Avoid useless warnings about priority change in implicit search

Warn about priority change in implicit search only if one of the participating
candidates appears in the final result.

It could be that we have an priority change between two ranked candidates that
both are superseded by the result of the implicit search. In this case, no
warning needs to be reported.

compiler/src/dotty/tools/dotc/typer/Implicits.scala

@@ -419,6 +419,12 @@ object Implicits:
   sealed abstract class SearchResult extends Showable {
     def tree: Tree
     def toText(printer: Printer): Text = printer.toText(this)
+
+    /** The references that were found, there can be two of them in the case
+     *  of an AmbiguousImplicits failure
+     */
+    def found: List[TermRef]
+
     def recoverWith(other: SearchFailure => SearchResult): SearchResult = this match {
       case _: SearchSuccess => this
       case fail: SearchFailure => other(fail)
@@ -434,13 +440,17 @@ object Implicits:
    *  @param tstate The typer state to be committed if this alternative is chosen
    */
   case class SearchSuccess(tree: Tree, ref: TermRef, level: Int, isExtension: Boolean = false)(val tstate: TyperState, val gstate: GadtConstraint)
-  extends SearchResult with RefAndLevel with Showable
+  extends SearchResult with RefAndLevel with Showable:
+    final def found = ref :: Nil
 
   /** A failed search */
   case class SearchFailure(tree: Tree) extends SearchResult {
     require(tree.tpe.isInstanceOf[SearchFailureType], s"unexpected type for ${tree}")
     final def isAmbiguous: Boolean = tree.tpe.isInstanceOf[AmbiguousImplicits | TooUnspecific]
     final def reason: SearchFailureType = tree.tpe.asInstanceOf[SearchFailureType]
+    final def found = tree.tpe match
+      case tpe: AmbiguousImplicits => tpe.alt1.ref :: tpe.alt2.ref :: Nil
+      case _ => Nil
   }
 
   object SearchFailure {
@@ -1290,6 +1300,11 @@ trait Implicits:
     /** Search a list of eligible implicit references */
     private def searchImplicit(eligible: List[Candidate], contextual: Boolean): SearchResult =
 
+      // A map that associates a priority change warning (between -source 3.4 and 3.6)
+      // with a candidate ref mentioned in the warning. We report the associated
+      // message if the candidate ref is part of the result of the implicit search
+      var priorityChangeWarnings = mutable.ListBuffer[(TermRef, Message)]()
+
       /** Compare `alt1` with `alt2` to determine which one should be chosen.
        *
        *  @return  a number > 0   if `alt1` is preferred over `alt2`
@@ -1306,6 +1321,8 @@ trait Implicits:
        */
       def compareAlternatives(alt1: RefAndLevel, alt2: RefAndLevel): Int =
         def comp(using Context) = explore(compare(alt1.ref, alt2.ref, preferGeneral = true))
+        def warn(msg: Message) =
+          priorityChangeWarnings += (alt1.ref -> msg) += (alt2.ref -> msg)
         if alt1.ref eq alt2.ref then 0
         else if alt1.level != alt2.level then alt1.level - alt2.level
         else
@@ -1319,16 +1336,16 @@ trait Implicits:
                 case  1 => "the first alternative"
                 case _  => "none - it's ambiguous"
               if sv.stable == SourceVersion.`3.5` then
-                report.warning(
+                warn(
                   em"""Given search preference for $pt between alternatives ${alt1.ref} and ${alt2.ref} will change
                       |Current choice           : ${choice(prev)}
-                      |New choice from Scala 3.6: ${choice(cmp)}""", srcPos)
+                      |New choice from Scala 3.6: ${choice(cmp)}""")
                 prev
               else
-                report.warning(
+                warn(
                   em"""Change in given search preference for $pt between alternatives ${alt1.ref} and ${alt2.ref}
                       |Previous choice          : ${choice(prev)}
-                      |New choice from Scala 3.6: ${choice(cmp)}""", srcPos)
+                      |New choice from Scala 3.6: ${choice(cmp)}""")
                 cmp
             else cmp
           else cmp
@@ -1578,7 +1595,10 @@ trait Implicits:
             validateOrdering(ord)
             throw ex
 
-      rank(sort(eligible), NoMatchingImplicitsFailure, Nil)
+      val result = rank(sort(eligible), NoMatchingImplicitsFailure, Nil)
+      for (ref, msg) <- priorityChangeWarnings do
+        if result.found.contains(ref) then report.warning(msg, srcPos)
+      result
     end searchImplicit
 
     def isUnderSpecifiedArgument(tp: Type): Boolean =