Fix -Xmulti-platform dropped from args when freeCompilerArgs is assigned

in user build script

Issue #KT-15371 Fixed

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/kotlin/org/jetbrains/kotlin/gradle/BaseGradleIT.kt

@@ -465,6 +465,9 @@ abstract class BaseGradleIT {
     private fun Project.createBuildCommand(wrapperDir: File, params: Array<out String>, options: BuildOptions): List<String> =
             createGradleCommand(wrapperDir, createGradleTailParameters(options, params))
 
+    fun Project.gradleBuildScript(subproject: String? = null): File =
+        File(projectDir, subproject?.plus("/").orEmpty() + "build.gradle")
+
     private fun Project.createGradleTailParameters(options: BuildOptions, params: Array<out String> = arrayOf()): List<String> =
             params.toMutableList().apply {
                 add("--stacktrace")

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/kotlin/org/jetbrains/kotlin/gradle/MultiplatformGradleIT.kt

@@ -185,4 +185,20 @@ class MultiplatformGradleIT : BaseGradleIT() {
             }
         }
     }
+
+    @Test
+    fun testFreeCompilerArgsAssignment(): Unit = with(Project("multiplatformProject")) {
+        setupWorkingDir()
+
+        val overrideCompilerArgs = "kotlinOptions.freeCompilerArgs = ['-verbose']"
+
+        gradleBuildScript("lib").appendText("\ncompileKotlinCommon.$overrideCompilerArgs")
+        gradleBuildScript("libJvm").appendText("\ncompileKotlin.$overrideCompilerArgs")
+        gradleBuildScript("libJs").appendText("\ncompileKotlin2Js.$overrideCompilerArgs")
+
+        build("build") {
+            assertSuccessful()
+            assertTasksExecuted(listOf(":lib:compileKotlinCommon", ":libJvm:compileKotlin", ":libJs:compileKotlin2Js"))
+        }
+    }
 }

libraries/tools/kotlin-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/plugin/KotlinMultiplatformPlugin.kt

@@ -54,10 +54,6 @@ open class KotlinPlatformImplementationPluginBase(platformName: String) : Kotlin
     private val platformKotlinTasksBySourceSetName = hashMapOf<String, AbstractKotlinCompile<*>>()
 
     override fun apply(project: Project) {
-        project.tasks.withType(AbstractKotlinCompile::class.java).all {
-            (it as KotlinCompile<*>).kotlinOptions.freeCompilerArgs += listOf("-Xmulti-platform")
-        }
-
         project.tasks.filterIsInstance<AbstractKotlinCompile<*>>().associateByTo(platformKotlinTasksBySourceSetName) { it.sourceSetName }
 
         val implementConfig = project.configurations.create(IMPLEMENT_CONFIG_NAME)

libraries/tools/kotlin-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/tasks/Tasks.kt

@@ -233,6 +233,8 @@ abstract class AbstractKotlinCompile<T : CommonCompilerArguments>() : AbstractKo
             args.verbose = true
         }
 
+        args.multiPlatform = project.plugins.any { it is KotlinPlatformPluginBase }
+
         setupPlugins(args)
     }