A left-biased variant for implicit/given pairs

We now use a left-biased scheme, as follows.

From 3.6 on:

 - A given x: X is better than a given or implicit y: Y if y can be instantiated/widened to X.
 - An implicit x: X is better than a given or implicit y: Y if y can be instantiated to a supertype of X.
 - Use owner score for givens as a tie breaker if after all other tests we still have an ambiguity.

This is not transitive, so we need a separate scheme to work around that.

Other change:

 - Drop special handling of NotGiven in prioritization. The previous logic pretended to do so,
   but was ineffective.

Author: odersky

community-build/community-projects/PPrint

@@ -2,8 +2,6 @@ package dotty.tools
 package dotc
 package printing
 
-import scala.language.unsafeNulls
-
 import scala.collection.mutable
 
 import core.*
@@ -52,7 +50,11 @@ object Formatting {
     object ShowAny extends Show[Any]:
       def show(x: Any): Shown = x
 
-    class ShowImplicits3:
+    class ShowImplicits4:
+      given [X: Show]: Show[X | Null] with
+        def show(x: X | Null) = if x == null then "null" else CtxShow(toStr(x.nn))
+
+    class ShowImplicits3 extends ShowImplicits4:
       given Show[Product] = ShowAny
 
     class ShowImplicits2 extends ShowImplicits3:
@@ -77,15 +79,10 @@ object Formatting {
       given [K: Show, V: Show]: Show[Map[K, V]] with
         def show(x: Map[K, V]) =
           CtxShow(x.map((k, v) => s"${toStr(k)} => ${toStr(v)}"))
-      end given
 
       given [H: Show, T <: Tuple: Show]: Show[H *: T] with
         def show(x: H *: T) =
           CtxShow(toStr(x.head) *: toShown(x.tail).asInstanceOf[Tuple])
-      end given
-
-      given [X: Show]: Show[X | Null] with
-        def show(x: X | Null) = if x == null then "null" else CtxShow(toStr(x.nn))
 
       given Show[FlagSet] with
         def show(x: FlagSet) = x.flagsString
@@ -148,8 +145,8 @@ object Formatting {
     private def treatArg(arg: Shown, suffix: String)(using Context): (String, String) = arg.runCtxShow match {
       case arg: Seq[?] if suffix.indexOf('%') == 0 && suffix.indexOf('%', 1) != -1 =>
         val end = suffix.indexOf('%', 1)
-        val sep = StringContext.processEscapes(suffix.substring(1, end))
-        (arg.mkString(sep), suffix.substring(end + 1))
+        val sep = StringContext.processEscapes(suffix.substring(1, end).nn)
+        (arg.mkString(sep), suffix.substring(end + 1).nn)
       case arg: Seq[?] =>
         (arg.map(showArg).mkString("[", ", ", "]"), suffix)
       case arg =>

compiler/src/dotty/tools/dotc/printing/Formatting.scala

@@ -2988,7 +2988,7 @@ class MissingImplicitArgument(
 
     /** Default error message for non-nested ambiguous implicits. */
     def defaultAmbiguousImplicitMsg(ambi: AmbiguousImplicits) =
-      s"Ambiguous given instances: ${ambi.explanation}${location("of")}"
+      s"Ambiguous given instances: ${ambi.explanation}${location("of")}${ambi.priorityChangeWarningNote}"
 
     /** Default error messages for non-ambiguous implicits, or nested ambiguous
      *  implicits.

compiler/src/dotty/tools/dotc/reporting/messages.scala

@@ -1762,6 +1762,17 @@ trait Applications extends Compatibility {
       else if sym2.is(Module) then compareOwner(sym1, cls2)
       else 0
 
+  enum CompareScheme:
+    case Old          // Normal specificity test for overloading resolution (where `preferGeneral` is false)
+                      // and in mode Scala3-migration when we compare with the old Scala 2 rules.
+
+    case Intermediate // Intermediate rules: better means specialize, but map all type arguments downwards
+                      // These are enabled for 3.0-3.4, or if OldImplicitResolution
+                      // is specified, and also for all comparisons between old-style implicits,
+
+    case New          // New rules: better means generalize, givens (and extensions) always beat implicits
+  end CompareScheme
+
   /** Compare two alternatives of an overloaded call or an implicit search.
    *
    *  @param  alt1, alt2      Non-overloaded references indicating the two choices
@@ -1788,6 +1799,15 @@ trait Applications extends Compatibility {
    */
   def compare(alt1: TermRef, alt2: TermRef, preferGeneral: Boolean = false)(using Context): Int = trace(i"compare($alt1, $alt2)", overload) {
     record("resolveOverloaded.compare")
+    val scheme =
+      val oldResolution = ctx.mode.is(Mode.OldImplicitResolution)
+      if !preferGeneral || Feature.migrateTo3 && oldResolution then
+        CompareScheme.Old
+      else if Feature.sourceVersion.isAtMost(SourceVersion.`3.4`)
+        || oldResolution
+        || alt1.symbol.is(Implicit) && alt2.symbol.is(Implicit)
+      then CompareScheme.Intermediate
+      else CompareScheme.New
 
