Fix several build warnings

- Opt-in to expect-actual classes experimental feature
- Suppress default hierarchy template warning
- Remove obsolete kotlin.Experimental opt-in

core/build.gradle.kts

@@ -5,6 +5,7 @@ import javax.xml.parsers.DocumentBuilderFactory
 import java.io.ByteArrayOutputStream
 import java.io.PrintWriter
 import org.jetbrains.dokka.gradle.AbstractDokkaLeafTask
+import org.jetbrains.kotlin.gradle.ExperimentalKotlinGradlePluginApi
 
 plugins {
     kotlin("multiplatform")
@@ -116,6 +117,11 @@ kotlin {
         }
     }
 
+    @OptIn(ExperimentalKotlinGradlePluginApi::class)
+    compilerOptions {
+        freeCompilerArgs.add("-Xexpect-actual-classes")
+    }
+
     sourceSets.all {
         val suffixIndex = name.indexOfLast { it.isUpperCase() }
         val targetName = name.substring(0, suffixIndex)

core/native/src/Parser.kt

@@ -32,7 +32,6 @@ internal fun <T, S> Parser<T>.chainIgnoring(other: Parser<S>): Parser<T> =
 internal fun <T, S> Parser<T>.chainSkipping(other: Parser<S>): Parser<S> =
     chain(other).map { (_, s) -> s }
 
-@SharedImmutable
 internal val eofParser: Parser<Unit> = { str, pos ->
     if (str.length > pos) {
         throw parseException("extraneous input", pos)

gradle.properties

@@ -15,6 +15,7 @@ java.modularToolchainVersion=11
 kotlin.js.compiler=ir
 kotlin.native.ignoreIncorrectDependencies=true
 kotlin.native.ignoreDisabledTargets=true
+kotlin.mpp.applyDefaultHierarchyTemplate=false
 
 # Workaround for Bintray treating .sha512 files as artifacts
 # https://github.com/gradle/gradle/issues/11412

serialization/build.gradle.kts

@@ -63,9 +63,6 @@ kotlin {
         val suffix = name.substring(suffixIndex).toLowerCase(Locale.ROOT).takeIf { it != "main" }
         kotlin.srcDir("$targetName/${suffix ?: "src"}")
         resources.srcDir("$targetName/${suffix?.let { it + "Resources" } ?: "resources"}")
-        languageSettings {
-            optIn("kotlin.Experimental")
-        }
     }
 
     targets.withType<org.jetbrains.kotlin.gradle.plugin.mpp.KotlinNativeTarget> {