[lang] disallow MetaLanguage-s as base languages

No language can have MetaLanguage as its base language.
This implies that MetaLanguage-s cannot have dialects.

platform/core-api/src/com/intellij/lang/Language.java

@@ -1,18 +1,4 @@
-/*
- * Copyright 2000-2016 JetBrains s.r.o.
- *
- * Licensed under the Apache License, Version 2.0 (the "License");
- * you may not use this file except in compliance with the License.
- * You may obtain a copy of the License at
- *
- * http://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
- */
+// Copyright 2000-2018 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license that can be found in the LICENSE file.
 package com.intellij.lang;
 
 import com.intellij.diagnostic.ImplementationConflictException;
@@ -71,6 +57,14 @@ public abstract class Language extends UserDataHolderBase {
   }
 
   protected Language(@Nullable Language baseLanguage, @NotNull String ID, @NotNull String... mimeTypes) {
+    if (baseLanguage instanceof MetaLanguage) {
+      throw new ImplementationConflictException(
+        "MetaLanguage cannot be a base language.\n" +
+        "This language: '" + ID + "'\n" +
+        "Base language: '" + baseLanguage.getID() + "'",
+        null, this, baseLanguage
+      );
+    }
     myBaseLanguage = baseLanguage;
     myID = ID;
     myMimeTypes = mimeTypes.length == 0 ? ArrayUtil.EMPTY_STRING_ARRAY : mimeTypes;