Make overloading resolution changes apply in 3.7 and report warnings in 3.6

This is similar to the warnings for changes in given preference.
In this case, however, the changes only affect a part of disambiguation used
relatively late in the process, as a "last resort" disambiguation mechanism.
We can therefore accept running resolution independently with both schemes
in these cases to detect and report changes.

Having a source position for the warning messages requires passing the tree
source position to resolveOverloaded from the Typer.
It could previously be avoided this since any potential error in overloading
could be determined from its result. Clearly, this cannot be done for the new
warnings, although I am open to an alternative design.

Author: EugeneFlesselle

compiler/src/dotty/tools/dotc/ast/Trees.scala

@@ -1908,7 +1908,7 @@ object Trees {
               case MethodTpe(_, _, x: MethodType) => !x.isImplicitMethod
               case _ => true
             }}
-          val alternatives = ctx.typer.resolveOverloaded(allAlts, proto)
+          val alternatives = ctx.typer.resolveOverloaded(allAlts, proto, receiver.srcPos)
           assert(alternatives.size == 1,
             i"${if (alternatives.isEmpty) "no" else "multiple"} overloads available for " +
               i"$method on ${receiver.tpe.widenDealiasKeepAnnots} with targs: $targs%, %; args: $args%, %; expectedType: $expectedType." +

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

@@ -2116,7 +2116,7 @@ trait Applications extends Compatibility {
    *  Two trials: First, without implicits or SAM conversions enabled. Then,
    *  if the first finds no eligible candidates, with implicits and SAM conversions enabled.
    */
-  def resolveOverloaded(alts: List[TermRef], pt: Type)(using Context): List[TermRef] =
+  def resolveOverloaded(alts: List[TermRef], pt: Type, srcPos: SrcPos)(using Context): List[TermRef] =
     record("resolveOverloaded")
 
     /** Is `alt` a method or polytype whose result type after the first value parameter
@@ -2154,7 +2154,7 @@ trait Applications extends Compatibility {
           case Nil => chosen
           case alt2 :: Nil => alt2
           case alts2 =>
-            resolveOverloaded(alts2, pt) match {
+            resolveOverloaded(alts2, pt, srcPos) match {
               case alt2 :: Nil => alt2
               case _ => chosen
             }
@@ -2169,12 +2169,12 @@ trait Applications extends Compatibility {
             val alts0 = alts.filterConserve(_.widen.stripPoly.isImplicitMethod)
             if alts0 ne alts then return resolve(alts0)
           else if alts.exists(_.widen.stripPoly.isContextualMethod) then
-            return resolveMapped(alts, alt => stripImplicit(alt.widen), pt)
+            return resolveMapped(alts, alt => stripImplicit(alt.widen), pt, srcPos)
         case _ =>
 
-      var found = withoutMode(Mode.ImplicitsEnabled)(resolveOverloaded1(alts, pt))
+      var found = withoutMode(Mode.ImplicitsEnabled)(resolveOverloaded1(alts, pt, srcPos))
       if found.isEmpty && ctx.mode.is(Mode.ImplicitsEnabled) then
-        found = resolveOverloaded1(alts, pt)
+        found = resolveOverloaded1(alts, pt, srcPos)
       found match
         case alt :: Nil => adaptByResult(alt, alts) :: Nil
         case _ => found
@@ -2221,10 +2221,44 @@ 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, srcPos: SrcPos)(using Context): List[TermRef] =
     trace(i"resolve over $alts%, %, pt = $pt", typr, show = true) {
     record(s"resolveOverloaded1", alts.length)
 
+    val sv = Feature.sourceVersion
+    val isOldPriorityVersion: Boolean = sv.isAtMost(SourceVersion.`3.6`)
+    val isWarnPriorityChangeVersion = sv == SourceVersion.`3.6` || sv == SourceVersion.`3.7-migration`
+
+    inline def warnOnPriorityChange(oldCands: List[TermRef], newCands: List[TermRef])(f: List[TermRef] => List[TermRef]): List[TermRef] =
+
+      def doWarn(oldChoice: String, newChoice: String): Unit =
+        val (change, whichChoice) =
+          if isOldPriorityVersion
+          then ("will change", "Current choice ")
+          else ("has changed", "Previous choice")
+
+        val msg = // uses oldCands as the list of alternatives since they should be a superset of newCands
+          em"""Overloading resolution for ${err.expectedTypeStr(pt)} between alternatives
+              | ${oldCands map (_.info)}%\n %
+              |$change.
+              |$whichChoice          : $oldChoice
+              |New choice from Scala 3.7: $newChoice"""
+
+        report.warning(msg, srcPos)
+      end doWarn
+
+      lazy val oldRes = f(oldCands)
+      val newRes = f(newCands)
+
+      if isWarnPriorityChangeVersion then (oldRes, newRes) match
+        case (oldAlt :: Nil, newAlt :: Nil) if oldAlt != newAlt => doWarn(oldAlt.info.show, newAlt.info.show)
+        case (oldAlt :: Nil, Nil) => doWarn(oldAlt.info.show, "none")
+        case (Nil, newAlt :: Nil) => doWarn("none", newAlt.info.show)
+        case _ => // neither scheme has determined an alternative
+
+      if isOldPriorityVersion then oldRes else newRes
+    end warnOnPriorityChange
+
     def isDetermined(alts: List[TermRef]) = alts.isEmpty || alts.tail.isEmpty
 
