Add checks for value classes

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

@@ -708,7 +708,19 @@ object RefChecks {
       if (clazz.is(Abstract))
         ctx.error("`abstract' modifier cannot be used with value classes", clazz.pos)
       if (!clazz.isStatic)
-        ctx.error("value class cannot be an inner class", clazz.pos)
+        ctx.error(s"value class may not be a ${if (clazz.owner.isTerm) "local class" else "member of another class"}", clazz.pos)
+      else {
+        val clParamAccessors = clazz.asClass.paramAccessors.filter(sym => sym.isTerm && !sym.is(Method))
+        clParamAccessors match {
+          case List(param) =>
+            if (param.is(Mutable))
+              ctx.error("value class parameter must not be a var", param.pos)
+            if (param.is(PrivateLocal))
+              ctx.error("value class parameter must not be private[this]", param.pos)
+          case _ =>
+            ctx.error("value class needs to have exactly one val parameter", clazz.pos)
+        }
+      }
       stats.foreach(checkValueClassMember)
     }
   }

test/dotc/tests.scala

@@ -185,6 +185,7 @@ class tests extends CompilerTest {
   @Test def neg_validateRefchecks = compileFile(negDir, "validate-refchecks", xerrors = 2)
   @Test def neg_skolemize = compileFile(negDir, "skolemize", xerrors = 2)
   @Test def neg_nested_bounds = compileFile(negDir, "nested_bounds", xerrors = 1)
+  @Test def neg_valueClasses = compileFile(negDir, "valueClasses", xerrors = 4)
 
   @Test def run_all = runFiles(runDir)
 

tests/neg/valueClasses.scala

@@ -0,0 +1,10 @@
+class A1 {
+  class A2(x: Int) extends AnyVal // error: value class may not be a member of another class
+}
+class B1 {
+  def test = {
+    class B2(x: Int) extends AnyVal // error: value class may not be a local class
+  }
+}
+class C(private[this] val u: Int) extends AnyVal // error: value class parameter must not be private[this]
+class D(u: Int) extends AnyVal // error: value class parameter must not be private[this]