Broken OpenFeign with Kotlin 2.2 and default interface method

[rouazana]

Describe the bug

While migrating to Kotlin 2.2, our OpenFeign calls stopped working. They were calling the default interface method instead of the OpenFeign proxy.
It happens with the following versions:

• Spring Boot: 3.5.7
• Spring Cloud: 2025.0.0
• Kotlin: 2.2.0 (broken) / 1.9.25 (working)
• Java: 21
• Build Tool: Gradle 8.x with Kotlin DSL

Sample

I was able to write a complete reproducer (see commit history and README for full explanations):
https://codeberg.org/Malt/reproducer-feign-jvm-default-kotlin-2-2

Workaround

As explained in the README, the -Xjvm-default=disable compiler option restores the correct behaviour.

[ryanjbaxter]

I am not a Kolin expert by any means but we started managing the kotlin-maven-plugin in spring-cloud-build and moved to version 2.2 in 5.0.0 (main). I wonder if you can try that version of openfeign and see if there are any differences?
spring-cloud/spring-cloud-build@c5c70bf

[rouazana]

I tried with the last version with this commit: https://codeberg.org/Malt/reproducer-feign-jvm-default-kotlin-2-2/commit/0838220b683da9ce98fbb0957f3594f437f41d5b

But the test is still failing. This is still the result from the default method that is returned.