Add appropriate script extension to the source name, if not provided

#KT-31704 fixed

plugins/scripting/scripting-compiler-impl/src/org/jetbrains/kotlin/scripting/resolve/refineCompilationConfiguration.kt

@@ -20,6 +20,7 @@ import org.jetbrains.kotlin.psi.KtAnnotationEntry
 import org.jetbrains.kotlin.psi.KtFile
 import org.jetbrains.kotlin.scripting.definitions.KotlinScriptDefinition
 import org.jetbrains.kotlin.scripting.definitions.ScriptDefinition
+import org.jetbrains.kotlin.scripting.withCorrectExtension
 import java.io.File
 import java.net.URL
 import kotlin.reflect.KClass
@@ -290,7 +291,7 @@ fun SourceCode.getVirtualFile(definition: ScriptDefinition): VirtualFile {
         val vFile = LocalFileSystem.getInstance().findFileByIoFile(file)
         if (vFile != null) return vFile
     }
-    val scriptName = name ?: "script.${definition.fileExtension}"
+    val scriptName = withCorrectExtension(name ?: "script", definition.fileExtension)
     val scriptPath = when (this) {
         is FileScriptSource -> file.path
         is ExternalSourceCode -> externalLocation.toString()

plugins/scripting/scripting-compiler-impl/src/org/jetbrains/kotlin/scripting/utils.kt

@@ -0,0 +1,27 @@
+/*
+ * Copyright 2010-2019 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+package org.jetbrains.kotlin.scripting
+
+import kotlin.script.experimental.api.ScriptCompilationConfiguration
+import kotlin.script.experimental.api.SourceCode
+import kotlin.script.experimental.api.fileExtension
+
+fun SourceCode.scriptFileName(
+    mainScript: SourceCode,
+    scriptCompilationConfiguration: ScriptCompilationConfiguration
+): String {
+    val mainExtension = scriptCompilationConfiguration[ScriptCompilationConfiguration.fileExtension]
+    return when {
+        name != null -> withCorrectExtension(name!!, mainExtension)
+        mainScript == this -> withCorrectExtension("script", mainExtension)
+        else -> throw Exception("Unexpected script without name: $this")
+    }
+}
+
+internal fun withCorrectExtension(name: String, mainExtension: String?): String =
+    // TODO: consider checking for all registered extensions
+    if ((mainExtension != null && name.endsWith(".$mainExtension")) || name.endsWith(".kts")) name else "$name.${mainExtension ?: "kts"}"
+