Test extract from reactive-mongo

This will now be ambiguous.

Author: odersky

tests/warn/bson.check

@@ -0,0 +1,10 @@
+-- Warning: tests/warn/bson/Test.scala:5:60 ----------------------------------------------------------------------------
+5 |def typedMapHandler[K, V: BSONHandler] = stringMapHandler[V] // warn
+  |                                                            ^
+  |Given search preference for bson.BSONWriter[Map[String, V]] between alternatives (bson.BSONWriter.mapWriter : [V²](using x$1: bson.BSONWriter[V²]): bson.BSONDocumentWriter[Map[String, V²]]) and (bson.BSONWriter.collectionWriter :
+  |  [T, Repr <: Iterable[T]](using x$1: bson.BSONWriter[T], x$2: Repr ¬ Option[T]): bson.BSONWriter[Repr]) will change
+  |Current choice           : the first alternative
+  |New choice from Scala 3.6: none - it's ambiguous
+  |
+  |where:    V  is a type in method typedMapHandler
+  |          V² is a type variable

tests/warn/bson/Test.scala

@@ -0,0 +1,5 @@
+//> using options -source 3.5
+import bson.*
+
+def stringMapHandler[V](using writer: BSONWriter[Map[String, V]]): BSONHandler[Map[String, V]] = ???
+def typedMapHandler[K, V: BSONHandler] = stringMapHandler[V] // warn

tests/warn/bson/bson.scala

@@ -0,0 +1,29 @@
+package bson
+
+trait BSONWriter[T]
+trait BSONDocumentWriter[T] extends BSONWriter[T]
+object BSONWriter extends BSONWriterInstances
+
+trait BSONHandler[T] extends BSONWriter[T]
+
+private[bson] trait BSONWriterInstances {
+  given mapWriter[V](using BSONWriter[V]): BSONDocumentWriter[Map[String, V]] = bson.mapWriter[V]
+  export bson.collectionWriter
+}
+
+final class ¬[A, B]
+object ¬ {
+  implicit def defaultEvidence[A, B]: ¬[A, B] = new ¬[A, B]()
+  @annotation.implicitAmbiguous("Could not prove type ${A} is not (¬) ${A}")
+  implicit def ambiguousEvidence1[A]: ¬[A, A] = null
+  implicit def ambiguousEvidence2[A]: ¬[A, A] = null
+}
+
+private[bson] trait DefaultBSONHandlers extends LowPriorityHandlers
+private[bson] trait LowPriorityHandlers{
+  given collectionWriter[T, Repr <: Iterable[T]](using BSONWriter[T], Repr ¬ Option[T]): BSONWriter[Repr] = ???
+  private[bson] def mapWriter[V](implicit valueWriter: BSONWriter[V]): BSONDocumentWriter[Map[String, V]] = ???
+}
+
+// ---
+package object bson extends DefaultBSONHandlers