I have project with mixed plugins (regulars and conventions). I have issue with accessing my extension inside my convention plugin. This works:

myExtension.mySubExtentions.variable.set("test")

but this throws errors:

myExtension {
  mySubExtension {
    variable.set("test")
  }
}

I use code like this to add my base extension as property:

@Suppress("UNCHECKED_CAST")
fun <T : Any> Any.requiredExtension(name: String): T =
    (this as ExtensionAware).extensions.getByName(name) as T

val Project.alpha: AlphaExtension
    get() = this.requiredExtension(ALPHA_EXTENSION_NAME)

And then in another plugin:

val AlphaExtension.version: VersionExtension
    get() = this.requiredExtension(ALPHA_VERSION_EXTENSION_NAME)

It seems like it's missing accessor in format:

fun myExtension(configure: Action<MyExtension>): Unit {do something...}

because at least first level is accessible when I add:

fun Project.alpha(configure: Action<AlphaExtension>) {
    configure.execute(alpha)
}

Is this expected behavior? Am I missing something or did something wrong? It works properly when I import my plugin in external project so it seems that normally Gradle adds those Accessors automatically.

Where you write "I use code like this to add my base extension as property", this code is not adding any extension anywhere. It is a Kotlin extension property that just gets the extension by name and type, but that is not the accessor consumers of that plugin get.