src/Tools/VSCode/src/lsp.scala

 
   object Message
   {
     val empty: JSON.Object.T = JSON.Object("jsonrpc" -> "2.0")
 
-    def log(prefix: String, json: JSON.T, logger: Logger, verbose: Boolean)
+    def log(prefix: String, json: JSON.T, logger: Logger, verbose: Boolean): Unit =
     {
       val header =
         json match {
           case JSON.Object(obj) => obj -- (obj.keySet - "method" - "id")
           case _ => JSON.Object.empty