Fix #2430: Make Erasure pass Ycheck

compiler/src/dotty/tools/dotc/transform/Erasure.scala

@@ -239,26 +239,44 @@ object Erasure {
      *  Casts from and to ErasedValueType are special, see the explanation
      *  in ExtensionMethods#transform.
      */
-    def cast(tree: Tree, pt: Type)(implicit ctx: Context): Tree = {
+    def cast(tree: Tree, pt: Type)(implicit ctx: Context): Tree = ctx.traceIndented(i"cast ${tree.tpe.widen} --> $pt", show = true) {
+      def wrap(tycon: TypeRef) =
+        ref(u2evt(tycon.typeSymbol.asClass)).appliedTo(tree)
+      def unwrap(tycon: TypeRef) =
+        ref(evt2u(tycon.typeSymbol.asClass)).appliedTo(tree)
+
+
       assert(!pt.isInstanceOf[SingletonType], pt)
       if (pt isRef defn.UnitClass) unbox(tree, pt)
-      else (tree.tpe, pt) match {
+      else (tree.tpe.widen, pt) match {
         case (JavaArrayType(treeElem), JavaArrayType(ptElem))
         if treeElem.widen.isPrimitiveValueType && !ptElem.isPrimitiveValueType =>
           // See SI-2386 for one example of when this might be necessary.
           cast(ref(defn.runtimeMethodRef(nme.toObjectArray)).appliedTo(tree), pt)
-        case (_, ErasedValueType(tycon, _)) =>
-          ref(u2evt(tycon.symbol.asClass)).appliedTo(tree)
-        case _ =>
-          tree.tpe.widen match {
-            case ErasedValueType(tycon, _) =>
-              ref(evt2u(tycon.symbol.asClass)).appliedTo(tree)
-            case _ =>
-              if (pt.isPrimitiveValueType)
-                primitiveConversion(tree, pt.classSymbol)
-              else
-                tree.asInstance(pt)
+
+        // When casting between two EVTs, we need to check which one underlies the other to determine
+        // wheter u2evt or evt2u should be used.
+        case (tp1 @ ErasedValueType(tycon1, underlying1), tp2 @ ErasedValueType(tycon2, underlying2)) =>
+          if (tp1 <:< underlying2)
+            // Cast EVT(tycon1, underlying1) to EVT(tycon2, EVT(tycon1, underlying1))
+            wrap(tycon2)
+          else {
+            assert(underlying1 <:< tp2, i"Non-sensical cast between unrelated types $tp1 and $tp2")
+            // Cast EVT(tycon1, EVT(tycon2, underlying2)) to EVT(tycon2, underlying2)
+            unwrap(tycon1)
           }
+
+        // When only one type is an EVT then we already know that the other one is the underlying
+        case (_, ErasedValueType(tycon2, _)) =>
+          wrap(tycon2)
+        case (ErasedValueType(tycon1, _), _) =>
+          unwrap(tycon1)
+
+        case _ =>
+          if (pt.isPrimitiveValueType)
+            primitiveConversion(tree, pt.classSymbol)
+          else
+            tree.asInstance(pt)
       }
     }
 

compiler/test/dotc/tests.scala

@@ -68,7 +68,7 @@ class tests extends CompilerTest {
   }
 
   implicit val defaultOptions: List[String] = noCheckOptions ++ {
-    if (dotty.Properties.isRunByDrone) List("-Ycheck:tailrec,resolveSuper,mixin,getClass,restoreScopes,labelDef") // should be Ycheck:all, but #725
+    if (dotty.Properties.isRunByDrone) List("-Ycheck:tailrec,resolveSuper,erasure,mixin,getClass,restoreScopes,labelDef") // should be Ycheck:all, but #725
     else List("-Ycheck:tailrec,resolveSuper,mixin,restoreScopes,labelDef,simplify")
   } ++ checkOptions ++ classPath
 

compiler/test/dotty/tools/vulpix/TestConfiguration.scala

@@ -50,7 +50,7 @@ object TestConfiguration {
     Array("-classpath", paths)
   }
 
-  private val yCheckOptions = Array("-Ycheck:tailrec,resolveSuper,mixin,getClass,restoreScopes,labelDef")
+  private val yCheckOptions = Array("-Ycheck:tailrec,resolveSuper,erasure,mixin,getClass,restoreScopes,labelDef")
 
   val defaultUnoptimised = noCheckOptions ++ checkOptions ++ yCheckOptions ++ classPath
   val defaultOptimised = defaultUnoptimised :+ "-optimise"