@ConfigurationProperties Spring Boot Configuration Annotation Processor not found in classpath

I had the same problem. I use idea 2017.2 and gradle 4.1,
and some blog said you should add:

dependencies {
    optional "org.springframework.boot:spring-boot-configuration-processor"
}

But I changed it to this:

dependencies {
    compile "org.springframework.boot:spring-boot-configuration-processor"
}

And the warning is gone.