Add LANGUAGE_VERSION test directive.

compiler/test-infrastructure/tests/org/jetbrains/kotlin/test/builders/LanguageVersionSettingsBuilder.kt

@@ -62,6 +62,19 @@ class LanguageVersionSettingsBuilder {
             val languageVersion = maxOf(LanguageVersion.LATEST_STABLE, LanguageVersion.fromVersionString(apiVersion.versionString)!!)
             this.languageVersion = languageVersion
         }
+        val languageVersionDirective = directives.singleOrZeroValue(LanguageSettingsDirectives.LANGUAGE_VERSION)
+        if (languageVersionDirective != null) {
+            languageVersion = languageVersionDirective
+            if (languageVersion < LanguageVersion.fromVersionString(this.apiVersion.versionString)!!) {
+                error(
+                    """
+                        Language version must be larger than or equal to the API version.
+                        Language version: '$languageVersion'.
+                        API version: '$apiVersion'.
+                    """.trimIndent()
+                )
+            }
+        }
         when {
             useK2 && this.languageVersion < LanguageVersion.KOTLIN_2_0 -> this.languageVersion = LanguageVersion.KOTLIN_2_0
             !useK2 && this.languageVersion > LanguageVersion.KOTLIN_1_9 -> this.languageVersion = LanguageVersion.KOTLIN_1_9

compiler/test-infrastructure/tests/org/jetbrains/kotlin/test/directives/LanguageSettingsDirectives.kt

@@ -5,9 +5,7 @@
 
 package org.jetbrains.kotlin.test.directives
 
-import org.jetbrains.kotlin.config.ApiVersion
+import org.jetbrains.kotlin.config.*
-import org.jetbrains.kotlin.config.ExplicitApiMode
-import org.jetbrains.kotlin.config.JvmDefaultMode
 import org.jetbrains.kotlin.test.directives.model.SimpleDirectivesContainer
 
 object LanguageSettingsDirectives : SimpleDirectivesContainer() {
@@ -22,6 +20,12 @@ object LanguageSettingsDirectives : SimpleDirectivesContainer() {
         description = "Version of Kotlin API",
         parser = this::parseApiVersion
     )
+
+    val LANGUAGE_VERSION by valueDirective<LanguageVersion>(
+        description = "Kotlin language version",
+        parser = this::parseLanguageVersion
+    )
+
     // --------------------- Analysis Flags ---------------------
 
     val OPT_IN by stringDirective(
@@ -89,4 +93,9 @@ object LanguageSettingsDirectives : SimpleDirectivesContainer() {
         "LATEST_STABLE" -> ApiVersion.LATEST_STABLE
         else -> ApiVersion.parse(versionString) ?: error("Unknown API version: $versionString")
     }
+
+    fun parseLanguageVersion(versionString: String): LanguageVersion = when (versionString) {
+        "LATEST_STABLE" -> LanguageVersion.LATEST_STABLE
+        else -> LanguageVersion.fromVersionString(versionString) ?: error("Unknown language version: $versionString")
+    }
 }