fix #22833: allow unroll annotation in methods of final class

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

@@ -3430,12 +3430,12 @@ extends DeclarationMsg(IllegalUnrollPlacementID):
       val isCtor = method.isConstructor
       def what = if isCtor then i"a ${if method.owner.is(Trait) then "trait" else "class"} constructor" else i"method ${method.name}"
       val prefix = s"Cannot unroll parameters of $what"
-      if method.is(Deferred) then
-        i"$prefix: it must not be abstract"
+      if method.isLocal then
+        i"$prefix because it is a local method"
+      else if !method.isEffectivelyFinal then
+        i"$prefix because it can be overridden"
       else if isCtor && method.owner.is(Trait) then
         i"implementation restriction: $prefix"
-      else if !(isCtor || method.is(Final) || method.owner.is(ModuleClass)) then
-        i"$prefix: it is not final"
       else if method.owner.companionClass.is(CaseClass) then
         i"$prefix of a case class companion object: please annotate the class constructor instead"
       else

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

@@ -132,9 +132,9 @@ class PostTyper extends MacroTransform with InfoTransformer { thisPhase =>
         then
           false // not an error, but not an expandable unrolled method
         else if
-          method.is(Deferred)
+            method.isLocal
+          || !method.isEffectivelyFinal
           || isCtor && method.owner.is(Trait)
-          || !(isCtor || method.is(Final) || method.owner.is(ModuleClass))
           || method.owner.companionClass.is(CaseClass)
             && (method.name == nme.apply || method.name == nme.fromProduct)
           || method.owner.is(CaseClass) && method.name == nme.copy

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

@@ -79,7 +79,8 @@ class UnrollDefinitions extends MacroTransform, IdentityDenotTransformer {
               else Some((paramClauseIndex, annotationIndices))
           if indices.nonEmpty then
             // pre-validation should have occurred in posttyper
-            assert(annotated.is(Final, butNot = Deferred) || annotated.isConstructor || annotated.owner.is(ModuleClass) || annotated.name.is(DefaultGetterName),
+            assert(!annotated.isLocal, i"$annotated is local")
+            assert(annotated.isEffectivelyFinal || annotated.name.is(DefaultGetterName),
               i"$annotated is not final&concrete, or a constructor")
           indices
       })

tests/neg/unroll-abstractMethod.check

@@ -1,8 +1,8 @@
 -- [E207] Declaration Error: tests/neg/unroll-abstractMethod.scala:6:41 ------------------------------------------------
 6 |  def foo(s: String, n: Int = 1, @unroll b: Boolean = true): String // error
   |                                         ^
-  |                                         Cannot unroll parameters of method foo: it must not be abstract
+  |                                         Cannot unroll parameters of method foo because it can be overridden
 -- [E207] Declaration Error: tests/neg/unroll-abstractMethod.scala:10:41 -----------------------------------------------
 10 |  def foo(s: String, n: Int = 1, @unroll b: Boolean = true): String // error
    |                                         ^
-   |                                         Cannot unroll parameters of method foo: it must not be abstract
+   |                                         Cannot unroll parameters of method foo because it can be overridden

tests/neg/unroll-illegal3.check

@@ -1,12 +1,12 @@
 -- [E207] Declaration Error: tests/neg/unroll-illegal3.scala:7:31 ------------------------------------------------------
 7 |    def foo(s: String, @unroll y: Boolean) = s + y // error
   |                               ^
-  |                               Cannot unroll parameters of method foo: it is not final
+  |                               Cannot unroll parameters of method foo because it is a local method
 -- [E207] Declaration Error: tests/neg/unroll-illegal3.scala:12:29 -----------------------------------------------------
 12 |  def foo(s: String, @unroll y: Boolean) = s + y // error
    |                             ^
-   |                             Cannot unroll parameters of method foo: it is not final
+   |                             Cannot unroll parameters of method foo because it can be overridden
 -- [E207] Declaration Error: tests/neg/unroll-illegal3.scala:16:29 -----------------------------------------------------
 16 |  def foo(s: String, @unroll y: Boolean): String // error
    |                             ^
-   |                             Cannot unroll parameters of method foo: it must not be abstract
+   |                             Cannot unroll parameters of method foo because it can be overridden

tests/pos/i22833-unroll-final-class.scala

@@ -0,0 +1,5 @@
+import scala.annotation.{experimental,unroll}
+
+@experimental final class Foo {
+  def bar(@unroll x: Int = 0) = x + 1
+}