Add synthetic casts to and from ErasedValueType

For a value class V, let U be the underlying type after
erasure. We add to the companion object of V two cast methods:

  def u2evt$(x0: U): ErasedValueType(V, U)
  def evt2u$(x0: ErasedValueType(V, U)): U

The casts are used in Erasure to make it typecheck, they are then removed
in ElimErasedValueType (not yet present in this commit). This is
different from the implementation of value classes in Scala 2 (see
SIP-15) which uses `asInstanceOf` which does not typecheck.

1 files changed, 14 insertions, 0 deletions

diff --git a/src/dotty/tools/dotc/transform/ValueClasses.scala b/src/dotty/tools/dotc/transform/ValueClasses.scala
index 9cd0e1ef7..8969b9321 100644
--- a/src/dotty/tools/dotc/transform/ValueClasses.scala
+++ b/src/dotty/tools/dotc/transform/ValueClasses.scala
@@ -35,6 +35,20 @@ object ValueClasses {
       .map(_.symbol)
       .getOrElse(NoSymbol)
 
+  /** For a value class `d`, this returns the synthetic cast from the underlying type to
+   *  ErasedValueType defined in the companion module. This method is added to the module
+   *  and further described in [[ExtensionMethods]].
+   */
+  def u2evt(d: ClassDenotation)(implicit ctx: Context): Symbol =
+    d.linkedClass.info.decl(nme.U2EVT).symbol
+
+  /** For a value class `d`, this returns the synthetic cast from ErasedValueType to the
+   *  underlying type defined in the companion module. This method is added to the module
+   *  and further described in [[ExtensionMethods]].
+   */
+  def evt2u(d: ClassDenotation)(implicit ctx: Context): Symbol =
+    d.linkedClass.info.decl(nme.EVT2U).symbol
+
   /** The unboxed type that underlies a derived value class */
   def underlyingOfValueClass(d: ClassDenotation)(implicit ctx: Context): Type =
     valueClassUnbox(d).info.resultType