The CompileOptions.bootClasspath property has been deprecated

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/");
}

My Gradle is using OpenJDK 8 from /usr/lib/jvm/java-8-openjdk-amd64/jre on Ubuntu (see by adding println System.getProperty("java.home") in build.gradle. No Android Studio involved.

I can trigger the the warning by explicitly setting sourceCompatibility and targetCompatibility to 1.7 (the default value). By changing the Java version to

android {

    compileOptions {
        sourceCompatibility 1.8
        targetCompatibility 1.8
    }
}

the warning disappears.

The reason why the warning is shown for only one project is that it is not repeated. commons is configured before core because of alphanumerical ordering. When I set sourceCompatibility and targetCompatibility to 1.8 for commons, the warning moves to core.

Setting sourceCompatibility and targetCompatibility to 1.8 for all projects removes the warning entirely. If this is what you want for your project and why Gradle 4 cannot be used warning-free with 1.7 are two separate questions.