Defining instances of a third-party typeclass, implicit not found but explicit works fine

Question

I\'m working with Slick\'s GetResult typeclass and wanted to use Shapeless to derive instances of GetResult[Option[(A, B, C...)]]

What I want:

Answer 1

The thing is that slick.jdbc.GetResult is covariant. If it were invariant, types would be inferred correctly and implicits would be resolved.

A workaround is hiding covariant slick.jdbc.GetResult with custom invariant type alias GetResult. Remove import slick.jdbc.GetResult and write in your source file

type GetResult[T] = slick.jdbc.GetResult[T]

object GetResult {
  def apply[T](implicit f: PositionedResult => T): GetResult[T] = slick.jdbc.GetResult.apply
}

Now implicitly[GetResult[Option[(Int, Int)]]] compiles. Tested in Scala 2.12.7 + Shapeless 2.3.3 + Slick 3.2.3.

Variance often makes troubles for implicit resolution:

https://github.com/scala/bug/issues/10099

https://github.com/locationtech/geotrellis/issues/1292

Implicit resolution with covariance