Don't generate mirrors for local classes

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

@@ -346,7 +346,10 @@ class Synthesizer(typer: Typer)(using @constructorOnly c: Context):
       (using Context): Tree =
     if checkFormal(formal) then
       formal.member(tpnme.MirroredType).info match
-        case TypeBounds(mirroredType, _) => synth(mirroredType.stripTypeVar, formal, span)
+        case TypeBounds(mirroredType, _)
+        if mirroredType.classSymbol.maybeOwner.isType =>
+          // local classes do not support mirrors, see neg/12328.scala for a test case.
+          synth(mirroredType.stripTypeVar, formal, span)
         case other => EmptyTree
     else EmptyTree
 

docs/docs/reference/contextual/derivation.md

@@ -29,7 +29,7 @@ We say that `Tree` is the _deriving type_ and that the `Eq`, `Ordering` and `Sho
 
 All data types can have a `derives` clause. This document focuses primarily on data types which also have a given instance
 of the `Mirror` type class available. Instances of the `Mirror` type class are generated automatically by the compiler
-for,
+for:
 
 + enums and enum cases
 + case classes and case objects
@@ -39,6 +39,8 @@ for,
 They also provide minimal term level infrastructure to allow higher level libraries to provide comprehensive
 derivation support.
 
+*Note*: Mirror instances are not generated for classes in a local context (i.e. classes that are not members of packages or other classes).
+
 ```scala
 sealed trait Mirror:
 

tests/neg/i12328.scala

@@ -0,0 +1,35 @@
+import scala.deriving.Mirror
+import scala.compiletime._
+
+trait Schema[T]
+
+object Schema {
+  inline def recurse[A <: Tuple]: List[Schema[Any]] =
+    inline erasedValue[A] match {
+      case _: (t *: ts) => summonInline[Schema[t]].asInstanceOf[Schema[Any]] :: recurse[ts]
+      case EmptyTuple   => Nil
+    }
+
+  inline def derived[T]: Schema[T] =
+    inline summonInline[Mirror.Of[T]] match {
+      case m: Mirror.SumOf[T] =>
+        val subTypes = recurse[m.MirroredElemTypes]
+        new Schema[T] { }
+      case m: Mirror.ProductOf[T] =>
+        val fields = recurse[m.MirroredElemTypes]
+        new Schema[T] { }
+  }
+
+  inline given gen[T]: Schema[T] = derived
+}
+
+@main def hello: Unit = {
+
+  sealed trait Item
+  object Item {
+    case object A extends Item
+    case object B extends Item
+  }
+
+  summon[Schema[Item]] // error
+}

tests/neg/multi-param-derives.scala

@@ -1,7 +1,7 @@
 import scala.deriving.*
 
 object Test extends App {
-  {
+  object test1 {
     trait Show[T]
     object Show {
       given Show[Int]()
@@ -19,7 +19,7 @@ object Test extends App {
     case class Poly3[A, B, C](a: A, b: B, c: C) derives Show
   }
 
-  {
+  object test2 {
     trait Functor[F[_]]
     object Functor {
       given [C]: Functor[[T] =>> C]()
@@ -37,7 +37,7 @@ object Test extends App {
     case class Poly3[A, B, C](a: A, b: B, c: C) derives Functor
   }
 
-  {
+  object test3 {
     trait FunctorK[F[_[_]]]
     object FunctorK {
       given [C]: FunctorK[[F[_]] =>> C]()
@@ -53,7 +53,7 @@ object Test extends App {
     case class Poly3[A, B, C](a: A, b: B, c: C) derives FunctorK // error
   }
 
-  {
+  object test4 {
     trait Bifunctor[F[_, _]]
     object Bifunctor {
       given [C]: Bifunctor[[T, U] =>> C]()

tests/run/poly-kinded-derives.scala

@@ -1,7 +1,7 @@
 import scala.deriving.*
 
 object Test extends App {
-  {
+  object test1 {
     trait Show[T]
     object Show {
       given Show[Int]()
@@ -19,7 +19,7 @@ object Test extends App {
     case class Poly3[A, B, C](a: A, b: B, c: C) derives Show
   }
 
-  {
+  object test2 {
     trait Functor[F[_]]
     object Functor {
       given [C]: Functor[[T] =>> C]()
@@ -37,7 +37,7 @@ object Test extends App {
     case class Poly3[A, B, C](a: A, b: B, c: C) derives Functor
   }
 
-  {
+  object test3 {
     trait FunctorK[F[_[_]]]
     object FunctorK {
       given [C]: FunctorK[[F[_]] =>> C]()
@@ -53,7 +53,7 @@ object Test extends App {
     //case class Poly3[A, B, C](a: A, b: B, c: C) derives FunctorK
   }
 
-  {
+  object test4 {
     trait Bifunctor[F[_, _]]
     object Bifunctor {
       given [C]: Bifunctor[[T, U] =>> C]()