fix scala#12919: case class companion is never a singleton mirror

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

@@ -592,7 +592,7 @@ class SyntheticMembers(thisPhase: DenotTransformer) {
     }
 
     if (clazz.is(Module)) {
-      if (clazz.is(Case)) makeSingletonMirror()
+      if (clazz.is(Case) && !linked.isGenericProduct) makeSingletonMirror()
       else if (linked.isGenericProduct) makeProductMirror(linked)
       else if (linked.isGenericSum(clazz)) makeSumMirror(linked)
       else if (linked.is(Sealed))

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

@@ -282,7 +282,7 @@ class Synthesizer(typer: Typer)(using @constructorOnly c: Context):
         if mirroredType.termSymbol.is(CaseVal) then
           val module = mirroredType.termSymbol
           val modulePath = pathFor(mirroredType).withSpan(span)
-          if module.info.classSymbol.is(Scala2x) then
+          if module.info.classSymbol.is(Scala2x) || module.companionClass.isGenericProduct then
             val mirrorType = mirrorCore(defn.Mirror_SingletonProxyClass, mirroredType, mirroredType, module.name, formal)
             val mirrorRef = New(defn.Mirror_SingletonProxyClass.typeRef, modulePath :: Nil)
             mirrorRef.cast(mirrorType)

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)
+}