Why does this gradle buildscript (kotlin dsl) fail to compile, when I introduce an unrelated custom task?

So I've been playing around with gradle's kotlin dsl and my buildscript has grown to a point where I wanted to introduce custom tasks for better organization.

Here's a MWE of my build.gradle.kts:

open class CustomTask : DefaultTask() {
  var dir = File(".")

  @TaskAction
  fun action() {
    println("CustomTask.dir: ${dir.absolutePath}")
  }
}
tasks.create<CustomTask>("debug") {
  //dir = File(".gradle") // Script compilation fails if I uncomment this line
}

tasks.create("traverseDir") {
  doLast {
    traverseDir(File("gradle/wrapper/gradle-wrapper.jar"))
  }
}
fun traverseDir(file: File) {
  var dir: File? = file.parentFile
  while (dir != null) {
    println(dir)

    dir = dir.parentFile
  }
}

Output of ./gradlew debug traverseDir without task configuration (meaning commented dir assignment):

> Task :debug
CustomTask.dir: /home/.../gradle-kotlin_dsl-smart_cast_error/.

> Task :traverseDir
gradle/wrapper
gradle

BUILD SUCCESSFUL in 0s
2 actionable tasks: 2 executed

Output of ./gradlew debug traverseDir with task configuration (meaning uncommented dir assignment):

> Configure project :
e: /home/.../gradle-kotlin_dsl-smart_cast_error/build.gradle.kts:32:11: Smart cast to 'File' is impossible, because 'dir' is a local variable that is captured by a changing closure

FAILURE: Build failed with an exception.

* Where:
Build file '/home/.../gradle-kotlin_dsl-smart_cast_error/build.gradle.kts' line: 32

* What went wrong:
Script compilation error:

  Line 32:     dir = dir.parentFile
                     ^ Smart cast to 'File' is impossible, because 'dir' is a local variable that is captured by a changing closure

1 error

* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output. Run with --scan to get full insights.

* Get more help at https://help.gradle.org

BUILD FAILED in 0s

Those two tasks have no connection to each other (other than them being defined in the same buildscript file), yet when I try to configure an instance of my custom task (i.e. uncomment the dir = ... line) the build fails because of a script compilation error in the - seemingly unrelated - traverseDir function.

Further weirdness:

• The error does not occur if I rename either of the dir variables (i.e. to dir_) - then the build succeeds.
• Changing the line dir = dir.parentFile to dir = dir?.parentFile makes the build succeed.

Why does this happen?

Output of ./gradlew --version:

------------------------------------------------------------
Gradle 4.10
------------------------------------------------------------

Build time:   2018-08-27 18:35:06 UTC
Revision:     ee3751ed9f2034effc1f0072c2b2ee74b5dce67d

Kotlin DSL:   1.0-rc-3
Kotlin:       1.2.60
Groovy:       2.4.15
Ant:          Apache Ant(TM) version 1.9.11 compiled on March 23 2018
JVM:          1.8.0_181 (Oracle Corporation 25.181-b13)
OS:           Linux 4.15.0-33-generic amd64

Update:

This seems to be a bug, I've created issues on:

• gradle/kotlin-dsl: https://github.com/gradle/kotlin-dsl/issues/1115
• jetbrains/kotlin: https://youtrack.jetbrains.com/issue/KT-26983