[BUG][R] Date and DateTime not parsed correctly

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

OpenAPI type: string, format: date and format: date-time fields to plain character in generated R models, not the proper classes. As a result, these arrive as a character string in the generated R client code, so callers cannot automatically use R's date/time API (as.Date, format, etc.) without explicit conversion. Even the schema default is emitted as a string literal.

No validation of dates or date times, so anything goes, e.g. "not-a-date". 🫢

openapi-generator version

7.26.0, but I know this is valid at least as far back as 6.1.0, maybe even 5.3.0.

OpenAPI declaration file content or url

Minimal reproduction (also attached as a full spec at https://gist.github.com/ajalon1/3ac3bdcb68fa7c5b9c66ad2625bf3ba4):

openapi: 3.0.1
components:
  schemas:
    DateTimeObject:
      type: object
      properties:
        requiredTime:
          type: string
          format: date-time
        optionalTime:
          type: string
          format: date-time
          nullable: true
          default: '2015-10-28T14:38:02Z'
        optionalDate:
          type: string
          format: date
          nullable: true
          default: '2015-10-28'

Generation Details

java -jar modules/openapi-generator-cli/target/openapi-generator-cli.jar generate \
  -g r -i rproblems.yaml -o ./out/r-client \
  --additional-properties=packageName=rproblems

Steps to reproduce

1. Generate the R client from the spec above.
2. Open R/date_time_object.R:
   • @field requiredTime is documented as plain character.
   • initialize() accepts any string for the field without validation.
   • the optionalTime default is the literal string "2015-10-28T14:38:02Z", not a POSIXct.
3. Round-trip: $fromJSON(toJSON(...)) and ApiClient$deserialize("POSIXct", "2015-10-28T14:38:02Z") both return the untouched string.

Related issues/PRs

Suggest a fix

Map date → R Date and date-time → R POSIXct and convert at the boundaries. I will open a PR shortly with the fix for this.

Generate the R client with the supplied OpenAPI YAML and inspect R/date_time_object.R, then trace ApiClient$deserialize and the $fromJSON(toJSON(...)) round trip. Done means date maps to Date and date-time to POSIXct, defaults use those types, and invalid date strings are no longer accepted as plain character values.