Type inference bug in Scala 3.1.2

I’m unsure if I’m reporting this issue in the right place.

Compiler version

3.1.2 — the issue does not happen in 3.0.x or in 3.1.1.

First bad commit db5956b36a7e62b77f4b3a9335ab1587e0e47e2c in #13886

Minimized code

Noticed it while upgrading to 3.1.2 in monix/newtypes:

trait Eq[A] {
  def eqv(a1: A, a2: A): Boolean
}

given stringEq: Eq[String] with {
  def eqv(a1: String, a2: String) = a1 == a2
}

abstract class Newtype[Src] {
  opaque type Type = Src

  protected final def derive[F[_]](using ev: F[Src]): F[Type] = ev
}

object Sample extends Newtype[String] {
  given eq: Eq[Type] = derive
}

Output

[error] -- Error: .../Sample.scala:16:29
[error] 16 |  given eq: Eq[Type] = derive
[error]    |                             ^
[error]    |no implicit argument of type F[String] was found for parameter ev of method derive in class Newtype
[error]    |
[error]    |where:    F is a type variable with constraint <: Eq
[error] one error found

Expectation

The code should compile just fine, as it does with 3.1.1, 3.0.2, or 2.13.x.

[alexandru]

👋 from my perspective as a user, if you say there are technical challenges that make this impractical, I believe you. It was surprising to me, as it used to work, and now it doesn’t.

If this helps compile times, then that may be a useful trade-off.

But maybe you can improve the error message a little? I’m beginning to be spoiled by Scala 3’s cool error message. IDK 🤷‍♂️

[odersky]

It looks like the implementation only looks for type variables in MethodTypes right now, but it seems like it could look in PolyTypes too.

Not sure I understand. Type variables bound in polytypes are usually meant to be instantiated by the implicit search, no?