Gradle artifactory plugin saying “Cannot cast object ‘org.jfrog.gradle.plugin.artifactory.dsl.ArtifactoryPluginConvention’…”

The problem was that when I added the various bits to the sibling project that meant I had two projects defining the buildscript {} section.

buildscript {
    ...
    dependencies {
        classpath group:'org.jfrog.buildinfo', name: 'build-info-extractor-gradle', version: '3.0.1'
    }
}

Apparently that caused two different versions of the dependency to exist in the classpath, hence the error.

The solution was to move the buildscript bit into the master project so those dependencies are only defined once:

buildscript {
    repositories {
        maven { url "https://plugins.gradle.org/m2/" }
    }
    dependencies {
        classpath group:'org.jfrog.buildinfo', name: 'build-info-extractor-gradle', version: '3.0.1'
    }
}