Add empty case class params check. Add test.

Author: ashwinbhaskar

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -2396,7 +2396,15 @@ object Parsers {
         }
         else Nil
       }
-      recur(firstClause = true, 0, ofInstance)
+
+      val start = in.lastOffset
+      val params = recur(firstClause = true, 0, ofInstance)
+
+      if(ofCaseClass && params.isEmpty)
+        syntaxError( "case classes without a parameter list are not allowed;\n"+
+          "use either case objects or case classes with an explicit `()' as a parameter list.", start)
+
+      params
     }
 
 /* -------- DEFS ------------------------------------------- */