[Circe] Unclear implicit prioritisation rules

[vpavkin]

Reproduction:
https://github.com/vpavkin/magnolia/blob/e278256e713ddf3b1aa3365898e930eff168274b/tests/src/main/scala/PriorityIssueTest.scala

Magnolia derivers sometimes override default instances, available from companion. For example, for this type

case class MapContainer(theMap: Map[String, List[Int]])

magnolia will derive coproduct codec for the List on rhs of the Map.

This only happens, when default instances are provided by means of being located in companion. If we import them explicitly (with import io.circe.Encoder._), then magnolia starts to see them.

This is confusing, and hard to notice and debug within deeply nested hierarchies.

[joroKr21]

In the test you import import MagnoliaEncoder.genEncoder which gives it higher priority than implicits in the companion object Encoder according to the regular implicit resolution rules in Scala. Even if we try to change it (which I'm not sure is possible), the net result will probably still be confusing because then people will be surprised why it doesn't follow the standard rules.

[vpavkin]

@joroKr21 makes sense, but when using circe auto we do kinda the same:

import io.circe.generic.auto._

And it doesn't stay in the way of default codecs.

Although, I have to admit, circe does some tricks with Exported, from end-user perspective it looks the same.

[joroKr21]

Sure, but there is also generic.semiauto which is arguably more useful. I guess the problem here is that you would want to support both use cases.

[vpavkin]

support for both would be nice, yes, but I actually also mostly use semiauto.
Will explore how magnolia behaves for such purpose, thanks for suggestion!

[vpavkin]

Closing in favor of #107