move 'extend final class' to error class

compiler/src/dotty/tools/dotc/reporting/diagnostic/ErrorMessageID.java

@@ -100,6 +100,7 @@ public enum ErrorMessageID {
     NoReturnFromInlineID,
     ReturnOutsideMethodDefinitionID,
     UncheckedTypePatternID,
+    ExtendFinalClassID,
     ;
 
     public int errorNumber() {

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

@@ -1770,4 +1770,12 @@ object messages {
           |"""
   }
 
+  case class ExtendFinalClass(clazz:Symbol, finalClazz: Symbol)(implicit ctx: Context) 
+    extends Message(ExtendFinalClassID) {
+    val kind = "Syntax"
+    val msg = hl"$clazz cannot extend ${"final"} $finalClazz"
+    val explanation =
+      hl"""A class marked with the ${"final"} keyword cannot be extended"""
+  }
+
 }

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

@@ -876,7 +876,7 @@ class Namer { typer: Typer =>
           else {
             val pclazz = pt.typeSymbol
             if (pclazz.is(Final))
-              ctx.error(em"cannot extend final $pclazz", cls.pos)
+              ctx.error(ExtendFinalClass(cls, pclazz), cls.pos)
             if (pclazz.is(Sealed) && pclazz.associatedFile != cls.associatedFile)
               ctx.error(em"cannot extend sealed $pclazz in different compilation unit", cls.pos)
             pt

compiler/test/dotty/tools/dotc/reporting/ErrorMessagesTests.scala

@@ -998,4 +998,16 @@ class ErrorMessagesTests extends ErrorMessagesTest {
       val ReturnOutsideMethodDefinition(owner) :: Nil = messages
       assertEquals("object A", owner.show)
     }
+  @Test def extendFinalClass = checkMessagesAfter("refchecks") {
+    """final class A
+      |
+      |class B extends A
+    """.stripMargin
+  }.expect { (ictx, messages) =>
+    implicit val ctx: Context = ictx
+    assertMessageCount(1, messages)
+    val ExtendFinalClass(extender, parent) :: Nil = messages
+    assertEquals(extender.show, "class B")
+    assertEquals(parent.show, "class A")
+  }
 }