Regression in `mjakubowski84/parquet4s` for type inference

[WojciechMazur]

Based on OpenCB failure in mjakubowski84/parquet4s - logs

Compiler version

Last good release: 3.4.0-RC1-bin-20231113-0dfe593-NIGHTLY
First bad release: 3.4.0-RC1-bin-20231114-18ada51-NIGHTLY
Bisect points to 6c556aa

Minimized code

// //> using dep "co.fs2::fs2-core:3.10.2"
sealed abstract class Pull[+F[_], +O, +R]:
  def >>[F2[x] >: F[x], O2 >: O, S](post: => Pull[F2, O2, S]): Pull[F2, O2, S]
object Pull:
  def eval[F[_], R](fr: F[R]): Pull[F, Nothing, R] = ???
  val done: Pull[Nothing, Nothing, Unit] = ???

class Test[F[_], T]:
  def dispose(path: String): F[Unit] = ???
  private def rotatePull(partitions: Iterable[String]): Pull[F, T, Unit] =
    partitions
      .map(partition => Pull.eval(dispose(partition)))
      .reduceOption(_ >> _)
      .getOrElse(Pull.done)

Output

-- [E007] Type Mismatch Error: /Users/wmazur/projects/sandbox/src/main/scala/usage.scala:14:16 
11 |    partitions
12 |      .map(partition => Pull.eval(dispose(partition)))
13 |      .reduceOption(_ >> _)
14 |      .getOrElse(Pull.done)
   |    ^
   |    Found:    Pull[F, Any, Unit]
   |    Required: Pull[F, T, Unit]
   |
   |    where:    F is a type in class Test with bounds <: [_] =>> Any
   |

Expectation

Should compile (probably)

Workaround:

Usage type parameter explicitly

.reduceOption[Pull[F, T, Unit]](_ >> _)

[KacperFKorban]

A slightly smaller minimization:

//> using scala 3.nightly

class Box[+O]:
  def ++[O2 >: O](other: Box[O2]): Box[O2] = ???
object Box:
  val empty: Box[Nothing] = ???

def test[T]: Box[T] =
  List(Box.empty, Box.empty)
    // .reduceOption[Box[T]](_ ++ _) // works
    .reduceOption(_ ++ _) // fails
    .getOrElse(Box.empty)

Output:

-- [E007] Type Mismatch Error: ~/bugs/i21275.scala:13:14 ---------------
10 |  List(Box.empty, Box.empty)
11 |    // .reduceOption[Box[T]](_ ++ _) // works
12 |    .reduceOption(_ ++ _) // fails
13 |    .getOrElse(Box.empty)
   |  ^
   |  Found:    Box[Any]
   |  Required: Box[T]
   |
   | longer explanation available when compiling with `-explain`

[Gedochao]

cc @odersky @dwijnand