reproduction steps

Using either Scala 2.12.14 or 2.13.6, and two source files:

Classes.scala
===========
package testwarning

sealed trait Trait1
sealed trait Trait2

class Class1 extends Trait1
// No class extends Trait1 with Trait2

Tester.scala
=========
package testwarning

object Tester {
  def test(x: Trait1): String = x match {
    case y: Trait2 => "Also Trait2"
    case _ => "Only Trait1"
  }
}

I expect to get the warning:

[warn] /home/danarmak/test/scala-testwarning-repro/src/main/scala/testwarning/Tester.scala:6:13: fruitless type test: a value of type testwarning.Trait1 cannot also be a testwarning.Trait2
[warn]     case y: Trait2 => "Also Trait2"
[warn]             ^

Running sbt clean compile produces no warning. But if I then change Tester.scala (e.g. add some whitespace) and run sbt compile (incrementally), I get the warning. I believe the same happens with the warning "match may not be exhaustive".

Because the reproduction requires two source files, I also uploaded it here with the necessary sbt project.

problem

Incremental compilation and clean / full compilation should produce the same result. And if they don't, then a clean build should be the 'correct' one.

This is interfering with fatal-warnings on my CI, because the CI's clean build doesn't catch the actual warnings. This has also caused a @nowarn annotation does not suppress any warnings error on the CI.