Support JDK 16

[C-Otto]

According to https://errorprone.info/docs/installation some flags seem to be necessary when using JDK 16. I have no idea how to configure those with Gradle and I'd appreciate some plugin support.

$ ./gradlew build

> Task :subproject-one:compileJava FAILED
An exception has occurred in the compiler (16). Please file a bug against the Java compiler via the Java bug reporting page (http://bugreport.java.com) after checking the Bug Database (http://bugs.java.com) for duplicates. Include your program, the following diagnostic, and the parameters passed to the Java compiler in your report. Thank you.
java.lang.IllegalAccessError: class com.google.errorprone.BaseErrorProneJavaCompiler (in unnamed module @0x3b727f48) cannot access class com.sun.tools.javac.api.BasicJavacTask (in module jdk.compiler) because module jdk.compiler does not export com.sun.tools.javac.api to unnamed module @0x3b727f48
        at com.google.errorprone.BaseErrorProneJavaCompiler.addTaskListener(BaseErrorProneJavaCompiler.java:94)
        at com.google.errorprone.ErrorProneJavacPlugin.init(ErrorProneJavacPlugin.java:34)
        at jdk.compiler/com.sun.tools.javac.api.BasicJavacTask.initPlugin(BasicJavacTask.java:255)
        at jdk.compiler/com.sun.tools.javac.api.BasicJavacTask.initPlugins(BasicJavacTask.java:229)
        at jdk.compiler/com.sun.tools.javac.api.JavacTaskImpl.prepareCompiler(JavacTaskImpl.java:204)
        at jdk.compiler/com.sun.tools.javac.api.JavacTaskImpl.lambda$doCall$0(JavacTaskImpl.java:101)
        at jdk.compiler/com.sun.tools.javac.api.JavacTaskImpl.invocationHelper(JavacTaskImpl.java:152)
        at jdk.compiler/com.sun.tools.javac.api.JavacTaskImpl.doCall(JavacTaskImpl.java:100)
        at jdk.compiler/com.sun.tools.javac.api.JavacTaskImpl.call(JavacTaskImpl.java:94)
        at org.gradle.internal.compiler.java.IncrementalCompileTask.call(IncrementalCompileTask.java:77)

[tbroyer]

This is in the works. In the mean time, you can use:

tasks.withType<JavaCompile>().configureEach {
    options.isFork = true
    options.forkOptions.jvmArgs!!.addAll(listOf(
        "--add-exports=jdk.compiler/com.sun.tools.javac.api=ALL-UNNAMED",
        "--add-exports=jdk.compiler/com.sun.tools.javac.file=ALL-UNNAMED",
        "--add-exports=jdk.compiler/com.sun.tools.javac.main=ALL-UNNAMED",
        "--add-exports=jdk.compiler/com.sun.tools.javac.tree=ALL-UNNAMED",
        "--add-exports=jdk.compiler/com.sun.tools.javac.code=ALL-UNNAMED",
        "--add-exports=jdk.compiler/com.sun.tools.javac.processing=ALL-UNNAMED",
        "--add-exports=jdk.compiler/com.sun.tools.javac.util=ALL-UNNAMED",
        "--add-exports=jdk.compiler/com.sun.tools.javac.parser=ALL-UNNAMED",
        "--add-opens=jdk.compiler/com.sun.tools.javac.comp=ALL-UNNAMED",
        "--add-opens=jdk.compiler/com.sun.tools.javac.code=ALL-UNNAMED"
    ))
}

(this is with the Kotlin DSL, adapt for Groovy DSL)

[tbroyer]

Done, and released in v2.0.0