Merge pull request #4445 from dotty-staging/harden-ide-12

Avoid classcast exception in IDE

Author: odersky

language-server/src/dotty/tools/languageserver/DottyLanguageServer.scala

@@ -257,9 +257,12 @@ class DottyLanguageServer extends LanguageServer
         if (enclTree.isInstanceOf[MemberDef])
           (driver.allTreesContaining(sym.name.sourceModuleName.toString),
            Include.overriding | Include.overridden)
-        else
-          (SourceTree.fromSymbol(sym.topLevelClass.asClass).toList,
-           Include.overriding)
+        else sym.topLevelClass match {
+          case cls: ClassSymbol =>
+            (SourceTree.fromSymbol(cls).toList, Include.overriding)
+          case _ =>
+            (Nil, Include.overriding)
+        }
       val defs = Interactive.namedTrees(trees, include, sym)
       defs.map(d => location(d.namePos)).asJava
     }