Value classes: add support for private[this] parameter

src/dotty/tools/dotc/transform/Getters.scala

@@ -12,6 +12,7 @@ import Constants._
 import TreeTransforms._
 import Flags._
 import Decorators._
+import ValueClasses._
 
 /** Performs the following rewritings for fields of a class:
  *
@@ -34,7 +35,7 @@ import Decorators._
  *
  *  Omitted from the rewritings are
  *
- *   - private[this] fields in non-trait classes
+ *   - private[this] fields in classes (excluding traits, value classes)
  *   - fields generated for static modules (TODO: needed?)
  *   - parameters, static fields, and fields coming from Java
  *
@@ -53,7 +54,7 @@ class Getters extends MiniPhaseTransform with SymTransformer { thisTransform =>
   override def transformSym(d: SymDenotation)(implicit ctx: Context): SymDenotation = {
     def noGetterNeeded =
       d.is(NoGetterNeeded) ||
-      d.initial.asInstanceOf[SymDenotation].is(PrivateLocal) && !d.owner.is(Trait) && !d.is(Flags.Lazy) ||
+      d.initial.asInstanceOf[SymDenotation].is(PrivateLocal) && !d.owner.is(Trait) && !isDerivedValueClass(d.owner) && !d.is(Flags.Lazy) ||
       d.is(Module) && d.isStatic ||
       d.isSelfSym
     if (d.isTerm && (d.is(Lazy) || d.owner.isClass) && d.info.isValueType && !noGetterNeeded) {

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

@@ -715,8 +715,6 @@ object RefChecks {
           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)
         }

test/dotc/tests.scala

@@ -186,7 +186,7 @@ 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 neg_valueClasses = compileFile(negDir, "valueClasses", xerrors = 2)
 
   @Test def run_all = runFiles(runDir)
 

tests/neg/valueClasses.scala

@@ -6,5 +6,3 @@ class B1 {
     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]

tests/pos/valueclasses/privatethisparam.scala

@@ -0,0 +1,18 @@
+package privatethisparam
+
+class Meter[T](x: T) extends AnyVal {
+  def zero: T = x
+}
+
+class Meter2(private[this] val x: Int) extends AnyVal {
+  def foo = x
+}
+
+object Test {
+  def bar = new Meter2(42)
+  def useZero = new Meter(5).zero
+  def test: Unit = {
+    val m1 = new Meter(1)
+    m1.zero
+  }
+}