Another fix to the ranking logic

Author: odersky

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

@@ -1492,8 +1492,8 @@ trait Implicits:
                     // need a candidate better than the two ambiguous alternatives.
                     val ambi = fail.reason.asInstanceOf[AmbiguousImplicits]
                     healAmbiguous(fail, newCand =>
-                      compareAlternatives(newCand, ambi.alt1) > 0 &&
-                      compareAlternatives(newCand, ambi.alt2) > 0)
+                      compareAlternatives(newCand, ambi.alt1, disambiguate = true) > 0 &&
+                      compareAlternatives(newCand, ambi.alt2, disambiguate = true) > 0)
                 }
             }
           case nil =>

tests/warn/i15264.check

@@ -0,0 +1,20 @@
+-- Warning: tests/warn/i15264.scala:48:26 ------------------------------------------------------------------------------
+48 |    val a = summon[A[Int]] // warn
+   |                          ^
+   |                          Given search preference for repro.A[Int] between alternatives
+   |                            (repro.exports.given_A_V : [V](using x$1: priority.Prio2): repro.A[V])
+   |                          and
+   |                            (repro.exports.given_C_V : [V](using x$1: priority.Prio0): repro.C[V])
+   |                          has changed.
+   |                          Previous choice          : the second alternative
+   |                          New choice from Scala 3.7: the first alternative
+-- Warning: tests/warn/i15264.scala:55:29 ------------------------------------------------------------------------------
+55 |    val a = summon[A[Q[Int]]] // warn
+   |                             ^
+   |                Given search preference for repro.A[repro.Q[Int]] between alternatives
+   |                  (repro.qcontext.gcq : [V](using p0: priority.Prio0)(using c: repro.C[V]): repro.C[repro.Q[V]])
+   |                and
+   |                  (repro.qcontext.gaq : [V](using p2: priority.Prio2)(using a: repro.A[V]): repro.A[repro.Q[V]])
+   |                has changed.
+   |                Previous choice          : the first alternative
+   |                New choice from Scala 3.7: the second alternative