add test

Author: nox213

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

@@ -1815,7 +1815,7 @@ class ValueClassParameterMayNotBeCallByName(valueClass: Symbol, param: Symbol)(u
 
 class ValueClassCannotExtendAliasOfAnyVal(valueClass: Symbol, alias: Symbol)(using Context)
   extends SyntaxMsg(ValueClassCannotExtendAliasOfAnyValID) {
-  def msg(using Context) = i"""Value class cannot extend from alias $alias of ${hl("AnyVal")} """
+  def msg(using Context) = i"""Value class cannot extend from an alias $alias of ${hl("AnyVal")} """
   def explain(using Context) = ""
 }
 

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

@@ -762,7 +762,7 @@ object Checking {
     if (clazz.isDerivedValueClass && !clazz.isEnumAnonymClass) {
       val implTree @ Template(_, _, _, _) = cdef.rhs: @unchecked
       val parent = implTree.parents.head
-      val isExtendingAliasOfAnyVal = parent.tpe.nn.dealias =:= defn.AnyValType && !(parent.tpe.nn =:= defn.AnyValType)
+      val isExtendingAliasOfAnyVal = parent.tpe.nn.dealias =:= defn.AnyValType && parent.symbol.isAliasType
 
       if (clazz.is(Trait))
         report.error(CannotExtendAnyVal(clazz), clazz.srcPos)

tests/neg/i21918.scala

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