     /** Is alternative `alt1` with type `tp1` as good as alternative
      *  `alt2` with type `tp2` ?
@@ -1830,15 +1850,15 @@ 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))
+          def compareValues(tp2: Type)(using Context) =
+            isAsGoodValueType(tp1, tp2, alt1.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(compareValues(instantiateWithTypeVars(tp2)))
             case _ => // 3b)
-              compareValues(tp1, tp2)
+              compareValues(tp2)
     }
 
     /** Test whether value type `tp1` is as good as value type `tp2`.
@@ -1876,9 +1896,8 @@ trait Applications extends Compatibility {
      *  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 =
-      val oldResolution = ctx.mode.is(Mode.OldImplicitResolution)
-      if !preferGeneral || Feature.migrateTo3 && oldResolution then
+    def isAsGoodValueType(tp1: Type, tp2: Type, alt1IsImplicit: Boolean)(using Context): Boolean =
+      if scheme == CompareScheme.Old then
         // Normal specificity test for overloading resolution (where `preferGeneral` is false)
         // and in mode Scala3-migration when we compare with the old Scala 2 rules.
         isCompatible(tp1, tp2)
@@ -1892,13 +1911,7 @@ trait Applications extends Compatibility {
         val tp1p = prepare(tp1)
         val tp2p = prepare(tp2)
 
-        if Feature.sourceVersion.isAtMost(SourceVersion.`3.4`)
-            || oldResolution
-            || 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,
-          // and in 3.5 and 3.6-migration when we compare with previous rules.
+        if scheme == CompareScheme.Intermediate || alt1IsImplicit then
           val flip = new TypeMap:
             def apply(t: Type) = t match
               case t @ AppliedType(tycon, args) =>
@@ -1909,9 +1922,7 @@ 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)
+          (tp2p relaxed_<:< tp1p) || viewExists(tp2, tp1)
     end isAsGoodValueType
 
     /** Widen the result type of synthetic given methods from the implementation class to the
@@ -1982,13 +1993,19 @@ trait Applications extends Compatibility {
         // alternatives are the same after following ExprTypes, pick one of them
         // (prefer the one that is not a method, but that's arbitrary).
         if alt1.widenExpr =:= alt2 then -1 else 1
-      else ownerScore match
-        case  1 => if winsType1 || !winsType2 then  1 else 0
-        case -1 => if winsType2 || !winsType1 then -1 else 0
-        case  0 =>
-          if winsType1 != winsType2 then if winsType1 then 1 else -1
-          else if alt1.symbol == alt2.symbol then comparePrefixes
-          else 0
+      else
+        // For new implicit resolution, take ownerscore as more significant than type resolution
+        // Reason: People use owner hierarchies to explicitly prioritize, we should not
+        // break that by changing implicit priority of types.
+        def drawOrOwner =
+          if scheme == CompareScheme.New then ownerScore else 0
+        ownerScore match
+          case  1 => if winsType1 || !winsType2 then  1 else drawOrOwner
+          case -1 => if winsType2 || !winsType1 then -1 else drawOrOwner
+          case  0 =>
+            if winsType1 != winsType2 then if winsType1 then 1 else -1
+            else if alt1.symbol == alt2.symbol then comparePrefixes
+            else 0
     end compareWithTypes
 
     if alt1.symbol.is(ConstructorProxy) && !alt2.symbol.is(ConstructorProxy) then -1

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

@@ -549,6 +549,11 @@ object Implicits:
   /** An ambiguous implicits failure */
   class AmbiguousImplicits(val alt1: SearchSuccess, val alt2: SearchSuccess, val expectedType: Type, val argument: Tree, val nested: Boolean = false) extends SearchFailureType:
 
