Change implicit resolution rule wrt implicit parameters

compiler/src/dotty/tools/dotc/config/Printers.scala

@@ -17,7 +17,7 @@ object Printers {
   val checks: Printer = noPrinter
   val config: Printer = noPrinter
   val cyclicErrors: Printer = noPrinter
-  val debug = noPrinter             // no type annotion here to force inlining
+  val debug = noPrinter             // no type annotation here to force inlining
   val derive: Printer = noPrinter
   val dottydoc: Printer = noPrinter
   val exhaustivity: Printer = noPrinter

compiler/src/dotty/tools/dotc/config/ScalaSettings.scala

@@ -70,6 +70,7 @@ class ScalaSettings extends Settings.SettingGroup {
   val XprintTypes: Setting[Boolean] = BooleanSetting("-Xprint-types", "Print tree types (debugging option).")
   val XprintDiff: Setting[Boolean] = BooleanSetting("-Xprint-diff", "Print changed parts of the tree since last print.")
   val XprintDiffDel: Setting[Boolean] = BooleanSetting("-Xprint-diff-del", "Print changed parts of the tree since last print including deleted parts.")
+  val XprintInline: Setting[Boolean] = BooleanSetting("-Xprint-inline", "Show  where inlined code comes from")
   val Xprompt: Setting[Boolean] = BooleanSetting("-Xprompt", "Display a prompt after each error (debugging option).")
   val XmainClass: Setting[String] = StringSetting("-Xmain-class", "path", "Class for manifest's Main-Class entry (only useful with -d <jar>)", "")
   val XnoValueClasses: Setting[Boolean] = BooleanSetting("-Xno-value-classes", "Do not use value classes. Helps debugging.")
@@ -157,7 +158,6 @@ class ScalaSettings extends Settings.SettingGroup {
   val YexplainLowlevel: Setting[Boolean] = BooleanSetting("-Yexplain-lowlevel", "When explaining type errors, show types at a lower level.")
   val YnoDoubleBindings: Setting[Boolean] = BooleanSetting("-Yno-double-bindings", "Assert no namedtype is bound twice (should be enabled only if program is error-free).")
   val YshowVarBounds: Setting[Boolean] = BooleanSetting("-Yshow-var-bounds", "Print type variables with their bounds")
-  val YshowNoInline: Setting[Boolean] = BooleanSetting("-Yshow-no-inline", "Show inlined code without the 'inlined from' info")
 
   val YnoDecodeStacktraces: Setting[Boolean] = BooleanSetting("-Yno-decode-stacktraces", "Show raw StackOverflow stacktraces, instead of decoding them into triggering operations.")
 

compiler/src/dotty/tools/dotc/core/SymDenotations.scala

@@ -1558,7 +1558,7 @@ object SymDenotations {
         case p :: parents1 =>
           p.classSymbol match {
             case pcls: ClassSymbol => builder.addAll(pcls.baseClasses)
-            case _ => assert(isRefinementClass || ctx.mode.is(Mode.Interactive), s"$this has non-class parent: $p")
+            case _ => assert(isRefinementClass || p.isError || ctx.mode.is(Mode.Interactive), s"$this has non-class parent: $p")
           }
           traverse(parents1)
         case nil =>

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

@@ -417,7 +417,7 @@ class RefinedPrinter(_ctx: Context) extends PlainPrinter(_ctx) {
         "[" ~ toTextGlobal(elems, ",") ~ " : " ~ toText(elemtpt) ~ "]"
       case tree @ Inlined(call, bindings, body) =>
         (("/* inlined from " ~ (if (call.isEmpty) "outside" else toText(call)) ~ " */ ") `provided`
-          !homogenizedView && !ctx.settings.YshowNoInline.value) ~
+          !homogenizedView && ctx.settings.XprintInline.value) ~
         blockText(bindings :+ body)
       case tpt: untpd.DerivedTypeTree =>
         "<derived typetree watching " ~ summarized(toText(tpt.watched)) ~ ">"

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

@@ -1344,9 +1344,6 @@ trait Applications extends Compatibility { self: Typer with Dynamic =>
         (flip(tp1) relaxed_<:< flip(tp2)) || viewExists(tp1, tp2)
       }
 
-    // # skipped implicit parameters in tp1  -  # skipped implicit parameters in tp2
-    var implicitBalance: Int = 0
-
     /** Widen the result type of synthetic implied methods from the implementation class to the
      *  type that's implemented. Example
      *
@@ -1381,12 +1378,11 @@ trait Applications extends Compatibility { self: Typer with Dynamic =>
     }
 
