Allow case classes that only have type parameters

A case class like

    case class C[T]

is OK, it cannot be mistaken for an object.

Author: odersky

compiler/src/dotty/tools/dotc/ast/Desugar.scala

@@ -320,7 +320,8 @@ object desugar {
     val constrTparams = impliedTparams.map(toDefParam)
     val constrVparamss =
       if (originalVparamss.isEmpty) { // ensure parameter list is non-empty
-        if (isCaseClass) ctx.error(CaseClassMissingParamList(cdef), cdef.namePos)
+        if (isCaseClass && originalTparams.isEmpty)
+          ctx.error(CaseClassMissingParamList(cdef), cdef.namePos)
         ListOfNil
       }
       else originalVparamss.nestedMap(toDefParam)