Warning “Kotlin plugin version is not the same as library version” (but it is!)

I have an Android studio project in which I have added a Java library module, which I call core. My three Gradle build files look like this.

project/build.gradle

buildscript {
    ext.kotlin_version = '1.2.40'
    repositories {
        google()
        jcenter()
    }
    dependencies {
        classpath 'com.android.tools.build:gradle:3.0.1'
        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
    }
}

allprojects {
    repositories {
        google()
        jcenter()
    }
}

task clean(type: Delete) {
    delete rootProject.buildDir
}

core/build.gradle

apply plugin: 'java-library'
apply plugin: 'kotlin'

dependencies { 
    implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk7"
    ...
}

app/build.gradle

apply plugin: 'com.android.application'
apply plugin: 'kotlin-android'
apply plugin: 'kotlin-android-extensions'

android { ... }

dependencies {
    implementation project(':core')
    implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk7:$kotlin_version"
    implementation 'com.android.support.constraint:constraint-layout:1.1.0'
    implementation 'com.android.support:appcompat-v7:27.1.1'

    androidTestImplementation 'com.android.support.test:runner:1.0.1'
    androidTestImplementation 'com.android.support.test.espresso:espresso-core:3.0.1'
    testImplementation 'junit:junit:4.12'
}

The problem I have is that, in core/build.gradle, the kotlin-stdlib-jdk7 line is giving me the warning Plugin version (1.2.40) is not the same as library version (jdk7-1.2.40). I have tried changing it to:

implementation "org.jetbrains.kotlin:kotlin-stdlib"

implementation "org.jetbrains.kotlin:kotlin-stdlib:1.2.40"

implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk7:1.2.40"

implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk7:$kotlin_version"

implementation "org.jetbrains.kotlin:kotlin-stdlib-jre7:$kotlin_version"

But the warning is still there. The build still runs successfully, and I know I can surpress the warning without any problems and ignore it, but I really want to know why this is happening and how I can get rid of it. I am using Android Studio 3.0.1. Does anyone know of a solution to this?

Answer

Starting from Kotlin 1.4 dependency on the standard library added by default:

You no longer need to declare a dependency on the stdlib library in any Kotlin Gradle project, including a multiplatform one. The dependency is added by default.

The automatically added standard library will be the same version of the Kotlin Gradle plugin, since they have the same versioning.

For platform-specific source sets, the corresponding platform-specific variant of the library is used, while a common standard library is added to the rest. The Kotlin Gradle plugin will select the appropriate JVM standard library depending on the kotlinOptions.jvmTarget compiler option of your Gradle build script.

Link to Kotlin Gradle plugin documentation.

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