Incorrect inexhaustivity warning when leafs are defined in an unrelated sealed trait

Compiler version

3.1.1

Minimized code

Here is a Scastie.

The exhaustivity check fails when instances of Schema are defined in a trait.

sealed trait Schema[A]

object Schema extends RecordInstances

sealed trait RecordInstances:
  case class Field[A]() extends Schema[A]
  case object Thing extends Schema[Int]

import Schema._

// Match not exhaustive error! (with fatal warnings :P)
def handle[A](schema: Schema[A]) =
  schema match
    case Field() => println("field")
    case Thing   => println("thing")

Expectation

While it is a bit odd to define these instances in a trait, I would still expect this to correctly check exhaustivity. Scala 2 works as expected here.

[smarter]

It causes another similar compiler error here (scastie.scala-lang.org/2ME3I1HIQuyUAe67T2jziw)

That looks like an unrelated bug (missing substitution of RecordInstances.this into Schema), could you open a separate issue for it?

[kitlangton]

Also Scala 2 warns. It would fail on the following inputs: Field(), Thing, _

Oh yeah… Don’t know why that didn’t trigger fatal warnings for me in Scala 2. The solution for me was to replace these traits with // RECORD INSTANCES comment headers instead 😛. I think someone just wanted the organizational niceties of traits to organize a very large sealed trait file.