title: Kotlin Opt-in: Flag is not supported by this version of the compiler
body: <p>I followed the Kotlin documentation <a href="https://kotlinlang.org/docs/reference/opt-in-requirements.html#module-wide-opt-in" rel="nofollow noreferrer">here</a> and <a href="https://kotlinlang.org/docs/reference/basic-types.html#experimental-status-of-unsigned-integers" rel="nofollow noreferrer">here</a> to opt into <code>kotlin.ExperimentalUnsignedTypes</code> for the whole module. My module <code>build.gradle</code> looks like this now:</p>
  
  <pre><code>android {
  
      ...
  
      kotlinOptions {
          jvmTarget = "1.8"
          freeCompilerArgs += "-Xopt-in=kotlin.ExperimentalUnsignedTypes"
      }
  </code></pre>
  
  <p>Lint is still complaining about ExperimentalUnsignedTypes and when compiling I get this warning:</p>
  
  <pre><code>&gt; Task :myModule:compileDebugKotlin
  w: Flag is not supported by this version of the compiler: -Xopt-in=kotlin.ExperimentalUnsignedTypes
  </code></pre>
  
  <p>My kotlin version is: 1.3.61</p>