Don't cast to a value class as self type

Fixes #12462

Author: odersky

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

@@ -29,7 +29,10 @@ class ExplicitSelf extends MiniPhase {
     !cls.is(Package) && cls.givenSelfType.exists && !cls.derivesFrom(tree.symbol.owner)
 
   private def castQualifier(tree: RefTree, cls: ClassSymbol, thiz: Tree)(using Context) =
-    cpy.Select(tree)(thiz.cast(AndType(cls.classInfo.selfType, thiz.tpe)), tree.name)
+    val selfType = cls.classInfo.selfType
+    if selfType.classSymbols.exists(_.isValueClass) then
+      report.error(em"self type $selfType of $cls may not be a value class", thiz.srcPos)
+    cpy.Select(tree)(thiz.cast(AndType(selfType, thiz.tpe)), tree.name)
 
   override def transformIdent(tree: Ident)(using Context): Tree = tree.tpe match {
     case tp: ThisType =>

tests/neg/i12462.scala

@@ -0,0 +1,4 @@
+class B(x: Int) {
+  self: Int =>
+  def this(a: String) = this() // error
+}