Fix 12919 - summon anonymous mirror when companion is a case object

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

@@ -120,8 +120,9 @@ object SymUtils:
     def useCompanionAsSumMirror(using Context): Boolean =
       def companionExtendsSum(using Context): Boolean =
         self.linkedClass.isSubClass(defn.Mirror_SumClass)
-      self.linkedClass.exists
-        && !self.is(Scala2x)
+      !self.is(Scala2x)
+        && self.linkedClass.exists
+        && !self.linkedClass.is(Case)
         && (
           // If the sum type is compiled from source, and `self` is a "generic sum"
           // then its companion object will become a sum mirror in `posttyper`. (This method

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

@@ -307,7 +307,7 @@ class Synthesizer(typer: Typer)(using @constructorOnly c: Context):
               .refinedWith(tpnme.MirroredElemTypes, TypeAlias(elemsType))
               .refinedWith(tpnme.MirroredElemLabels, TypeAlias(elemsLabels))
           val mirrorRef =
-            if (cls.is(Scala2x)) anonymousMirror(monoType, ExtendsProductMirror, span)
+            if (cls.is(Scala2x) || cls.linkedClass.is(Case)) anonymousMirror(monoType, ExtendsProductMirror, span)
             else companionPath(mirroredType, span)
           mirrorRef.cast(mirrorType)
         else EmptyTree

tests/run/i12919.scala

@@ -0,0 +1,20 @@
+case class Normal(value: String)
+object Normal
+
+case class ClassWithCaseCompanion(value: String)
+case object ClassWithCaseCompanion
+
+def instantiate[T](product: Product)(implicit mirror: scala.deriving.Mirror.ProductOf[T]) =
+  mirror.fromProduct(product)
+
+@main def Test: Unit = {
+  assert(instantiate[Normal](Tuple1("a")) == Normal("a")) // works as expected
+
+  assert(instantiate[ClassWithCaseCompanion.type](EmptyTuple) == ClassWithCaseCompanion) // works as expected
+
+  val c = instantiate[ClassWithCaseCompanion](Tuple1("b")) // throws java.lang.ClassCastException: class ClassWithCaseCompanion$ cannot be cast to class ClassWithCaseCompanion
+  assert(c == ClassWithCaseCompanion("b")) // desired behaviour
+
+  val d = instantiate[ClassWithCaseCompanion.type](EmptyTuple)
+  assert(d == ClassWithCaseCompanion)
+}

tests/run/i12919a.scala

@@ -0,0 +1,18 @@
+import scala.deriving.Mirror
+
+case class Standalone(i: Int)
+object Standalone
+
+case class WithCompanionCaseClass(i: Int)
+case object WithCompanionCaseClass
+
+@main def Test: Unit =
+
+  val mStandalone = summon[Mirror.ProductOf[Standalone]]
+  assert(mStandalone eq Standalone) // the companion object is the mirror for the case class
+
+  val mWithCompanion = summon[Mirror.ProductOf[WithCompanionCaseClass]]
+  assert(mWithCompanion ne WithCompanionCaseClass) // A case object can not be the mirror of a companion case class.
+
+  val mWithCompanionCaseObject = summon[Mirror.ProductOf[WithCompanionCaseClass.type]]
+  assert(mWithCompanionCaseObject eq WithCompanionCaseClass) // A case object is its own mirror.

tests/run/i12919b.scala

@@ -0,0 +1,17 @@
+import scala.deriving.Mirror
+
+
+sealed trait WithCompanionSealedTrait
+case object WithCompanionSealedTrait:
+  case class FirstChild(x: Int) extends WithCompanionSealedTrait
+
+@main def Test: Unit =
+
+  val mWithCompanionSum = summon[Mirror.SumOf[WithCompanionSealedTrait]]
+  assert(mWithCompanionSum.ordinal(WithCompanionSealedTrait.FirstChild(1)) == 0)
+  assert(mWithCompanionSum ne WithCompanionSealedTrait) // A case object can not be the mirror of a companion case class.
+
+  val mWithCompanionSingleton = summon[Mirror.ProductOf[WithCompanionSealedTrait.type]]
+  assert(mWithCompanionSingleton.fromProduct(EmptyTuple) == WithCompanionSealedTrait)
+  assert(mWithCompanionSingleton.isInstanceOf[Mirror.Singleton]) // case object is its own mirror.
+  assert(mWithCompanionSingleton eq WithCompanionSealedTrait) // case object is its own mirror.

tests/run/i15101.scala

@@ -0,0 +1,10 @@
+trait Encoder[T]
+object Encoder:
+  def derived[T](using scala.deriving.Mirror.Of[T]): Encoder[T] = new Encoder[T] {}
+
+case object Bar
+enum Bar derives Encoder:
+  case A, B
+
+@main def Test: Unit =
+  summon[Encoder[Bar]]