+    private[Implicits] var priorityChangeWarnings: List[Message] = Nil
+
+    def priorityChangeWarningNote(using Context): String =
+      priorityChangeWarnings.map(msg => s"\n\nNote: $msg").mkString
+
     def msg(using Context): Message =
       var str1 = err.refStr(alt1.ref)
       var str2 = err.refStr(alt2.ref)
@@ -1330,7 +1335,7 @@ 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())
+          val cmp = comp(using searchContext())
           val sv = Feature.sourceVersion
           if isWarnPriorityChangeVersion(sv) then
             val prev = comp(using searchContext().addMode(Mode.OldImplicitResolution))
@@ -1345,13 +1350,21 @@ trait Implicits:
                 case _  => "none - it's ambiguous"
               if sv.stable == SourceVersion.`3.5` then
                 warn(
-                  em"""Given search preference for $pt between alternatives ${alt1.ref} and ${alt2.ref} will change
+                  em"""Given search preference for $pt between alternatives
+                      |  ${alt1.ref}
+                      |and
+                      |  ${alt2.ref}
+                      |will change.
                       |Current choice           : ${choice(prev)}
                       |New choice from Scala 3.6: ${choice(cmp)}""")
                 prev
               else
                 warn(
-                  em"""Change in given search preference for $pt between alternatives ${alt1.ref} and ${alt2.ref}
+                  em"""Given search preference for $pt between alternatives
+                      |  ${alt1.ref}
+                      |and
+                      |  ${alt2.ref}
+                      |has changed.
                       |Previous choice          : ${choice(prev)}
                       |New choice from Scala 3.6: ${choice(cmp)}""")
                 cmp
@@ -1610,9 +1623,23 @@ trait Implicits:
             throw ex
 
       val result = rank(sort(eligible), NoMatchingImplicitsFailure, Nil)
-      for (critical, msg) <- priorityChangeWarnings do
-        if result.found.exists(critical.contains(_)) then
-          report.warning(msg, srcPos)
+
+      // Issue all priority change warnings that can affect the result
+      val shownWarnings = priorityChangeWarnings.toList.collect:
+        case (critical, msg) if result.found.exists(critical.contains(_)) =>
+          msg
+      result match
+        case result: SearchFailure =>
+          result.reason match
+            case ambi: AmbiguousImplicits =>
+              // Make warnings part of error message because otherwise they are suppressed when
+              // the error is emitted.
+              ambi.priorityChangeWarnings = shownWarnings
+            case _ =>
+        case _ =>
+      for msg <- shownWarnings do
+        report.warning(msg, srcPos)
+
       result
     end searchImplicit
 

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

@@ -23,6 +23,7 @@ class StringFormatterTest extends AbstractStringFormatterTest:
   @Test def flagsTup   = check("(<static>,final)", i"${(JavaStatic, Final)}")
   @Test def seqOfTup2  = check("(final,given), (private,lazy)", i"${Seq((Final, Given), (Private, Lazy))}%, %")
   @Test def seqOfTup3  = check("(Foo,given, (right is approximated))", i"${Seq((Foo, Given, TypeComparer.ApproxState.None.addHigh))}%, %")
+  @Test def tupleNull   = check("(1,null)", i"${(1, null: String | Null)}")
 
   class StorePrinter extends Printer:
     var string: String = "<never set>"

compiler/test/dotty/tools/dotc/StringFormatterTest.scala

@@ -2,3 +2,11 @@
 15 |@main def Test = f  // error
    |                  ^
    |Ambiguous given instances: both given instance given_B and given instance given_C match type A of parameter a of method f
+   |
+   |Note: Given search preference for A between alternatives
+   |  (given_A : A)
+   |and
+   |  (given_B : B)
+   |will change.
+   |Current choice           : the second alternative
+   |New choice from Scala 3.6: the first alternative

tests/neg/given-triangle.check

@@ -0,0 +1,4 @@
+-- [E172] Type Error: tests/neg/i21212.scala:8:52 ----------------------------------------------------------------------
+8 |  def test2(using a2: A)(implicit b2: B) = summon[A] // error: ambiguous
+  |                                                    ^
+  |Ambiguous given instances: both parameter b2 and parameter a2 match type Minimization.A of parameter x of method summon in object Predef

tests/neg/i21212.check

@@ -0,0 +1,11 @@
+
+object Minimization:
+
+  trait A
+  trait B extends A
+
+  def test1(using a1: A)(using    b1: B) = summon[A] // picks (most general) a1
+  def test2(using a2: A)(implicit b2: B) = summon[A] // error: ambiguous
+  def test3(implicit       a3: A, b3: B) = summon[A] // picks (most specific) b3
+
+end Minimization

tests/neg/i21212.scala

@@ -0,0 +1 @@
+public enum JavaEnum { ABC, DEF, GHI }