Fix #1342: Handle value classes for type parameters and don't unbox value classes returned as interface/supertype by ianbrandt · Pull Request #1442 · mockk/mockk

ianbrandt · 2025-10-25T19:34:11Z

Handle generic type parameters of value classes. Don't unbox value classes that are returned as their interface or supertype to avoid a ClassCastException. See added KDoc for details. Fixes #1342.

ianbrandt · 2025-10-25T21:00:30Z

modules/mockk-core/src/jvmMain/kotlin/io/mockk/core/ValueClassSupport.kt

        if (!resultType.isValue_safe) {
            return this
        }
+


I reworked the function to remove duplication and hopefully improve clarity.

ianbrandt · 2025-10-26T22:33:04Z

.gitignore


 .gradle/

+.kotlin/


Created by Kotlin Gradle Plugin during tests.

ianbrandt · 2025-10-27T05:28:24Z

test-modules/client-tests/src/jvmTest/kotlin/io/mockk/core/ValueClassSupportTest.kt

        assertEquals(JavaEnum.A, result)
    }

+    @Test


Added several test cases covering value class handling.

ianbrandt · 2025-10-27T23:41:05Z

I see there was a build failure:

https://github.com/mockk/mockk/actions/runs/18830786451/job/53742959648?pr=1442#step:9:630

I'm not particularly familiar with Android development. Is this something I need to fix in my PR, or an issue with the build infrastructure?

Raibaz · 2025-10-28T11:01:44Z

@ianbrandt , can you please rebase on top of the master branch? I should have fixed the build failure.

ianbrandt · 2025-10-29T05:28:14Z

can you please rebase on top of the master branch? I should have fixed the build failure.

@Raibaz, Done. Thank you for considering this PR.

Additional notes:

As I dug into this with test cases, I realized the issue I described in #1342 (comment) turned out to be a separate, much simpler to fix bug than the generics issue originally reported by @micheljung. I’ve attempted to fix both in this PR.

Admittedly, not being deeply familiar with MockK or Kotlin value class internals, I leaned on Junie quite a bit to help develop the code and documentation for the cached "*unbox-impl" MethodHandle approach that ultimately got the generic value class tests passing. I did review and refine it, so it’s not just blindly used AI-generated code. That said, I’m not sure whether it's the best possible solution. Feedback welcomed. If there's a more recommended approach regarding MockK’s architecture or long-term Kotlin value class compatibility, I’m happy to take stab at that. I'm also on the fence about the added KDoc and comments. They do explain what's going on, but they're also a bit much. If you want me to pare them down, I can.

Raibaz · 2025-10-30T11:27:39Z

Thanks for this! I like the test coverage particularly.

I agree the KDoc and comments in ValueClassSupport.kt are a bit too much, can you clean them up a bit?

ianbrandt · 2025-10-31T15:37:20Z

I agree the KDoc and comments in ValueClassSupport.kt are a bit too much, can you clean them up a bit?

You got it. Will do.

ianbrandt · 2025-12-29T23:54:38Z

@Raibaz, I finally got a chance to clean up the KDoc and inline comments for this change. I also rebased the changes onto the latest master (hence the force push to this branch). Let me know if you see anything else that needs fixing.

Raibaz · 2026-01-06T10:32:35Z

Can you also please run ./gradlew spotlessApply to make the builds pass? 🙏

Thanks!

ianbrandt · 2026-01-07T00:30:55Z

Done. I wasn't familiar with Spotless. I'll be sure to run that after any other changes going forward. Thanks!

Raibaz · 2026-01-07T13:03:10Z

Looks like there are some test failures in older Kotlin versions and in instrumented tests. Can you PTAL?

Thanks! 🙏

…ed as their interface or supertype to avoid a `ClassCastException` (fixes mockk#1342).

…g of value classes.

…Plugin temporarily during `gradlew check`).

…ty types.

…operty types, and added KDoc.

…ockk#1308 to ensure those fixes are preserved by the mockk#1342 changes.