     /** The shape of given tree as a type; cannot handle named arguments. */
@@ -2372,7 +2406,7 @@ trait Applications extends Compatibility {
               TypeOps.boundsViolations(targs1, tp.paramInfos, _.substParams(tp, _), NoType).isEmpty
           val alts2 = alts1.filter(withinBounds)
           if isDetermined(alts2) then alts2
-          else resolveMapped(alts1, _.widen.appliedTo(targs1.tpes), pt1)
+          else resolveMapped(alts1, _.widen.appliedTo(targs1.tpes), pt1, srcPos)
 
       case pt =>
         val compat = alts.filterConserve(normalizedCompatible(_, pt, keepConstraint = false))
@@ -2430,37 +2464,37 @@ trait Applications extends Compatibility {
       candidates
     else
       val found = narrowMostSpecific(candidates)
-      if found.length <= 1 then found
+      if isDetermined(found) then 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(found,
-              skipParamClause(pt.typedArgs().tpes, targs.tpes), resType)
+            warnOnPriorityChange(candidates, found):
+              resolveMapped(_, skipParamClause(pt.typedArgs().tpes, targs.tpes), resType, srcPos)
           case pt @ FunProto(_, resType: FunOrPolyProto) =>
             // try to narrow further with snd argument list
-            resolveMapped(found,
-              skipParamClause(pt.typedArgs().tpes, Nil), resType)
+            warnOnPriorityChange(candidates, found):
+              resolveMapped(_, skipParamClause(pt.typedArgs().tpes, Nil), resType, srcPos)
           case _ =>
             // prefer alternatives that need no eta expansion
             val noCurried = alts.filterConserve(!resultIsMethod(_))
             val noCurriedCount = noCurried.length
             if noCurriedCount == 1 then
               noCurried
             else if noCurriedCount > 1 && noCurriedCount < alts.length then
-              resolveOverloaded1(noCurried, pt)
+              resolveOverloaded1(noCurried, pt, srcPos)
             else
               // prefer alternatves that match without default parameters
               val noDefaults = alts.filterConserve(!_.symbol.hasDefaultParams)
               val noDefaultsCount = noDefaults.length
               if noDefaultsCount == 1 then
                 noDefaults
               else if noDefaultsCount > 1 && noDefaultsCount < alts.length then
-                resolveOverloaded1(noDefaults, pt)
+                resolveOverloaded1(noDefaults, pt, srcPos)
               else if deepPt ne pt then
                 // try again with a deeper known expected type
-                resolveOverloaded1(alts, deepPt)
+                resolveOverloaded1(alts, deepPt, srcPos)
               else
                 candidates
     }
@@ -2494,7 +2528,7 @@ trait Applications extends Compatibility {
    *  type is mapped with `f`, alternatives with non-existing types or symbols are dropped, and the
    *  expected type is `pt`. Map the results back to the original alternatives.
    */
-  def resolveMapped(alts: List[TermRef], f: TermRef => Type, pt: Type)(using Context): List[TermRef] =
+  def resolveMapped(alts: List[TermRef], f: TermRef => Type, pt: Type, srcPos: SrcPos)(using Context): List[TermRef] =
     val reverseMapping = alts.flatMap { alt =>
       val t = f(alt)
       if t.exists && alt.symbol.exists then
@@ -2517,7 +2551,7 @@ trait Applications extends Compatibility {
     }
     val mapped = reverseMapping.map(_._1)
     overload.println(i"resolve mapped: ${mapped.map(_.widen)}%, % with $pt")
-    resolveOverloaded(mapped, pt)(using ctx.retractMode(Mode.SynthesizeExtMethodReceiver))
+    resolveOverloaded(mapped, pt, srcPos)(using ctx.retractMode(Mode.SynthesizeExtMethodReceiver))
       .map(reverseMapping.toMap)
 
   /** Try to typecheck any arguments in `pt` that are function values missing a

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

@@ -4108,7 +4108,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
       def altRef(alt: SingleDenotation) = TermRef(ref.prefix, ref.name, alt)
       val alts = altDenots.map(altRef)
 
-      resolveOverloaded(alts, pt) match
+      resolveOverloaded(alts, pt, tree.srcPos) match
         case alt :: Nil =>
           readaptSimplified(tree.withType(alt))
         case Nil =>