Spring Webflux, R2DBC: class cannot be cast to class kotlin.Result

[msotho]

Problem:
The below code throws the following error on Spring Boot 3.4.0 but not on Spring Boot 3.3.6:

java.lang.ClassCastException: class kotlin.Unit cannot be cast to class kotlin.Result (kotlin.Unit and kotlin.Result are in unnamed module of loader 'app')

interface Repo {
    suspend fun fetch(): Result<Unit>
}

@Repository
class RepoImpl : Repo {
    override suspend fun fetch(): Result<Unit> {
        return Result.success(Unit)
    }
}

@RestController
class HomeAPI(private val repo: Repo) {

    @GetMapping("/")
    suspend fun index(): ResponseEntity<Unit> {
        repo.fetch().getOrThrow()

        return ResponseEntity(HttpStatus.OK)
    }

}

More details:
This seems to occur when I add the spring-boot-starter-data-r2dbc related dependencies

Minimal Repo: https://github.com/msotho/castissue.git

Includes a Unit Test:

@SpringBootTest(webEnvironment = SpringBootTest.WebEnvironment.RANDOM_PORT)
@AutoConfigureWebTestClient
internal class HomeAPITest @Autowired constructor(private val webTestClient: WebTestClient) {

    @Test
    fun `should return http status OK`() {
        webTestClient
            .get()
            .uri("/")
            .accept(MediaType.APPLICATION_JSON)
            .exchange()
            .expectStatus().isOk
    }

}

[wilkinsona]

Thanks for the sample.

As far as I can tell, this is a regression in Spring Framework 6.2. The problem occurs with Spring Boot 3.3.6 when the Spring Framework version is overridden to 6.2.0:

ext {
    set("spring-framework.version", "6.2.0")
}

We'll transfer this issue so that the Framework team can continue the investigation.

[msotho]

Setting Spring Boot 3.3.6 then it works. So it seems it's introduced by 3.4.0