Fix implicit resolution conflict when MongoPolyDataCompanion is used with custom implicits

commons-mongo/jvm/src/main/scala-2.13/com/avsystem/commons/mongo/typed/MongoPolyDataCompanion.scala

@@ -9,18 +9,26 @@ import com.avsystem.commons.serialization.{GenCodec, GenObjectCodec}
 import scala.annotation.compileTimeOnly
 
 trait MongoPolyAdtInstances[D[_]] {
-  // needed by MongoAdtFormat.materialize for generic type
-  protected final implicit def codecFromFormat[T: MongoFormat]: GenCodec[T] = MongoFormat[T].codec
-
   def codec[T: GenCodec]: GenObjectCodec[D[T]]
-  def format[T: MongoFormat]: MongoAdtFormat[D[T]]
+
+  /**
+    * We need to accept an implicit `GenCodec[T]` because materialization of
+    * [[MongoAdtFormat]] requires a [[GenObjectCodec]] ([[MongoAdtFormat.codec]]). In practice, it can be derived
+    * from the `MongoFormat[T]` that is already accepted by this method but we have to be careful about priority of
+    * implicits. Because of that, this implicit is actually provided by [[AbstractMongoPolyDataCompanion.format]].
+    */
+  def format[T: MongoFormat : GenCodec]: MongoAdtFormat[D[T]]
 }
 
 abstract class AbstractMongoPolyDataCompanion[Implicits, D[_]](implicits: Implicits)(
-  implicit instances: MacroInstances[Implicits, MongoPolyAdtInstances[D]]
+  implicit instances: MacroInstances[Implicits, MongoPolyAdtInstances[D]],
 ) {
   implicit def codec[T: GenCodec]: GenObjectCodec[D[T]] = instances(implicits, this).codec[T]
-  implicit def format[T: MongoFormat]: MongoAdtFormat[D[T]] = instances(implicits, this).format[T]
+
+  implicit def format[T: MongoFormat]: MongoAdtFormat[D[T]] = {
+    implicit def tCodec: GenCodec[T] = MongoFormat[T].codec
+    instances(implicits, this).format[T]
+  }
 
   implicit def isMongoAdtOrSubtype[C <: D[_]]: IsMongoAdtOrSubtype[C] = null
 
@@ -49,5 +57,5 @@ abstract class AbstractMongoPolyDataCompanion[Implicits, D[_]](implicits: Implic
   * }}}
   */
 abstract class MongoPolyDataCompanion[D[_]](
-  implicit instances: MacroInstances[BsonGenCodecs.type, MongoPolyAdtInstances[D]]
+  implicit instances: MacroInstances[BsonGenCodecs.type, MongoPolyAdtInstances[D]],
 ) extends AbstractMongoPolyDataCompanion[BsonGenCodecs.type, D](BsonGenCodecs)

commons-mongo/jvm/src/test/scala-2.13/com/avsystem/commons/mongo/typed/PolyDataWithCustomImplicits.scala

@@ -0,0 +1,23 @@
+package com.avsystem.commons
+package mongo.typed
+
+import com.avsystem.commons.meta.MacroInstances
+import com.avsystem.commons.serialization.GenCodec
+
+case class CustomWrappy(value: String)
+
+object CustomImplicits {
+  implicit val customWrappyCodec: GenCodec[CustomWrappy] =
+    GenCodec.transformed[CustomWrappy, String](_.value, CustomWrappy)
+}
+
+abstract class CustomPolyDataCompanion[D[_]](
+  implicit instances: MacroInstances[CustomImplicits.type, MongoPolyAdtInstances[D]],
+) extends AbstractMongoPolyDataCompanion[CustomImplicits.type, D](CustomImplicits)
+
+/**
+  * This class tests (through its compilation) if implicit resolution conflicts that were
+  * previously present in [[MongoPolyAdtInstances]] are fixed.
+  */
+case class PolyDataWithCustomImplicits[+T](wrappy: CustomWrappy, value: List[T])
+object PolyDataWithCustomImplicits extends CustomPolyDataCompanion[PolyDataWithCustomImplicits]