sum mirror is available when mirror for child can't be summoned

The fix for https://github.com/lampepfl/dotty/pull/14986 does not consider when a sum type has a child for which the mirror can not be summoned.

Compiler version

3.2.0-RC1-bin-20220517-e5abec0-NIGHTLY

First bad commit d6e17a23e65e8efa0dea608eb5dbfc490a977bd1 in #15008

Minimized code

import scala.deriving.Mirror

package lib {
  sealed trait Foo
  object Foo // Mirror.Of[Foo] cached in companion
  case class Bar private[lib] () extends Foo
  case object Bar // force mirror for Bar to be anonymous.
}

package app {
  val mFoo = summon[Mirror.SumOf[lib.Foo]]
  val mBar = summon[Mirror.ProductOf[lib.Bar]] // error
}

Output

-- Error: test.scala:11:49 -------------------------------------------------------------
11 |  val mBar = summon[Mirror.ProductOf[lib.Bar]] // error
   |                                                 ^
   |No given instance of type deriving.Mirror.ProductOf[lib.Bar] was found for parameter x of method summon in object Predef. Failed to synthesize an instance of type deriving.Mirror.ProductOf[lib.Bar]: class Bar is not a generic product because the constructor of class Bar is innaccessible from the calling scope.

Expectation

summon[Mirror.SumOf[lib.Foo]] should also fail because in the calling scope, the child of Foo, i.e. Bar can not have a mirror - this affects signatures of Foo, because object Foo should not cache the mirror.

[bishabosha]

mirrors are meant to represent an algebraic data type, and that all parts mentioned by the mirror should be reachable. The mirror for Foo will mention Bar in its MirroredElemTypes. the user should then be able to iterate through MirroredElemTypes (with inline method or macro) to summon the mirror for Bar programatically. This operation is meant to be total

[joroKr21]

Hmm I didn’t know that - what is the reason for this limitation?