a failure on the current master:

1. format: uuid on a header

parameters:
  - name: x-fapi-interaction-id
    in: header
    required: true
    schema:
      type: string
      format: uuid

.headers(HeaderSerializable.serialize("x-fapi-interaction-id", xFapiInteractionId, false))
[error] No given instance of type scala.deriving.Mirror.Of[java.util.UUID] was found.
[error] Failed to synthesize an instance of type scala.deriving.Mirror.Of[java.util.UUID]:
[error]   * class UUID is not a generic product because it is not a case class
[error]   * class UUID is not a generic sum because it is not a sealed class

2. format: date-time on a header

parameters:
  - name: x-fapi-auth-date
    in: header
    required: false
    schema:
      type: string
      format: date-time

.headers(HeaderSerializable.serialize("x-fapi-auth-date", xFapiAuthDate, false))
[error] No given instance of type scala.deriving.Mirror.Of[java.time.OffsetDateTime] was found.
[error] Failed to synthesize an instance of type scala.deriving.Mirror.Of[java.time.OffsetDateTime]:
[error]   * class OffsetDateTime is not a generic product because it is not a case class
[error]   * class OffsetDateTime is not a generic sum because it is not a sealed class

3. format: date on a query parameter

parameters:
  - name: fromBookingDate
    in: query
    schema:
      type: string
      format: date

.addParams(FormSerializable.serialize("fromBookingDate", fromBookingDate, FormStyleFormat.FORM, true): _*)
[error] No given instance of type scala.deriving.Mirror.Of[java.time.LocalDate] was found.
[error] Failed to synthesize an instance of type scala.deriving.Mirror.Of[java.time.LocalDate]:
[error]   * class LocalDate is not a generic product because it is not a case class
[error]   * class LocalDate is not a generic sum because it is not a sealed class

Root cause

helpers.mustache declares:

type Primitive = String | Short | Int | Long | Float | Double | BigDecimal | Boolean

but the generator emits UUID / OffsetDateTime / LocalDate for the three formats above. The inline match falls through to the summonInline[Mirror.Of[T]] branch, which can't synthesize a Mirror for a Java class.

[cubic-dev-ai]

No issues found across 2 files

_{Re-trigger cubic}

[wing328]

tested locally and the result is good

[info] Fetched artifacts of
[info] compiling 13 Scala sources to C:\Users\User\code\openapi-generator\samples\client\petstore\scala-sttp4-jsoniter\target\scala-3.3.7\classes ...
[success] Total time: 12 s, completed Jun 5, 2026, 9:58:08 AM

will update integration tests to cover this change moving forward after merging this PR

[wing328]

FYI. Merged #23947 to add tests