Fix #1644: Disallow inner classes in value classes

Author: smarter

compiler/src/dotty/tools/dotc/typer/Checking.scala

@@ -410,6 +410,8 @@ object Checking {
   /** Verify classes extending AnyVal meet the requirements */
   def checkDerivedValueClass(clazz: Symbol, stats: List[Tree])(implicit ctx: Context) = {
     def checkValueClassMember(stat: Tree) = stat match {
+      case _: TypeDef if stat.symbol.isClass =>
+        ctx.error(s"value class may not define an inner class", stat.pos)
       case _: ValDef if !stat.symbol.is(ParamAccessor) =>
         ctx.error(s"value class may not define non-parameter field", stat.pos)
       case d: DefDef if d.symbol.isConstructor =>

tests/neg/valueclasses-impl-restrictions.scala

@@ -0,0 +1,19 @@
+class X1(val s: String) extends AnyVal {
+  trait I2 { // error: value class may not define an inner class or trait
+   val q: String
+   def z = s + q
+  }
+}
+
+class X2(val s: String) extends AnyVal {
+  private[this] class I2(val q: String) // error: value class may not define an inner class or trait
+
+  def y(i: Int) = {
+    val i2 = new I2(i.toString)
+    i2.q + s
+  }
+}
+
+class X3(val s: String) extends AnyVal {
+  object I3 // error: value class may not define non-parameter field
+}