[Experiment] Drop prioritization of givens over implicits

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

@@ -1830,15 +1830,13 @@ trait Applications extends Compatibility {
             isAsGood(alt1, tp1.instantiate(tparams.map(_.typeRef)), alt2, tp2)
           }
         case _ => // (3)
-          def compareValues(tp1: Type, tp2: Type)(using Context) =
-            isAsGoodValueType(tp1, tp2, alt1.symbol.is(Implicit), alt2.symbol.is(Implicit))
           tp2 match
             case tp2: MethodType => true // (3a)
             case tp2: PolyType if tp2.resultType.isInstanceOf[MethodType] => true // (3a)
             case tp2: PolyType => // (3b)
-              explore(compareValues(tp1, instantiateWithTypeVars(tp2)))
+              explore(isAsGoodValueType(tp1, instantiateWithTypeVars(tp2)))
             case _ => // 3b)
-              compareValues(tp1, tp2)
+              isAsGoodValueType(tp1, tp2)
     }
 
     /** Test whether value type `tp1` is as good as value type `tp2`.
@@ -1868,15 +1866,14 @@ trait Applications extends Compatibility {
      *     for overloading resolution (when `preferGeneral is false), and the opposite relation
      *     `U <: T` or `U convertible to `T` for implicit disambiguation between givens
      *     (when `preferGeneral` is true). For old-style implicit values, the 3.4 behavior is kept.
-     *     If one of the alternatives is an implicit and the other is a given (or an extension), the implicit loses.
      *
      *   - In Scala 3.5 and Scala 3.6-migration, we issue a warning if the result under
      *     Scala 3.6 differ wrt to the old behavior up to 3.5.
      *
      *  Also and only for given resolution: If a compared type refers to a given or its module class, use
      *  the intersection of its parent classes instead.
      */
-    def isAsGoodValueType(tp1: Type, tp2: Type, alt1IsImplicit: Boolean, alt2IsImplicit: Boolean)(using Context): Boolean =
+    def isAsGoodValueType(tp1: Type, tp2: Type)(using Context): Boolean =
       val oldResolution = ctx.mode.is(Mode.OldImplicitResolution)
       if !preferGeneral || Feature.migrateTo3 && oldResolution then
         // Normal specificity test for overloading resolution (where `preferGeneral` is false)
@@ -1892,10 +1889,7 @@ trait Applications extends Compatibility {
         val tp1p = prepare(tp1)
         val tp2p = prepare(tp2)
 
-        if Feature.sourceVersion.isAtMost(SourceVersion.`3.4`)
-            || oldResolution
-            || alt1IsImplicit && alt2IsImplicit
-        then
+        if Feature.sourceVersion.isAtMost(SourceVersion.`3.4`) || oldResolution then
           // Intermediate rules: better means specialize, but map all type arguments downwards
           // These are enabled for 3.0-3.5, and for all comparisons between old-style implicits,
           // and in 3.5 and 3.6-migration when we compare with previous rules.
@@ -1909,9 +1903,8 @@ trait Applications extends Compatibility {
               case _ => mapOver(t)
           (flip(tp1p) relaxed_<:< flip(tp2p)) || viewExists(tp1, tp2)
         else
-          // New rules: better means generalize, givens (and extensions) always beat implicits
-          if alt1IsImplicit != alt2IsImplicit then alt2IsImplicit
-          else (tp2p relaxed_<:< tp1p) || viewExists(tp2, tp1)
+          // New rules: better means generalize
+          (tp2p relaxed_<:< tp1p) || viewExists(tp2, tp1)
     end isAsGoodValueType
 
     /** Widen the result type of synthetic given methods from the implementation class to the

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

@@ -1330,10 +1330,13 @@ trait Implicits:
         if alt1.ref eq alt2.ref then 0
         else if alt1.level != alt2.level then alt1.level - alt2.level
         else
-          var cmp = comp(using searchContext())
+          lazy val prev = comp(using searchContext().addMode(Mode.OldImplicitResolution))
+          val cmp = comp(using searchContext()) match
+            // if we get an ambiguity with new rules for a pair of old-style implicits, fall back to old rules
+            case 0 if alt1.ref.symbol.is(Implicit) && alt2.ref.symbol.is(Implicit) => prev
+            case cmp => cmp
           val sv = Feature.sourceVersion
           if isWarnPriorityChangeVersion(sv) then
-            val prev = comp(using searchContext().addMode(Mode.OldImplicitResolution))
             if disambiguate && cmp != prev then
               def warn(msg: Message) =
                 val critical = alt1.ref :: alt2.ref :: Nil

tests/neg/i2974.scala

@@ -0,0 +1,28 @@
+
+trait Foo[-T]
+trait Bar[-T] extends Foo[T]
+
+object Test {
+
+  locally:
+    implicit val fa: Foo[Int] = ???
+    implicit val ba: Bar[Int] = ???
+    summon[Foo[Int]] // ok
+
+  locally:
+    implicit val fa: Foo[Int] = ???
+    implicit val ba: Bar[Any] = ???
+    summon[Foo[Int]] // ok
+
+  locally:
+    given fa: Foo[Any] = ???
+    given ba: Bar[Int] = ???
+    summon[Foo[Int]] // error: now ambiguous,
+    // was resolving to `ba` when using intermediate rules:
+    // better means specialize, but map all type arguments downwards
+
+  locally:
+    implicit val fa: Foo[Any] = ???
+    implicit val ba: Bar[Int] = ???
+    summon[Foo[Int]] // is OK since we fall back to old rules for old-style implicits as a tie breaker
+}

tests/run/enrich-gentraversable.check

@@ -4,5 +4,5 @@ List(2, 4)
 HW
 ArraySeq(72, 108, 108, 32, 114, 108, 100)
 Map(bar -> 2)
-TreeMap(bar -> 2)
+Map(bar -> 2)
 Map(bar -> 2)

tests/run/enrich-gentraversable.scala

@@ -47,7 +47,8 @@ object Test extends App {
 
     val tm = TreeMap(1 -> "foo", 2 -> "bar")
     val tmm = tm.filterMap { case (k, v) => if (k % 2 == 0) Some(v -> k) else None }
-    typed[TreeMap[String, Int]](tmm)
+    typed[Map[String, Int]](tmm) // was TreeMap, now Map,
+    // since `BuildFrom` is covariant in `That` and `TreeMap[String, Int] <:< Map[String, Int]`
     println(tmm)
 
     val mv = m.view

tests/run/implicits_poly.check

@@ -1 +1 @@
-barFoo
+fooFoo

tests/warn/i15503a.scala

@@ -149,8 +149,8 @@ object GivenImportOrderAtoB:
   object A { implicit val x: X = new X }
   object B { implicit val y: Y = new Y }
   class C {
-    import A._ // warn
-    import B._ // OK
+    import A._ // OK
+    import B._ // warn
     def t = implicitly[X]
   }
 
@@ -160,8 +160,8 @@ object GivenImportOrderBtoA:
   object A { implicit val x: X = new X }
   object B { implicit val y: Y = new Y }
   class C {
-    import B._ // OK
-    import A._ // warn
+    import B._ // warn
+    import A._ // OK
     def t = implicitly[X]
   }
 /* END : tests on given import order */