Don't cast to a value class as self type

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

@@ -3,6 +3,7 @@ package transform
 
 import core._
 import Contexts._, Types._, MegaPhase._, ast.Trees._, Symbols._, Decorators._, Flags._
+import SymUtils.*
 
 /** Transform references of the form
  *
@@ -29,7 +30,10 @@ 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) && !cls.isUniversalTrait 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 =>

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

@@ -66,6 +66,9 @@ object SymUtils:
 
     def isNoValue(using Context): Boolean = self.is(Package) || self.isAllOf(JavaModule)
 
+    def isUniversalTrait(using Context): Boolean =
+      self.is(Trait) && self.asClass.parentSyms.head == defn.AnyClass
+
     /** Is this a type or term parameter or a term parameter accessor? */
     def isParamOrAccessor(using Context): Boolean =
       self.is(Param) || self.is(ParamAccessor)

tests/neg/i12462.scala

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