Fix #21918: Disallow value classes extending type aliases of AnyVal

compiler/src/dotty/tools/dotc/reporting/ErrorMessageID.scala

@@ -223,6 +223,7 @@ enum ErrorMessageID(val isActive: Boolean = true) extends java.lang.Enum[ErrorMe
   case IllegalUnrollPlacementID // errorNumber: 207
   case ExtensionHasDefaultID // errorNumber: 208
   case FormatInterpolationErrorID // errorNumber: 209
+  case ValueClassCannotExtendAliasOfAnyValID // errorNumber: 210
 
   def errorNumber = ordinal - 1
 

compiler/src/dotty/tools/dotc/reporting/messages.scala

@@ -1813,6 +1813,12 @@ class ValueClassParameterMayNotBeCallByName(valueClass: Symbol, param: Symbol)(u
   def explain(using Context) = ""
 }
 
+class ValueClassCannotExtendAliasOfAnyVal(valueClass: Symbol, alias: Symbol)(using Context)
+  extends SyntaxMsg(ValueClassCannotExtendAliasOfAnyValID) {
+  def msg(using Context) = i"""A value class cannot extend a type alias ($alias) of ${hl("AnyVal")}"""
+  def explain(using Context) = ""
+}
+
 class SuperCallsNotAllowedInlineable(symbol: Symbol)(using Context)
   extends SyntaxMsg(SuperCallsNotAllowedInlineableID) {
   def msg(using Context) = i"Super call not allowed in inlineable $symbol"

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

@@ -244,7 +244,7 @@ object TreeChecker {
     private val everDefinedSyms = MutableSymbolMap[untpd.Tree]()
 
     // don't check value classes after typer, as the constraint about constructors doesn't hold after transform
-    override def checkDerivedValueClass(clazz: Symbol, stats: List[Tree])(using Context): Unit = ()
+    override def checkDerivedValueClass(cdef: untpd.TypeDef, clazz: Symbol, stats: List[Tree])(using Context): Unit = ()
 
     def withDefinedSyms[T](trees: List[untpd.Tree])(op: => T)(using Context): T = {
       var locally = List.empty[Symbol]

compiler/src/dotty/tools/dotc/typer/Checking.scala

@@ -743,7 +743,7 @@ object Checking {
   }
 
   /** Verify classes extending AnyVal meet the requirements */
-  def checkDerivedValueClass(clazz: Symbol, stats: List[Tree])(using Context): Unit = {
+  def checkDerivedValueClass(cdef: untpd.TypeDef, clazz: Symbol, stats: List[Tree])(using Context): Unit = {
     def checkValueClassMember(stat: Tree) = stat match {
       case _: TypeDef if stat.symbol.isClass =>
         report.error(ValueClassesMayNotDefineInner(clazz, stat.symbol), stat.srcPos)
@@ -760,6 +760,15 @@ object Checking {
     // enum extending a value class type (AnyVal or an alias of it)
     // The error message 'EnumMayNotBeValueClassesID' will take care of generating the error message (See #22236)
     if (clazz.isDerivedValueClass && !clazz.isEnumAnonymClass) {
+      val parentOpt = cdef.rhs match {
+        case impl: Template =>
+          impl.parents.headOption
+        case _ => None
+      }
+      val isExtendingAliasOfAnyVal = parentOpt.exists { parent =>
+        parent.symbol.isAliasType && parent.tpe.nn.dealias =:= defn.AnyValType
+      }
+
       if (clazz.is(Trait))
         report.error(CannotExtendAnyVal(clazz), clazz.srcPos)
       if clazz.is(Module) then
@@ -770,6 +779,8 @@ object Checking {
         report.error(ValueClassesMayNotBeAbstract(clazz), clazz.srcPos)
       if (!clazz.isStatic)
         report.error(ValueClassesMayNotBeContainted(clazz), clazz.srcPos)
+      if (isExtendingAliasOfAnyVal)
+        report.error(ValueClassCannotExtendAliasOfAnyVal(clazz, parentOpt.get.symbol), clazz.srcPos)
       if (isDerivedValueClass(underlyingOfValueClass(clazz.asClass).classSymbol))
         report.error(ValueClassesMayNotWrapAnotherValueClass(clazz), clazz.srcPos)
       else {
@@ -1307,8 +1318,8 @@ trait Checking {
     else tpt
 
   /** Verify classes extending AnyVal meet the requirements */
-  def checkDerivedValueClass(clazz: Symbol, stats: List[Tree])(using Context): Unit =
-    Checking.checkDerivedValueClass(clazz, stats)
+  def checkDerivedValueClass(cdef: untpd.TypeDef, clazz: Symbol, stats: List[Tree])(using Context): Unit =
+    Checking.checkDerivedValueClass(cdef, clazz, stats)
 
   /** Check that case classes are not inherited by case classes.
    */
@@ -1689,7 +1700,7 @@ trait NoChecking extends ReChecking {
   override def checkNoTargetNameConflict(stats: List[Tree])(using Context): Unit = ()
   override def checkParentCall(call: Tree, caller: ClassSymbol)(using Context): Unit = ()
   override def checkSimpleKinded(tpt: Tree)(using Context): Tree = tpt
-  override def checkDerivedValueClass(clazz: Symbol, stats: List[Tree])(using Context): Unit = ()
+  override def checkDerivedValueClass(cdef: untpd.TypeDef, clazz: Symbol, stats: List[Tree])(using Context): Unit = ()
   override def checkCaseInheritance(parentSym: Symbol, caseCls: ClassSymbol, pos: SrcPos)(using Context): Unit = ()
   override def checkNoForwardDependencies(vparams: List[ValDef])(using Context): Unit = ()
   override def checkMembersOK(tp: Type, pos: SrcPos)(using Context): Type = tp

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -3243,7 +3243,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
       checkNonCyclicInherited(cls.thisType, cls.info.parents, cls.info.decls, cdef.srcPos)
 
       // check value class constraints
-      checkDerivedValueClass(cls, body1)
+      checkDerivedValueClass(cdef, cls, body1)
 
       val effectiveOwner = cls.owner.skipWeakOwner
       if cls.is(ModuleClass)

tests/neg/i21918.scala

@@ -0,0 +1,2 @@
+type AliasToAnyVal = AnyVal
+class Foo(a: Int) extends AliasToAnyVal // error