Another fix to the ranking logic

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

@@ -1481,8 +1481,8 @@ 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 =>
@@ -1621,7 +1621,7 @@ trait Implicits:
             throw ex
 
       val sorted = sort(eligible)
-      val result = sorted match
+      val res = sorted match
         case first :: rest =>
           val firstIsImplicit = first.ref.symbol.is(Implicit)
           if rest.exists(_.ref.symbol.is(Implicit) != firstIsImplicit) then
@@ -1638,11 +1638,11 @@ trait Implicits:
 
       // Issue all priority change warnings that can affect the result
       val shownWarnings = priorityChangeWarnings.toList.collect:
-        case (critical, msg) if result.found.exists(critical.contains(_)) =>
+        case (critical, msg) if res.found.exists(critical.contains(_)) =>
           msg
-      result match
-        case result: SearchFailure =>
-          result.reason match
+      res match
+        case res: SearchFailure =>
+          res.reason match
             case ambi: AmbiguousImplicits =>
               // Make warnings part of error message because otherwise they are suppressed when
               // the error is emitted.
@@ -1652,7 +1652,7 @@ trait Implicits:
       for msg <- shownWarnings do
         report.warning(msg, srcPos)
 
-      result
+      res
     end searchImplicit
 
     def isUnderSpecifiedArgument(tp: Type): Boolean =

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_C_V : [V](using x$1: priority.Prio0): repro.C[V])
+   |                          and
+   |                            (repro.exports.given_A_V : [V](using x$1: priority.Prio2): repro.A[V])
+   |                          has changed.
+   |                          Previous choice          : the first alternative
+   |                          New choice from Scala 3.7: the second 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

tests/warn/i15264.scala

@@ -0,0 +1,55 @@
+import language.`3.7-migration`
+object priority:
+    // lower number = higher priority
+    class Prio0 extends Prio1
+    object Prio0 { given Prio0() }
+
+    class Prio1 extends Prio2
+    object Prio1 { given Prio1() }
+
+    class Prio2
+    object Prio2 { given Prio2() }
+
+object repro:
+    // analogous to cats Eq, Hash, Order:
+    class A[V]
+    class B[V] extends A[V]
+    class C[V] extends A[V]
+
+    class Q[V]
+
+    object context:
+        // prios work here, which is cool
+        given[V](using priority.Prio0): C[V] = new C[V]
+        given[V](using priority.Prio1): B[V] = new B[V]
+        given[V](using priority.Prio2): A[V] = new A[V]
+
+    object exports:
+        // so will these exports
+        export context.given
+
+    // if you import these don't import from 'context' above
+    object qcontext:
+        // base defs, like what you would get from cats
+        given ga: A[Int] = new B[Int] // added so that we don't get an ambiguity in test2
+        given gb: B[Int] = new B[Int]
+        given gc: C[Int] = new C[Int]
+
+        // these seem like they should work but don't
+        given gcq[V](using p0: priority.Prio0)(using c: C[V]): C[Q[V]] = new C[Q[V]]
+        given gbq[V](using p1: priority.Prio1)(using b: B[V]): B[Q[V]] = new B[Q[V]]
+        given gaq[V](using p2: priority.Prio2)(using a: A[V]): A[Q[V]] = new A[Q[V]]
+
+object test1:
+    import repro.*
+    import repro.exports.given
+
+    // these will work
+    val a = summon[A[Int]] // warn
+
+
+object test2:
+    import repro.*
+    import repro.qcontext.given
+
+    val a = summon[A[Q[Int]]] // warn