Support "// !API_VERSION: LATEST" directive in tests

Useful for tests that check behavior which depends on having a recent
API version enabled, and that don't want to depend on a specific fixed
API version (to prevent eventual obsoletion)

compiler/tests-common/tests/org/jetbrains/kotlin/checkers/CompilerTestLanguageVersionSettings.kt

@@ -76,8 +76,11 @@ fun parseLanguageVersionSettings(directives: Map<String, String>): CompilerTestL
         return null
     }
 
-    val apiVersion = (if (apiVersionString != null) ApiVersion.parse(apiVersionString) else ApiVersion.LATEST_STABLE)
-        ?: error("Unknown API version: $apiVersionString")
+    val apiVersion = when (apiVersionString) {
+        null -> ApiVersion.LATEST_STABLE
+        "LATEST" -> ApiVersion.LATEST
+        else -> ApiVersion.parse(apiVersionString) ?: error("Unknown API version: $apiVersionString")
+    }
 
     val languageVersion = maxOf(LanguageVersion.LATEST_STABLE, LanguageVersion.fromVersionString(apiVersion.versionString)!!)
 

compiler/util/src/org/jetbrains/kotlin/config/ApiVersion.kt

@@ -52,6 +52,9 @@ class ApiVersion private constructor(
         @JvmField
         val KOTLIN_1_3 = createByLanguageVersion(LanguageVersion.KOTLIN_1_3)
 
+        @JvmField
+        val LATEST: ApiVersion = createByLanguageVersion(LanguageVersion.values().last())
+
         @JvmField
         val LATEST_STABLE: ApiVersion = createByLanguageVersion(LanguageVersion.LATEST_STABLE)