Allow derivation for classes with no companions

Two problems needed to be fixed

 - deciding that the derived type is the companion, not
   the synthetic companion
 - Avoiding forcing `Shape` types in checkRealizable since
   these can compute `children` too early.

Author: odersky

compiler/src/dotty/tools/dotc/core/CheckRealizable.scala

@@ -105,7 +105,11 @@ class CheckRealizable(implicit ctx: Context) {
   /** `Realizable` if `tp` has good bounds, a `HasProblem...` instance
    *  pointing to a bad bounds member otherwise. "Has good bounds" means:
    *
-   *    - all type members have good bounds (except for opaque helpers)
+   *    - all non-synthetic type members have good bounds.
+   *      Synthetic members are unchecked, for two reasons:
+   *       - synthetic opaque aliases do have conflicting bounds, but this is OK
+   *       - we should not force synthesized Shape types because that might
+   *         query the `children` annotation too early.
    *    - all refinements of the underlying type have good bounds (except for opaque companions)
    *    - all base types are class types, and if their arguments are wildcards
    *      they have good bounds.
@@ -123,7 +127,8 @@ class CheckRealizable(implicit ctx: Context) {
     val memberProblems =
       for {
         mbr <- tp.nonClassTypeMembers
-        if !(mbr.info.loBound <:< mbr.info.hiBound) && !mbr.symbol.isOpaqueHelper
+        if !mbr.symbol.is(Synthetic)
+        if !(mbr.info.loBound <:< mbr.info.hiBound)
       }
       yield new HasProblemBounds(mbr.name, mbr.info)
 

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

@@ -988,7 +988,8 @@ class Namer { typer: Typer =>
 
       if (impl.derived.nonEmpty) {
         val derivingClass =
-          if (original.removeAttachment(DerivingCompanion).isDefined) cls.companionClass.asClass
+          if (original.removeAttachment(DerivingCompanion).isDefined ||
+              original.mods.is(Synthetic)) cls.companionClass.asClass
           else cls
         val deriver = new Deriver(derivingClass, impl.pos.startPos)(localCtx)
         deriver.enterDerived(impl.derived)

tests/run/typeclass-derivation3.scala

@@ -3,25 +3,24 @@ import scala.annotation.tailrec
 
 object datatypes {
   import typeclasses._
+
   // An algebraic datatype
   enum Lst[+T] derives Eq, Pickler, Show {
     case Cons(hd: T, tl: Lst[T])
     case Nil
   }
-
   object Lst {}
 
+
   // A simple product type
   case class Pair[T](x: T, y: T) derives Eq, Pickler, Show
-  object Pair
 
   // another ADT
   sealed trait Either[+L, +R] extends Product derives Eq, Pickler, Show
   case class Left[L](x: L) extends Either[L, Nothing]
   case class Right[R](x: R) extends Either[Nothing, R]
 
-  object Either {
-  }
+  //object Either {}
 }
 
 object typeclasses {