TL'DR: In this Android Kotlin library I updated from Gradle 5.6.4 to 6.6.1 (commit d5d8d2). Now I cannot build a project depending on the aar anymore.

Test setup

I build and deploy the aar to mavenLocal ...

$ ./gradlew clean :roadsigns:assemble
$ ./gradlew publishToMavenLocal

... and then reference the deployed library artifact in the sample Android app module. First I add mavenLocal() in the root build.gradle file:

allprojects {
    repositories {
        google()
        mavenCentral()
        jcenter()
        mavenLocal() // <-- Add this
    }
}

I reference the mavenLocal() dependency directly:

dependencies {
    implementation
    // implementation project(":roadsigns")
    implementation "info.metadude.kotlin.library.roadsigns:roadsigns:$version"
    implementation Libs.kotlinStdlib
    // ...

The error

When I build the sample app then I get the following build error:

$ ./gradlew clean assembleDebug

Execution failed for task ':checkDebugAarMetadata'.
> Multiple task action failures occurred:

   > A failure occurred while executing com.android.build.gradle.internal.tasks.CheckAarMetadataWorkAction
      > A dependency's AAR metadata (META-INF/com/android/build/gradle/aar-metadata.properties) does
        not specify an aarFormatVersion value, which is a required value.
        Dependency: info.metadude.kotlin.library.roadsigns:roadsigns:4.0.0.
        AAR metadata file: /home/USERNAME/.m2/repository/info/metadude/kotlin/library/roadsigns/roadsigns/4.0.0/roadsigns-4.0.0-javadoc.jar.

   > A failure occurred while executing com.android.build.gradle.internal.tasks.CheckAarMetadataWorkAction
      > A dependency's AAR metadata (META-INF/com/android/build/gradle/aar-metadata.properties) does
        not specify an aarFormatVersion value, which is a required value.
        Dependency: info.metadude.kotlin.library.roadsigns:roadsigns:4.0.0.
        AAR metadata file: /home/USERNAME/.m2/repository/info/metadude/kotlin/library/roadsigns/roadsigns/4.0.0/roadsigns-4.0.0-sources.jar.

   > A failure occurred while executing com.android.build.gradle.internal.tasks.CheckAarMetadataWorkAction
      > A dependency's AAR metadata (META-INF/com/android/build/gradle/aar-metadata.properties) does
        not specify an aarFormatVersion value, which is a required value.
        Dependency: info.metadude.kotlin.library.roadsigns:roadsigns:4.0.0.
        AAR metadata file: /home/USERNAME/.m2/repository/info/metadude/kotlin/library/roadsigns/roadsigns/4.0.0/roadsigns-4.0.0.aar.

Context information

When I try the same with Gradle 5.6.4 then there is no error.

In the Android Kotlin library I am using:

• digital.wup:android-maven-publish:3.6.3
• com.jfrog.bintray.gradle:gradle-bintray-plugin:1.8.5

I using Java 8 (OpenJDK) on my machine and verified that the same error occurs on a different computer.

Experiments

• When I deploy the library with Gradle 5 and build the app with Gradle 5 or Gradle 6 in both cases it works.

The question

What changed from Gradle 5 to Gradle 6 which causes the aar to be broken (?)

Findings

• I took a look at the files being deployed in mavenLocal():

  ~/.m2/repository/info/metadude/kotlin/library/roadsigns
  └── roadsigns
  ├── 4.0.0
  │   ├── roadsigns-4.0.0.aar
  │   ├── roadsigns-4.0.0-javadoc.jar
  │   ├── roadsigns-4.0.0.module
  │   ├── roadsigns-4.0.0.pom
  │   └── roadsigns-4.0.0-sources.jar
  └── maven-metadata-local.xml

• I compared the files deployed with Gradle 5 and Gradle 6. Interestingly, the roadsigns-4.0.0.aar files are binary equivalent. The pom file differs, though:

  

• Also there is a roadsigns-4.0.0.module file only when I deploy with Gradle 6.

• When I manually remove the "new" do_not_remove: published-with-gradle-metadata part from the file deployed with Gradle 6 then the app builds successfully! The question remains ... what's going on here?!

The problem very likely comes from the plugins you are using to publish the library (digital.wup:android-maven-publish:3.6.3) and the quite abandoned bintray Gradle plugin which hasn't been updated to support Gradle metadata.

I bet the .module file that takes precedence over the content of the pom.xml files in Gradle 6 consumer projects is improperly populated because of how these two plugins work.

Fortunately, there's now built-in maven-publish support in the Android Gradle Plugin (so first-party), and it's documented here: https://developer.android.com/studio/build/maven-publish-plugin

With it, you can publish to mavenLocal, bintray, and any other proper maven repo.

You should not have any issues if you use that instead of any third party alternatives.