No implicit argument of type F[T] was found (works in Scala 2)

[fdietze]

Compiler version

3.1.3-RC2

Minimized code

trait Observer[-T]
trait PublishSubject[A] extends Observer[A]

trait Source[H[_]] 
implicit object source extends Source[PublishSubject]


final def foo[F[_]: Source, T2 <: String](source: F[T2]) = ???
final def foo[F[_]: Source, T2 <: Option[String]](source: F[T2], dummy: Unit = ())  = ???


val x:PublishSubject[Option[String]] = ???
foo(x)

https://scastie.scala-lang.org/7HyQgDhzSgmRUO0mJPgPwA

Output

no implicit argument of type Playground.Source[Playground.Observer] was found for an implicit parameter of method foo in object Playground

Expectation

Should compile like in Scala 2.13

[bishabosha]

it compiles if you make PublishSubject contravariant:

trait Observer[-T]
trait PublishSubject[-A] extends Observer[A]

trait Source[H[_]] 
implicit object source extends Source[PublishSubject]


def foo[F[_]: Source, T2 <: String](source: F[T2]) = ()
def foo[F[_]: Source, T2 <: Option[String]](source: F[T2], dummy: Unit = ())  = ()


val x:PublishSubject[Option[String]] = new PublishSubject[Option[String]] {}
foo(x)

[fdietze]

it compiles if you make PublishSubject contravariant:

Yes, I noticed that too. It was quite difficult to find this reproducing piece of code.

[nicolasstucki]

It also fails with foo(x)(source) but not with foo(x, ())(source). There might be an issue with overload resolution related to implicit search.

[smarter]

Minimized from https://contributors.scala-lang.org/t/better-type-inference-for-scala-send-us-your-problematic-cases/2410/173, here's another case where overloading and implicit search seem to break in Scala 3 compared to Scala 2:

class BodyReadable[R]

trait CustomResponse {
  def body: String
  def body[T: BodyReadable]: T
}

class MustExpectable[T] {
  def must: Any = ???
}
trait MustExpectations {
  import scala.language.implicitConversions
  implicit def theValue[T](t: T): MustExpectable[T] = ???
}

object Test extends MustExpectations {

  val r: CustomResponse = ???
  r.body.must // error

  val rbody = r.body
  rbody.must // OK
}

This affects play where it is worked around using a lot of imports: playframework/playframework@8d48699