Fix error message when deriving an obstract type

Fixes #13808

Author: odersky

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

@@ -83,7 +83,10 @@ trait Deriving {
      */
     private def processDerivedInstance(derived: untpd.Tree): Unit = {
       val originalTypeClassType = typedAheadType(derived, AnyTypeConstructorProto).tpe
-      val typeClassType = checkClassType(underlyingClassRef(originalTypeClassType), derived.srcPos, traitReq = false, stablePrefixReq = true)
+      val underlyingClassType = underlyingClassRef(originalTypeClassType)
+      val typeClassType = checkClassType(
+          underlyingClassType.orElse(originalTypeClassType),
+          derived.srcPos, traitReq = false, stablePrefixReq = true)
       val typeClass = typeClassType.classSymbol
       val typeClassParams = typeClass.typeParams
       val typeClassArity = typeClassParams.length

tests/neg/i13808.check

@@ -0,0 +1,8 @@
+-- Error: tests/neg/i13808.scala:13:37 ---------------------------------------------------------------------------------
+13 |case class Boom[A](value: A) derives OpaqueType, Foo // error // error
+   |                                     ^^^^^^^^^^
+   |                                     OpaqueTypes.OpaqueType is not a class type
+-- Error: tests/neg/i13808.scala:13:49 ---------------------------------------------------------------------------------
+13 |case class Boom[A](value: A) derives OpaqueType, Foo // error // error
+   |                                                 ^^^
+   |                                                 FooModule.Foo is not a class type

tests/neg/i13808.scala

@@ -0,0 +1,14 @@
+object OpaqueTypes:
+  opaque type OpaqueType[A] = List[A]
+  object OpaqueType:
+    def derived[A]: OpaqueType[A] = Nil
+
+object FooModule:
+  type Foo[A]
+  object Foo:
+    def derived[A]: Foo[A] = Nil.asInstanceOf[Foo[A]]
+
+import FooModule.Foo
+import OpaqueTypes.OpaqueType
+case class Boom[A](value: A) derives OpaqueType, Foo // error // error
+