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Anyone else running into an issue with `-Xopt-in` and Kotlin 1.6.20-RC while using Gradle Kotlin DSL?

<https://youtrack.jetbrains.com/issue/KT-51708>

Use `-opt-in=kotlin.RequiresOptIn`. The `-opt-in kotlin.RequiresOptIn` syntax is only valid in CLI (i.e. when you invoke kotlin compiler on the command line).

presumably `listOf("-opt-in", "kotlin.RequiresOptIn")` would work like the CLI too. but a single argument with a space in it is definitely not right

also, the Gradle field is in `languageSettings`

e.g.
```kotlin.sourceSets.all {
    languageSettings.optIn(...)
}```

&gt; Use `-opt-in=kotlin.RequiresOptIn`
If I use `-opt-in=Kotlin.RequiresOptIn` I get an error that says:
```e: Invalid argument: -opt-in=kotlin.RequiresOptIn```
&gt; listOf("-opt-in", "kotlin.RequiresOptIn")
Results in an error that says:
```e: Invalid argument: -opt-in```
&gt; The Gradle field is in `languageSettings`
That was it, thanks! I thought it was in `kotlinOptions` because of <https://youtrack.jetbrains.com/issue/KT-47099>

so the change isn't in -RC, then. <https://github.com/JetBrains/kotlin/commit/d8417fd622749bdcef028e1d7f75d992725b0765|https://github.com/JetBrains/kotlin/commit/d8417fd622749bdcef028e1d7f75d992725b0765> should be in -RC2 though

Whoops I left that out in OP. I am using RC2.

It looks like the issue might be that Gradle Kotlin DSL forces Kotlin 1.5.31