The CompileOptions.bootClasspath property has been deprecated

Question

After upgrading to Gradle 4.x, I get the warning

The CompileOptions.bootClasspath property has been deprecated and is scheduled to be removed in Gradle 5.0. Please use the CompileOptions.bootstrapClasspath property instead.

in one of my projects. I don't see anything called bootClasspath or similar in my build.gradle. What does this warning mean?

the warning only appears in the commons subproject, not in core.

commons/build.gradle:

apply plugin: 'com.android.library'  ext {     PUBLISH_GROUP_ID = 'com.afollestad.material-dialogs'     PUBLISH_ARTIFACT_ID = 'commons'     PUBLISH_VERSION = '0.9.2.3'     BUILD_TOOLS = "26.0.3"     TARGET_SDK = 25 }  android {     compileSdkVersion TARGET_SDK     buildToolsVersion BUILD_TOOLS      defaultConfig {         minSdkVersion 16         targetSdkVersion TARGET_SDK         versionCode 1         versionName PUBLISH_VERSION     }     lintOptions {         checkReleaseBuilds false     } }  dependencies {     implementation project(':core') }  // Changes to this block must be applied in core/build.gradle and commons/build.gradle task("javadoc", type: Javadoc) {     description "Generates Javadoc API documentation for the main source code."     source = android.sourceSets.main.java.srcDirs     ext.androidJar = "${android.sdkDirectory}/platforms/${android.compileSdkVersion}/android.jar"     classpath += files(ext.androidJar)     exclude "**/BuildConfig.java"     exclude "**/R.java"     options.links("http://docs.oracle.com/javase/7/docs/api/");     options.links("http://d.android.com/reference/"); } 

core/build.gradle:

apply plugin: 'com.android.library'  ext {     PUBLISH_GROUP_ID = 'com.afollestad.material-dialogs'     PUBLISH_ARTIFACT_ID = 'core'     PUBLISH_VERSION = '0.9.2.3'     SUPPORT_LIBRARY_VERSION = '25.4.0'     BUILD_TOOLS = "26.0.3"     TARGET_SDK = 25 }  android {     compileSdkVersion TARGET_SDK     buildToolsVersion BUILD_TOOLS      defaultConfig {         minSdkVersion 16         targetSdkVersion TARGET_SDK         versionCode 1         versionName PUBLISH_VERSION         consumerProguardFiles 'progress-proguard.txt'     }     lintOptions {         checkReleaseBuilds false     } }  dependencies {     api "com.android.support:support-v13:$SUPPORT_LIBRARY_VERSION"     api "com.android.support:appcompat-v7:$SUPPORT_LIBRARY_VERSION"     api "com.android.support:recyclerview-v7:$SUPPORT_LIBRARY_VERSION"     api "com.android.support:support-annotations:$SUPPORT_LIBRARY_VERSION"     implementation "me.zhanghai.android.materialprogressbar:library:1.4.1" }  // Changes to this block must be applied in core/build.gradle and commons/build.gradle task("javadoc", type: Javadoc) {     description "Generates Javadoc API documentation for the main source code."     source = android.sourceSets.main.java.srcDirs     ext.androidJar = "${android.sdkDirectory}/platforms/${android.compileSdkVersion}/android.jar"     classpath += files(ext.androidJar)     exclude "**/BuildConfig.java"     exclude "**/R.java"     options.links("http://docs.oracle.com/javase/7/docs/api/");     options.links("http://d.android.com/reference/"); }