Prefer extensions over conversions and implicits for member selection

Before the changes, if
`isAsGoodValueType` was called with an extension and a given conversion,
it would prefer the conversion over the extension,
because only the former yielded true in `isGiven`.

Which contradicted the logic from searchImplicit which
preferred extension over conversions for member selection.

Author: EugeneFlesselle

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

@@ -1816,10 +1816,8 @@ trait Applications extends Compatibility {
             isAsGood(alt1, tp1.instantiate(tparams.map(_.typeRef)), alt2, tp2)
           }
         case _ => // (3)
-          def isGiven(alt: TermRef) =
-            alt1.symbol.is(Given) && alt.symbol != defn.NotGivenClass
           def compareValues(tp1: Type, tp2: Type)(using Context) =
-            isAsGoodValueType(tp1, tp2, isGiven(alt1), isGiven(alt2))
+            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)
@@ -1856,15 +1854,15 @@ 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 a given and the other is an implicit, the given wins.
+     *     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, alt1isGiven: Boolean, alt2isGiven: Boolean)(using Context): Boolean =
+    def isAsGoodValueType(tp1: Type, tp2: Type, alt1IsImplicit: Boolean, alt2IsImplicit: Boolean)(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)
@@ -1882,7 +1880,7 @@ trait Applications extends Compatibility {
 
         if Feature.sourceVersion.isAtMost(SourceVersion.`3.4`)
             || oldResolution
-            || !alt1isGiven && !alt2isGiven
+            || alt1IsImplicit && alt2IsImplicit
         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,
@@ -1897,8 +1895,8 @@ trait Applications extends Compatibility {
               case _ => mapOver(t)
           (flip(tp1p) relaxed_<:< flip(tp2p)) || viewExists(tp1, tp2)
         else
-          // New rules: better means generalize, givens always beat implicits
-          if alt1isGiven != alt2isGiven then alt1isGiven
+          // New rules: better means generalize, givens (and extensions) always beat implicits
+          if alt1IsImplicit != alt2IsImplicit then alt2IsImplicit
           else (tp2p relaxed_<:< tp1p) || viewExists(tp2, tp1)
     end isAsGoodValueType
 

tests/pos/i19715.scala

@@ -6,7 +6,8 @@ class NT(t: Tup):
 object NT:
   extension (x: NT)
     def app(n: Int): Boolean = true
-  given Conversion[NT, Tup] = _.toTup
+  given c1: Conversion[NT, Tup] = _.toTup
+  implicit def c2(t: NT): Tup = c1(t)
 
 def test =
   val nt = new NT(Tup())