Kotlin generics are 🍌🍌🍌

0_build.gradle.kts

// ...stuff...

tasks.withType<org.jetbrains.kotlin.gradle.tasks.KotlinCompile> {
    kotlinOptions {
        freeCompilerArgs = listOf(
            "-Xnew-inference",
            "-Xallow-kotlin-package"
        )
    }
}

1_KotlinInternalAnnotations.kt

// from: https://github.com/JetBrains/kotlin/blob/1.3.20/libraries/stdlib/src/kotlin/internal/Annotations.kt
package kotlin.internal

/**
 * Specifies that the constraint built for the type during type inference should be an equality one.
 */
@Target(AnnotationTarget.TYPE)
@Retention(AnnotationRetention.BINARY)
internal annotation class Exact

2_BANANAS.kt

fun <T> Set<@Exact T>.containsExactly(vararg elements: T): Boolean = TODO()
//          ^^^^^^                                                         
//          MAGIC!                                                         

fun shouldCompile(bananas: Set<String>) {
    bananas.containsExactly()
    bananas.containsExactly("one", "two")
    bananas.containsExactly<String>("one", "two")
}

fun shouldNotCompile(bananas: Set<String>) {

    // e: Unresolved reference. None of the following candidates is applicable because of receiver type mismatch
    bananas.containsExactly<Any?>()
    
    // e: Unresolved reference. None of the following candidates is applicable because of receiver type mismatch
    bananas.containsExactly<String?>()
    
    // e: The integer literal does not conform to the expected type String
    bananas.containsExactly(1, 2)
    
    // e: Unresolved reference. None of the following candidates is applicable because of receiver type mismatch
    bananas.containsExactly<Any?>(1, 2)
    
    // e: Null can not be a value of a non-null type String
    bananas.containsExactly("", null)
    
    // e: Unresolved reference. None of the following candidates is applicable because of receiver type mismatch
    bananas.containsExactly<String?>("", null)
}