     /** Drop any implicit parameter section */
-    def stripImplicit(tp: Type, weight: Int): Type = tp match {
+    def stripImplicit(tp: Type): Type = tp match {
       case mt: MethodType if mt.isImplicitMethod =>
-        implicitBalance += mt.paramInfos.length * weight
-        resultTypeApprox(mt)
+        stripImplicit(resultTypeApprox(mt))
       case pt: PolyType =>
-        pt.derivedLambdaType(pt.paramNames, pt.paramInfos, stripImplicit(pt.resultType, weight))
+        pt.derivedLambdaType(pt.paramNames, pt.paramInfos, stripImplicit(pt.resultType))
       case _ =>
         tp
     }
@@ -1412,18 +1408,16 @@ trait Applications extends Compatibility { self: Typer with Dynamic =>
 
     val fullType1 = widenImplied(alt1.widen, alt1)
     val fullType2 = widenImplied(alt2.widen, alt2)
-    val strippedType1 = stripImplicit(fullType1, -1)
-    val strippedType2 = stripImplicit(fullType2, +1)
+    val strippedType1 = stripImplicit(fullType1)
+    val strippedType2 = stripImplicit(fullType2)
 
     val result = compareWithTypes(strippedType1, strippedType2)
-    if (result != 0 || !ctx.typerState.test(implicit ctx => strippedType1 =:= strippedType2))
-      result
-    else if (implicitBalance != 0)
-      -implicitBalance.signum
-    else if ((strippedType1 `ne` fullType1) || (strippedType2 `ne` fullType2))
-      compareWithTypes(fullType1, fullType2)
-    else
-      0
+    if (result != 0) result
+    else if (strippedType1 eq fullType1)
+      if (strippedType2 eq fullType2) 0         // no implicits either side: its' a draw
+      else 1                                    // prefer 1st alternative with no implicits
+    else if (strippedType2 eq fullType2) -1     // prefer 2nd alternative with no implicits
+    else compareWithTypes(fullType1, fullType2) // continue by comparing implicits parameters
   }}
 
   def narrowMostSpecific(alts: List[TermRef])(implicit ctx: Context): List[TermRef] = track("narrowMostSpecific") {

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

@@ -2547,7 +2547,10 @@ class Typer extends Namer
               if (arg.tpe.isError) Nil else untpd.NamedArg(pname, untpd.TypedSplice(arg)) :: Nil
             }
             tryEither { implicit ctx =>
-              typed(untpd.Apply(untpd.TypedSplice(tree), namedArgs), pt, locked)
+              val app = cpy.Apply(tree)(untpd.TypedSplice(tree), namedArgs)
+              if (wtp.isContextual) app.pushAttachment(untpd.ApplyGiven, ())
+              typr.println(i"try with default implicit args $app")
+              typed(app, pt, locked)
             } { (_, _) =>
               issueErrors()
             }

docs/docs/reference/changed-features/implicit-resolution.md

@@ -114,11 +114,9 @@ affect implicits on the language level.
     An alternative A is _more specific_ than an alternative B if
 
      - the relative weight of A over B is greater than the relative weight of B over A, or
-     - the relative weights are the same, and the returned types of A and B are
-       unifiable, and A takes more inferable parameters than B, or
-     - the relative weights and the number of inferable parameters are the same, and
-       the returned types of A and B are unifiable, and
-       A is more specific than B if all inferable parameters in either alternative are
+     - the relative weights are the same, and A takes no implicit parameters but B does, or
+     - the relative weights are the same, both A and B take implicit parameters, and
+       A is more specific than B if all implicit parameters in either alternative are
        replaced by regular parameters.
 
 [//]: # todo: expand with precise rules

tests/pos/i5978.scala

@@ -63,14 +63,18 @@ package p3 {
       import implied TextParser._
       import TextParser._
 
-      val tp_v: TokenParser[Char, Position[CharSequence]] = TextParser.TP
-      val tp_i = the[TokenParser[Char, Position[CharSequence]]]
-      implicit val co_i: Conversion[Char, Position[CharSequence]] = the[Conversion[Char, Position[CharSequence]]]
-      val co_x : Position[CharSequence] = 'x'
+      val co_i: Conversion[Char, Position[CharSequence]] = the[Conversion[Char, Position[CharSequence]]]
 
       {
-        implied XXX for Conversion[Char, Position[CharSequence]] = co_i
-        val co_y : Position[CharSequence] = 'x'
+        val tp_v: TokenParser[Char, Position[CharSequence]] = TextParser.TP
+        val tp_i = the[TokenParser[Char, Position[CharSequence]]]
+        implied for Conversion[Char, Position[CharSequence]] = co_i
+        val co_x : Position[CharSequence] = 'x'
+
+        {
+          implied XXX for Conversion[Char, Position[CharSequence]] = co_i
+          val co_y : Position[CharSequence] = 'x'
+        }
       }
     }
   }

