New ProGuard implementation in #2983 causes R8 warning

[ypedegroot]

Describe the bug
Since kotlinx-serialization-json v1.9.0, I get an R8 warning when building a release version of my Android app using Gradle.

I get the following warning:

The type "<1>$*" is used in a field rule. The characters in this type are legal for the JVM, but unlikely to originate from a source language. Maybe this is not the rule you are looking for.

When checking the configuration.txt file in the mapping folder (when compiling using Android Studio), I see exactly one occurrence of the <1>$* string. It occurs only in the r8\kotlinx-serialization-common.pro file.

It looks like this is introduced since #2983 (specifically, in this commit: 6665953)

Is this a reason to rollback to the previous version? Or is there something I should change in my code to prevent this?

Environment

• Kotlin version: [2.0.21]
• Library version: [1.9.0]
• Kotlin platforms: [JVM 21.0.6]
• Gradle version: [8.13]
• IDE version [Android Studio 2025.1.1]
• Targeting Android SDK 36, compiling using SDK 36, minimum SDK 31

[sandwwraith]

cc @shanshin
Rule looks correct though, because class generated by serialization plugin is called $serializer. Maybe this warning should be suppressed somehow.

[sgjesse]

Interesting. The warning was introduced as a fix to https://issuetracker.google.com/110021323 in https://r8-review.googlesource.com/23190. It looks as if R8 think that the <>'s are not matching or is not a valid back reference.

Do you have a reproduction that the R8 team can test with?

[ypedegroot]

Interesting. The warning was introduced as a fix to https://issuetracker.google.com/110021323 in https://r8-review.googlesource.com/23190. It looks as if R8 think that the <>'s are not matching or is not a valid back reference.

Do you have a reproduction that the R8 team can test with?

I do not have a small reproduction code, but I think it is enough to create an empty project, add one Kotlin data class and mark it as @Serializable, and run a release build with isMinifyEnabled = true and isShrinkResources = true.

[angelix]

Anyone found a workaround about this or the only solution is the downgrade?

[shubhamsinghshubham777]

I'm also facing the same issue. Is there an update on this?

[sgjesse]

Opened an R8 issue on this https://issuetracker.google.com/433451526.

The fix is in https://r8-review.googlesource.com/c/r8/+/107501. I will ping this issue when it has landed.

The warning is benign, and can be ignored until the fix is ready.