Kotlin WebClient / RestClient nullability for Spring Boot 4 compatibility

[hiddewie]

For Kotlin WebClient and RestClient generated clients.
Using with Spring Boot 4.0.0 release https://github.com/spring-projects/spring-boot/releases/tag/v4.0.0.

The current implementation causes a generated client like

open class ApiClient(protected val client: RestClient) {

    protected inline fun <reified I : Any, reified T: Any?> request(requestConfig: RequestConfig<I>): ResponseEntity<T> {
        return prepare(defaults(requestConfig))
            .retrieve()
            .toEntity(object : ParameterizedTypeReference<T>() {})

    }

    // ...
}

while that gives errors:

Fixes #22411

Fixes #22368

Duplicate of #22369

Tagging Technical Spring and Kotlin committees:

• Spring: @cachescrubber (2022/02) @welshm (2022/02) @MelleD (2022/02) @atextor (2022/02) @manedev79 (2022/02) @javisst (2022/02) @borsch (2022/02) @banlevente (2022/02) @Zomzog (2022/09) @martin-mfg (2023/08)
• Kotlin: @karismann (2019/03) @Zomzog (2019/04) @andrewemery (2019/10) @4brunu (2019/11) @yutaka0m (2020/03) @stefankoppier (2022/06) @e5l (2024/10)

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package || exit
  ./bin/generate-samples.sh ./bin/configs/*.yaml || exit
  ./bin/utils/export_docs_generators.sh || exit
  

  (For Windows users, please run the script in WSL)
  Commit all changed files.
  This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master.
  These must match the expectations made by your contribution.
  You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*.
  IMPORTANT: Do NOT purge/delete any folders/files (e.g. tests) when regenerating the samples as manually written tests may be removed.
• File the PR against the correct branch: master (upcoming 7.x.0 minor release - breaking changes with fallbacks), 8.0.x (breaking changes without fallbacks)
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[skyfall174]

Hi,
I have the same problem, that this PR fix
Any idea when this changes will be done? In which version?

Thank you