Generating IDEA workspace files with language level setting is not working from this gradle

language level problem

buildscript {
    repositories {
        mavenCentral()
    }
}

group '...'
version '1.0-SNAPSHOT'

apply plugin: 'java'
apply plugin: 'idea'

repositories {
    mavenCentral()
}

allprojects {
    sourceCompatibility = 1.8
    targetCompatibility = 1.8
}

idea {
    project {
        jdkName = sourceCompatibility
        languageLevel = sourceCompatibility
    }

    module {
        excludeDirs -= file("$buildDir")
        sourceDirs += file("$buildDir/generated/source/...")
    }
}