Unresolved references for Kotlin (JVM) Standard Library Elements - intellij-idea

Question

After adding gradle to an existing Kotlin project in IntelliJ, I started having issues with references to some standard library elements. For instance, Kotlin String type is recognized, but the mutableMapOf is giving me
Error:(11, 60) Kotlin: Unresolved reference: mutableMapOf
another is:
Error:(9, 78) Kotlin: Unresolved reference: Array
during compilation. They are marked red in the IDE, as well (not isolated to only compilation)
Another error that appears in IntelliJ is Kotlin not configured, with no options to configure Kotlin
Here is my gradle build file:
apply plugin: 'kotlin'
group = "com.serguei.myproject"
version = "1.0"
buildscript {
ext.kotlin_version = '1.2.10'
repositories {
mavenCentral()
}
dependencies {
classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
}
}
repositories {
mavenCentral()
}
dependencies {
compile "org.jetbrains.kotlin:kotlin-stdlib-jdk8:$kotlin_version"
compile 'com.google.code.gson:gson:2.8.2'
}
compileKotlin {
kotlinOptions {
jvmTarget = "1.8"
}
}
compileTestKotlin {
kotlinOptions {
jvmTarget = "1.8"
}
}

Answer 1

Thank you for the comments Ice100 and Preston Garno, they have helped greatly in finding a solution.
The issue was that my project was not structured correctly. I had:
MyProject -> src -> [Source Files]
-> build.gradle
however, after consulting Using Gradle more closely, it appears that the recommended project structure should be
MyProject -> src -> main -> kotlin -> [Source Files]
-> build.gradle
Once the project structure is adjusted, go into File -> Invalidate Caches / Restart and click the Invalidate and Restart. Once the indexer is rerun, the Kotlin standard library types, functions, etc. should now be available.

Answer 2

For any one coming to this page because of problem that IntelliJ suddenly does not find stuff in kotlin standard library: File -> Invalidate Caches / Restart helped me in a situation which didn't even involve gradle.

Answer 3

After migrating my Java app to Kotlin I find myself in this situation.
After checking gradle structure here
You can sync your project with gradle using sync button from menu bar.
this will download necessary files for Kotlin
Then you can just validate cache & Restart if necessary.