Type inference bug in Scala 3.1.2

[alexandru]

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 db5956b 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.

[smarter]

@bishabosha assigned smarter 3 days ago

I won't be able to look at new issues for the next few months, so I recommend not assigning me, but feel free to re-assign me if a bisection points to one of my commits and no one else can figure it out.

[griggt]

Bisected to db5956b

[odersky]

This seems to be intentional. In the interest of not allowing compile-times to blow up, we do not perform implicit searches for uninstantiated top-level type variables.