…y version to align with the `io_mockk_kotlin_version` Gradle project property, if set.

…to support backwards compatibility with Android API level 21.

ianbrandt · 2026-01-19T02:41:42Z

Looks like there are some test failures in older Kotlin versions and in instrumented tests. Can you PTAL?

I believe the client test failures with earlier versions of Kotlin were due to them being run with the latest Kotlinx Coroutines version, which is not that backwards compatible. I believe any coEvery in :test-modules:client-tests would've failed, there just wasn't any such coverage previously. I changed the build to use the latest compatible Kotlinx Coroutines version for each Kotlin version. The corresponding coroutinesVersion logic in 'test-modules/client-tests/build.gradle.kts' may be a point of maintenance going forward.

I believe the instrumented tests were failing because they require compatibility with API level 21, and I had used java.lang.invoke.MethodHandle, which only became available from API level 26. I changed to using java.lang.reflect.Method instead. All the check tests still pass. I'm not familiar with how to setup to run connectedCheck locally yet. I'll be watching for whether they pass if you approve the next workflow run.

P.S. I also rebased against current master and checked spotlessApply.

Raibaz · 2026-01-19T08:18:38Z

Thanks a lot for all the follow ups, this looks very clean now! 🙏

ianbrandt commented Oct 25, 2025

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ianbrandt commented Oct 26, 2025

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Created by Kotlin Gradle Plugin during tests.

ianbrandt marked this pull request as draft October 27, 2025 00:52

ianbrandt commented Oct 27, 2025

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ianbrandt marked this pull request as ready for review October 27, 2025 05:28

ianbrandt changed the title ~~Fix #1342: Don't unbox value for value class returned as interface/supertype~~ Fix #1342: Handle value class type parameters and don't unbox value for value class returned as interface/supertype Oct 27, 2025

ianbrandt changed the title ~~Fix #1342: Handle value class type parameters and don't unbox value for value class returned as interface/supertype~~ Fix #1342: Handle value classes for type parameters and don't unbox value classes returned as interface/supertype Oct 27, 2025

ianbrandt force-pushed the issue-1342 branch from 1f41bab to 09d6bfa Compare October 29, 2025 04:01

ianbrandt force-pushed the issue-1342 branch from 91c0c79 to b4bd8ad Compare December 29, 2025 23:51

ianbrandt added 12 commits January 18, 2026 18:16

Don't unbox values for method return of value classes that are return…

2bcd228

…ed as their interface or supertype to avoid a `ClassCastException` (fixes mockk#1342).

Added more tests for maybeUnboxValueForMethodReturn(Method) handlin…

ea61fad

…g of value classes.

Added .kotlin/ directory to .gitignore (created by Kotlin Gradle …

a9ae191

…Plugin temporarily during `gradlew check`).

Improved test property naming.

6cf8c55

Added tests for suspend functions returning nullable value classes.

b1dd5f2

Added support for value classes as generic function return and proper…

660b0ce

…ty types.

Improved handling for value classes as generic function return and pr…

5a3d886

…operty types, and added KDoc.

Streamlined KDoc and code comments for clarity.

44b07b4

Reverted redundant .gitignore entry for .kotlin directory.

a161d7d

Restored some original comments and formatting after rebase.

1a032e4

Added more ValueClassSupportTest test cases covering mockk#1103 and m…

c170c1c

…ockk#1308 to ensure those fixes are preserved by the mockk#1342 changes.

Ran spotlessApply Gradle task to align formatting.

c0090c4

ianbrandt added 2 commits January 18, 2026 18:16

Updated the :test-modules:client-tests Kotlinx Coroutines dependenc…

a34d4d9

…y version to align with the `io_mockk_kotlin_version` Gradle project property, if set.

Replaced MethodHandle usage with Method for value class unboxing …

9249fbf

…to support backwards compatibility with Android API level 21.

ianbrandt force-pushed the issue-1342 branch from ee52a54 to 9249fbf Compare January 19, 2026 02:21

Raibaz merged commit db40a01 into mockk:master Jan 19, 2026
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