Exception: Unsupported metadata type: kotlinx.metadata.jvm.KotlinClassMetadata$SyntheticClass

Build fails with an exception from kapt processing step:

...
Caused by: org.jetbrains.kotlin.kapt3.base.util.KaptBaseError: Exception while annotation processing
        at org.jetbrains.kotlin.kapt3.base.AnnotationProcessingKt.doAnnotationProcessing(annotationProcessing.kt:83)
        at org.jetbrains.kotlin.kapt3.base.AnnotationProcessingKt.doAnnotationProcessing$default(annotationProcessing.kt:34)
        at org.jetbrains.kotlin.kapt3.base.Kapt.kapt(Kapt.kt:45)
        ... 28 more
Caused by: java.lang.IllegalStateException: Unsupported metadata type: kotlinx.metadata.jvm.KotlinClassMetadata$SyntheticClass@6928739
        at dagger.internal.codegen.kotlin.KotlinMetadata.metadataOf(KotlinMetadata.java:225)
        at dagger.internal.codegen.kotlin.KotlinMetadata.from(KotlinMetadata.java:196)
        at dagger.internal.codegen.kotlin.KotlinMetadataFactory.create(KotlinMetadataFactory.java:56)
        at dagger.internal.codegen.kotlin.KotlinMetadataUtil.isCompanionObjectClass(KotlinMetadataUtil.java:86)
        at dagger.internal.codegen.validation.BindingMethodValidator$MethodValidator.checkEnclosingElement(BindingMethodValidator.java:169)
        at dagger.internal.codegen.validation.BindingMethodValidator$MethodValidator.checkAdditionalProperties(BindingMethodValidator.java:151)
        at dagger.internal.codegen.validation.BindingElementValidator$ElementValidator.validate(BindingElementValidator.java:153)
        at dagger.internal.codegen.validation.BindingElementValidator$ElementValidator.access$000(BindingElementValidator.java:137)
        at dagger.internal.codegen.validation.BindingElementValidator.validateUncached(BindingElementValidator.java:86)
        at dagger.internal.codegen.base.Util.reentrantComputeIfAbsent(Util.java:33)
        at dagger.internal.codegen.validation.BindingElementValidator.validate(BindingElementValidator.java:82)
        at dagger.internal.codegen.validation.AnyBindingMethodValidator.validateUncached(AnyBindingMethodValidator.java:99)
        at dagger.internal.codegen.base.Util.reentrantComputeIfAbsent(Util.java:33)
        at dagger.internal.codegen.validation.AnyBindingMethodValidator.validate(AnyBindingMethodValidator.java:74)
        at dagger.internal.codegen.validation.BindingMethodProcessingStep.process(BindingMethodProcessingStep.java:58)
        at dagger.internal.codegen.validation.BindingMethodProcessingStep.process(BindingMethodProcessingStep.java:30)
        at dagger.internal.codegen.validation.TypeCheckingProcessingStep.lambda$process$0(TypeCheckingProcessingStep.java:51)
        at com.google.common.collect.RegularImmutableMap.forEach(RegularImmutableMap.java:185)
        at dagger.internal.codegen.validation.TypeCheckingProcessingStep.process(TypeCheckingProcessingStep.java:48)
        at dagger.internal.codegen.validation.TypeCheckingProcessingStep.process(TypeCheckin
...

Dagger version we use is 2.26.1, but the issue persists on 2.32. Kotlin version is 1.4.21

From the basic research I’ve done, it seems that Dagger still uses old org.jetbrains.kotlinx:kotlinx-metadata-jvm:0.1.0 library version, while 0.2.0 with some new API is released.

So the steps to fix this issue, as I see it, are:

1. Bump the version in org.jetbrains.kotlinx:kotlinx-metadata-jvm to 0.2.0
2. Support KotlinClassMetadata.SyntheticClass (and any other new metadata types) in dagger.internal.codegen.kotlin.KotlinMetadata and possibly adapt its use-sites.

[barry-dempsey]

This issue looks to have been addressed in release 2.34

[danysantiago]

The trace shows the SyntheticClass had to be inspected while validating binding method @Provides, @Binds, @Producer, etc. SyntheticClass are generated for a few things but the most likely the case you are running into is a binding method ending on a synthetic class because its an interface default method.

Something like this would cause this:

@Module
interface MyModule {
  @Provides
  fun provide() = "Hello"
}