Description
Compiler version
3.1.3 and 3.2.0-RC1
Minimized code
runnable: https://scastie.scala-lang.org/mIVAeZ5HSMCazZQlFl9qHQ
class _Monoid[A]
object _Monoid {
implicit val Monoid: _Monoid[Int] = new _Monoid[Int]
}
class Lifecycle[A]
object Lifecycle {
implicit def monoidForLifecycle[Monoid[_], A](
implicit
monoidType: GetMonoidType[Monoid],
monoidA: Monoid[A]
): Monoid[Lifecycle[A]] = new _Monoid().asInstanceOf[Monoid[Lifecycle[A]]]
}
sealed class GetMonoidType[C[_]]
object GetMonoidType {
implicit val getMonoid: GetMonoidType[_Monoid] = new GetMonoidType[_Monoid]
}
object App extends App {
println(implicitly[_Monoid[Lifecycle[Int]]])
}Output
No given instance of type _Monoid[Lifecycle[Int]] was found for parameter e of method implicitly in object Predef.
I found:
Lifecycle.monoidForLifecycle[Monoid, Int](GetMonoidType.getMonoid,
/* missing */summon[Monoid[Int]]
)
But no implicit values were found that match type Monoid[Int].Note that the type Monoid should be inferred to _Monoid as a result of picking up GetMonoidType.getMonoid, however it's not, and the search is performed on an uninstantiated type variable Monoid, instead of on _Monoid[Int] and cannot succeed.
Expectation
Expected to work, as in Scala 2: https://scastie.scala-lang.org/AXiXnf60R8K2CRaOMm7g8A
This is similar to #13986 in that it involves an HKT type variable not being instantiated correctly during implicit search.
Unlike that issue, there's a workaround for this one, via multiple using clauses:
implicit def monoidForLifecycle[Monoid[_], A](
using monoidType: GetMonoidType[Monoid]
)(using monoidA: Monoid[A]
): Monoid[Lifecycle[A]] = new _Monoid().asInstanceOf[Monoid[Lifecycle[A]]]Note that due to #13986, setting a bound on the type variable Monoid[x] <: _Monoid[x] does not workaround the error.
This code is a minimized version of real library code in https://github.com/izumi/izumi which we're trying to port to Scala 3