fix scala#1670: move the check of value class to typer

Author: liufengyun

src/dotty/tools/dotc/typer/RefChecks.scala

@@ -696,7 +696,7 @@ object RefChecks {
   }
 
   /** Verify classes extending AnyVal meet the requirements */
-  private def checkDerivedValueClass(clazz: Symbol, stats: List[Tree])(implicit ctx: Context) = {
+  def checkDerivedValueClass(clazz: Symbol, stats: List[Tree])(implicit ctx: Context) = {
     def checkValueClassMember(stat: Tree) = stat match {
       case _: ValDef if !stat.symbol.is(ParamAccessor) =>
         ctx.error(s"value class may not define non-parameter field", stat.pos)
@@ -843,7 +843,6 @@ class RefChecks extends MiniPhase { thisTransformer =>
       checkParents(cls)
       checkCompanionNameClashes(cls)
       checkAllOverrides(cls)
-      checkDerivedValueClass(cls, tree.body)
       tree
     } catch {
       case ex: MergeError =>

src/dotty/tools/dotc/typer/Typer.scala

@@ -1263,6 +1263,11 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
       ctx.featureWarning(nme.dynamics.toString, "extension of type scala.Dynamic", isScala2Feature = true,
           cls, isRequired, cdef.pos)
     }
+
+
+    // check value class constraints
+    RefChecks.checkDerivedValueClass(cls, body1)
+
     cdef1
 
     // todo later: check that

tests/neg/i1670.scala

@@ -0,0 +1 @@
+class A(a:Int, b:Int) extends AnyVal