Draft: Overloading result-based pruning shouldn't prefer inapplicable…

… alt

If an overloaded alternative in `alts` does not end up being part of the
`candidate` list in `resolveOverloaded1`, then it is not applicable to the
current arguments and should not be considered by `adaptByResult`. This avoids
discarding working solutions in favor of invalid ones when the working solution
involves a match type in the method result type that will fail `resultConforms`.

Fixes #21410.

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

@@ -2129,11 +2129,13 @@ trait Applications extends Compatibility {
             return resolveMapped(alts, alt => stripImplicit(alt.widen), pt)
         case _ =>
 
-      var found = withoutMode(Mode.ImplicitsEnabled)(resolveOverloaded1(alts, pt))
+      var candidatesAndFound = withoutMode(Mode.ImplicitsEnabled)(resolveOverloaded1(alts, pt))
+      def candidates = candidatesAndFound._1
+      def found = candidatesAndFound._2
       if found.isEmpty && ctx.mode.is(Mode.ImplicitsEnabled) then
-        found = resolveOverloaded1(alts, pt)
+        candidatesAndFound = resolveOverloaded1(alts, pt)
       found match
-        case alt :: Nil => adaptByResult(alt, alts) :: Nil
+        case alt :: Nil => adaptByResult(alt, candidates) :: Nil
         case _ => found
     end resolve
 
@@ -2178,7 +2180,7 @@ trait Applications extends Compatibility {
    *  It might be called twice from the public `resolveOverloaded` method, once with
    *  implicits and SAM conversions enabled, and once without.
    */
-  private def resolveOverloaded1(alts: List[TermRef], pt: Type)(using Context): List[TermRef] =
+  private def resolveOverloaded1(alts: List[TermRef], pt: Type)(using Context): (List[TermRef], List[TermRef]) =
     trace(i"resolve over $alts%, %, pt = $pt", typr, show = true) {
     record(s"resolveOverloaded1", alts.length)
 
@@ -2306,7 +2308,9 @@ trait Applications extends Compatibility {
           else
             record("resolveOverloaded.narrowedByShape", alts2.length)
             pretypeArgs(alts2, pt)
-            narrowByTrees(alts2, pt.typedArgs(normArg(alts2, _, _)), resultType)
+            val alts3 = narrowByTrees(alts2, pt.typedArgs(normArg(alts2, _, _)), resultType)
+            overload.println(i"narrowed by trees: ${alts3.map(_.symbol.showDcl)}%, %")
+            alts3
 
       case pt @ PolyProto(targs1, pt1) =>
         val alts1 = alts.filterConserve(pt.canInstantiate)
@@ -2372,42 +2376,42 @@ trait Applications extends Compatibility {
     if pt.unusableForInference then
       // `pt` might have become erroneous by typing arguments of FunProtos.
       // If `pt` is erroneous, don't try to go further; report the error in `pt` instead.
-      candidates
+      (candidates, candidates)
     else
       val found = narrowMostSpecific(candidates)
-      if found.length <= 1 then found
+      if found.length <= 1 then (candidates, found)
       else
         val deepPt = pt.deepenProto
         deepPt match
           case pt @ FunProto(_, PolyProto(targs, resType)) =>
             // try to narrow further with snd argument list and following type params
-            resolveMapped(candidates,
-              skipParamClause(pt.typedArgs().tpes, targs.tpes), resType)
+            (candidates, resolveMapped(candidates,
+              skipParamClause(pt.typedArgs().tpes, targs.tpes), resType))
           case pt @ FunProto(_, resType: FunOrPolyProto) =>
             // try to narrow further with snd argument list
-            resolveMapped(candidates,
-              skipParamClause(pt.typedArgs().tpes, Nil), resType)
+            (candidates, resolveMapped(candidates,
+              skipParamClause(pt.typedArgs().tpes, Nil), resType))
           case _ =>
             // prefer alternatives that need no eta expansion
             val noCurried = alts.filterConserve(!resultIsMethod(_))
             val noCurriedCount = noCurried.length
             if noCurriedCount == 1 then
-              noCurried
+              (candidates, noCurried)
             else if noCurriedCount > 1 && noCurriedCount < alts.length then
               resolveOverloaded1(noCurried, pt)
             else
               // prefer alternatves that match without default parameters
               val noDefaults = alts.filterConserve(!_.symbol.hasDefaultParams)
               val noDefaultsCount = noDefaults.length
               if noDefaultsCount == 1 then
-                noDefaults
+                (candidates, noDefaults)
               else if noDefaultsCount > 1 && noDefaultsCount < alts.length then
                 resolveOverloaded1(noDefaults, pt)
               else if deepPt ne pt then
                 // try again with a deeper known expected type
                 resolveOverloaded1(alts, deepPt)
               else
-                candidates
+                (candidates, candidates)
     }
   end resolveOverloaded1
 

tests/pos/i21410.scala

@@ -0,0 +1,12 @@
+class A
+object Test:
+  type F[X] <: Any = X match
+    case A => Int
+
+  def foo[T](x: String): T = ???
+  def foo[U](x: U): F[U] = ???
+
+  val x1 = foo(A())
+  val y: Int = x1
+
+  val x2: Int = foo(A()) // error