Regression: Explicit Nulls & Flexible Types

[marcinzh]

Compiler version

3.7.4 - warning
3.7.2 - ok
3.3.7 - ok

Minimized code

//> using scala "3.7.4"
//> using option "-Yexplicit-nulls"
import scala.annotation.nowarn
import java.time.Instant

sealed trait Eff[+A]:
  def run: A = this match
    case x: Eff.Impure[A] => x()

object Eff:
  sealed abstract class Impure[+A]() extends Eff[A] with Function0[A]

  @nowarn("msg=anonymous class definition")
  inline def impure[A](inline a: => A): Eff[A] =
    new Impure[A]:
      override def apply(): A = a

  def instant: Eff[Instant] = impure(Instant.now().nn)

@main def main =
  Eff.impure:
    println("hell world")
  .run

Output

[warn] method apply exposes a flexible type in its inferred result type (java.time.Instant)?. Consider annotating the type explicitly
[warn]   def instant: Eff[Instant] = impure(Instant.now().nn)
[warn] 

Expectation

Compiles without warning.

---

We can make the warning go away, by applying one of the following modifications:

1. Remove inlines
2. Remove covariance from Eff[+A] and Impure[+A]
3. Add explicit type parameter to the last impure call:

   def instant: Eff[Instant] = impure[Instant](Instant.now().nn)

[noti0na1]

Can yau also test 3 nightly? We improved the flexible type warning ar #24471

[marcinzh]

// using scala "3.8.0-RC1-bin-20250822-658c8bd-NIGHTLY"
It compiles without warning.

[Gedochao]

@noti0na1 @WojciechMazur seems like we might want to backport #24471 to 3.8.0-RC4 (labelled it already)

[Gedochao]

Otherwise, closing this, since it's fixed on main/latest nightly.