Filter out more false positives in priority change warnings

Author: odersky

Diff for: compiler/src/dotty/tools/dotc/typer/Implicits.scala

@@ -1318,16 +1318,14 @@ trait Implicits:
        *                  return new result with preferGeneral = true
        *  3.6 and higher: compare with preferGeneral = true
        *
+       *  @param disambiguate      The call is used to disambiguate two successes, not for ranking.
+       *                           When ranking, we are always filtering out either > 0 or <= 0 results.
+       *                           In each case a priority change from 0 to -1 or vice versa makes no difference.
        *  @param only2ndCritical   If true only the second alternative is critical in case
        *                           of a priority change.
        */
-      def compareAlternatives(alt1: RefAndLevel, alt2: RefAndLevel, only2ndCritical: Boolean = false): Int =
+      def compareAlternatives(alt1: RefAndLevel, alt2: RefAndLevel, disambiguate: Boolean = false, only2ndCritical: Boolean = false): Int =
         def comp(using Context) = explore(compare(alt1.ref, alt2.ref, preferGeneral = true))
-        def warn(msg: Message) =
-          val critical =
-            if only2ndCritical then alt2.ref :: Nil
-            else alt1.ref :: alt2.ref :: Nil
-          priorityChangeWarnings += ((critical, msg))
         if alt1.ref eq alt2.ref then 0
         else if alt1.level != alt2.level then alt1.level - alt2.level
         else
@@ -1336,6 +1334,12 @@ trait Implicits:
           if sv.stable == SourceVersion.`3.5` || sv == SourceVersion.`3.6-migration` then
             val prev = comp(using searchContext().addMode(Mode.OldImplicitResolution))
             if cmp != prev then
+              def warn(msg: Message) =
+                if disambiguate || cmp > 0 || prev > 0 then
+                  val critical =
+                    if only2ndCritical then alt2.ref :: Nil
+                    else alt1.ref :: alt2.ref :: Nil
+                  priorityChangeWarnings += ((critical, msg))
               def choice(c: Int) = c match
                 case -1 => "the second alternative"
                 case  1 => "the first alternative"
@@ -1362,7 +1366,7 @@ trait Implicits:
        */
       def disambiguate(alt1: SearchResult, alt2: SearchSuccess) = alt1 match
         case alt1: SearchSuccess =>
-          var diff = compareAlternatives(alt1, alt2)
+          var diff = compareAlternatives(alt1, alt2, disambiguate = true)
           assert(diff <= 0)   // diff > 0 candidates should already have been eliminated in `rank`
           if diff == 0 && alt1.ref =:= alt2.ref then
             diff = 1 // See i12951 for a test where this happens
@@ -1463,7 +1467,7 @@ trait Implicits:
                           // `retained`. Therefore, it's a priorty change only if the
                           // second alternative appears in the final search result. Otherwise
                           // we have the following scenario:
-                          //  - 1st alternative, bit not snd appears in final result
+                          //  - 1st alternative, but not snd appears in final result
                           //  - Hence, snd was eliminated either here, or otherwise by a direct
                           //    comparison later.
                           //  - Hence, no change in resolution.