Prefer extensions over conversions for member selection

Fixes #19715

[Cherry-picked c79bd93]

Author: odersky

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

@@ -1275,35 +1275,36 @@ trait Implicits:
         case alt1: SearchSuccess =>
           var diff = compareAlternatives(alt1, alt2)
           assert(diff <= 0)   // diff > 0 candidates should already have been eliminated in `rank`
-          if diff == 0 && alt1.isExtension && alt2.isExtension then
-            // Fall back: if both results are extension method applications,
-            // compare the extension methods instead of their wrappers.
-            def stripExtension(alt: SearchSuccess) = methPart(stripApply(alt.tree)).tpe
-            (stripExtension(alt1), stripExtension(alt2)) match
-              case (ref1: TermRef, ref2: TermRef) =>
-                // ref1 and ref2 might refer to type variables owned by
-                // alt1.tstate and alt2.tstate respectively, to compare the
-                // alternatives correctly we need a TyperState that includes
-                // constraints from both sides, see
-                // tests/*/extension-specificity2.scala for test cases.
-                val constraintsIn1 = alt1.tstate.constraint ne ctx.typerState.constraint
-                val constraintsIn2 = alt2.tstate.constraint ne ctx.typerState.constraint
-                def exploreState(alt: SearchSuccess): TyperState =
-                  alt.tstate.fresh(committable = false)
-                val comparisonState =
-                  if constraintsIn1 && constraintsIn2 then
-                    exploreState(alt1).mergeConstraintWith(alt2.tstate)
-                  else if constraintsIn1 then
-                    exploreState(alt1)
-                  else if constraintsIn2 then
-                    exploreState(alt2)
-                  else
-                    ctx.typerState
-
-                diff = inContext(ctx.withTyperState(comparisonState)) {
-                  compare(ref1, ref2)
-                }
-              case _ =>
+          if diff == 0 && alt2.isExtension then
+            if alt1.isExtension then
+              // Fall back: if both results are extension method applications,
+              // compare the extension methods instead of their wrappers.
+              def stripExtension(alt: SearchSuccess) = methPart(stripApply(alt.tree)).tpe
+              (stripExtension(alt1), stripExtension(alt2)) match
+                case (ref1: TermRef, ref2: TermRef) =>
+                  // ref1 and ref2 might refer to type variables owned by
+                  // alt1.tstate and alt2.tstate respectively, to compare the
+                  // alternatives correctly we need a TyperState that includes
+                  // constraints from both sides, see
+                  // tests/*/extension-specificity2.scala for test cases.
+                  val constraintsIn1 = alt1.tstate.constraint ne ctx.typerState.constraint
+                  val constraintsIn2 = alt2.tstate.constraint ne ctx.typerState.constraint
+                  def exploreState(alt: SearchSuccess): TyperState =
+                    alt.tstate.fresh(committable = false)
+                  val comparisonState =
+                    if constraintsIn1 && constraintsIn2 then
+                      exploreState(alt1).mergeConstraintWith(alt2.tstate)
+                    else if constraintsIn1 then
+                      exploreState(alt1)
+                    else if constraintsIn2 then
+                      exploreState(alt2)
+                    else
+                      ctx.typerState
+
+                  diff = inContext(ctx.withTyperState(comparisonState)):
+                    compare(ref1, ref2)
+            else // alt1 is a conversion, prefer extension alt2 over it
+              diff = -1
           if diff < 0 then alt2
           else if diff > 0 then alt1
           else SearchFailure(new AmbiguousImplicits(alt1, alt2, pt, argument), span)

Diff for: tests/pos/i19715.scala

@@ -0,0 +1,15 @@
+class Tup():
+  def app(n: Int): String = "a"
+
+class NT(t: Tup):
+  def toTup = t
+object NT:
+  extension (x: NT)
+    def app(n: Int): Boolean = true
+  given Conversion[NT, Tup] = _.toTup
+
+def test =
+  val nt = new NT(Tup())
+  val x = nt.app(3)
+  val _: Boolean = x
+