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 ------------------------------------------- */

tests/pos/gadt-GadtStlc.scala

@@ -10,7 +10,7 @@ object GadtStlc {
   // Var[M[W]]
   sealed trait Var[A]
   object VarW extends Var[W]
-  case class VarM[A] extends Var[M[A]]
+  case class VarM[A]() extends Var[M[A]]
 
   // \s.e
   sealed trait Abs[S, E]

tests/pos/i4176-gadt.scala

@@ -1,7 +1,7 @@
 object i4176 {
   sealed trait TNat
   case class TZero() extends TNat
-  case class TSucc[N <: TNat] extends TNat
+  case class TSucc[N <: TNat]() extends TNat
 
   object TNatSum {
     sealed trait TSum[M, N, R]

tests/pos/i4316.scala

@@ -1,6 +1,6 @@
 object Test {
 
-  case class Bar[A]
+  case class Bar[A]()
 
   def meth[A](consumer: A => Unit, s: Bar[A]): Unit = {
     s match {