tests/run/implicit-functors.check

@@ -0,0 +1,2 @@
+functorId
+funcorConst

tests/run/implicit-functors.scala

@@ -0,0 +1,25 @@
+object Utils {
+  type Id[t] = t
+  type Const[c] = [t] => c
+}
+
+import Utils._
+
+class Instances[F[_[_]], T[_]]
+
+trait Functor[F[_]]
+
+object Functor {
+  implicit val functorId: Functor[Id] = { println("functorId"); null }
+  implicit def functorGen[F[_]](implicit inst: Instances[Functor, F]): Functor[F] = { println("funcorGen"); null }
+  implicit def functorConst[T]: Functor[Const[T]] =  { println("funcorConst"); null }
+}
+
+case class Sm[A](value: A)
+object Sm {
+  implicit def smInstances[F[_[_]]]: Instances[F, Sm] = { println("smInstances"); null }
+}
+
+object Test extends App {
+  implicitly[Functor[Sm]]
+}

tests/run/implicit-specifity.scala

@@ -12,19 +12,27 @@ object Generic {
   implied showGen[T] given Generic for Show[T] = new Show[T](2)
 }
 
+class Generic2
+object Generic2 {
+  opaque type HiPriority = AnyRef
+  implied showGen[T] for (Show[T] & HiPriority) = new Show[T](2).asInstanceOf
+}
+
+class SubGen extends Generic
+object SubGen {
+  implied for SubGen
+}
 object Contextual {
   trait Context
   implied ctx for Context
   implied showGen[T] given Generic for Show[T] = new Show[T](2)
   implied showGen[T] given Generic, Context for Show[T] = new Show[T](3)
+  implied showGen[T] given SubGen for Show[T] = new Show[T](4)
 }
 
 object Test extends App {
   assert(Show[Int] == 0)
   assert(Show[String] == 1)
-  assert(Show[Generic] == 2) // showGen beats fallback due to longer argument list
-
-  { import implied Contextual._
-    assert(Show[Generic] == 3)
-  }
+  assert(Show[Generic] == 1)   // showGen loses against fallback due to longer argument list
+  assert(Show[Generic2] == 2)  // ... but the opaque type intersection trick works.
 }

tests/run/implied-divergence.scala

@@ -0,0 +1,13 @@
+// Checks that divergence checking works before going into
+// recursions.
+case class E(x: E | Null)
+
+implied e for E(null)
+
+object Test extends App {
+
+  implied f given (e: E) for E(e)
+
+  assert(the[E].toString == "E(E(null))")
+
+}

tests/run/implied-priority.scala

@@ -0,0 +1,161 @@
+/* These tests show various mechanisms available for implicit prioritization.
+ */
+
+class E[T](val str: String)  // The type for which we infer terms below
+
+class Arg[T]  // An argument that we use as a given for some implied instances below
+
+/* First, two schemes that require a pre-planned architecture for how and
+ * where implied instances are defined.
+ *
+ * Traditional scheme: prioritize with location in class hierarchy
+ */
+class LowPriorityImplicits {
+  implied t1[T] for E[T]("low")
+}
+
+object NormalImplicits extends LowPriorityImplicits {
+  implied t2[T] given Arg[T] for E[T]("norm")
+}
+
+def test1 = {
+  import implied NormalImplicits._
+  assert(the[E[String]].str == "low") // No Arg available, so only t1 applies
+
+  { implied for Arg[String]
+    assert(the[E[String]].str == "norm")  // Arg available, t2 takes priority
+  }
+}
+
+/* New scheme: dummy implicit arguments that indicate priorities
+ */
+object Priority {
+  class Low
+  object Low { implied for Low }
+
+  class High extends Low
+  object High { implied for High }
+}
+
+object Impl2 {
+  implied t1[T] given Priority.Low for E[T]("low")
+  implied t2[T] given Priority.High given Arg[T] for E[T]("norm")
+}
+
+def test2 = {
+  import implied Impl2._
+  assert(the[E[String]].str == "low") // No Arg available, so only t1 applies
+
+  { implied for Arg[String]
+    assert(the[E[String]].str == "norm") // Arg available, t2 takes priority
+  }
+}
+
+/* The remaining tests show how we can add an override of highest priority or
+ * a fallback of lowest priority to a group of existing implied instances, without
+ * needing to change the location or definition of those instances.
+ *
+ * First, consider the problem how to define an override of highest priority.
+ * If all of the alternatives in the existing hierarchy take implicit arguments,
+ * an alternative without implicit arguments would override all of them.
+ */
+object Impl2a {
+  implied t3[T] for E[T]("hi")
+}
+
+def test2a = {
+  import implied Impl2._
+  import implied Impl2a._
+
+  implied for Arg[String]
+  assert(the[E[String]].str == "hi")
+}
+
+/* If that solution is not applicable, we can define an override by refining the
+ * result type of the implied instance, e.g. like this:
+ */
+object Impl3 {
+  implied t1[T] for E[T]("low")
+}
+
+object Override {
+  trait HighestPriority  // A marker trait to indicate a higher priority
+
+  implied over[T] for E[T]("hi"), HighestPriority
+}
+
+def test3 = {
+  import implied Impl3._
+  assert(the[E[String]].str == "low")  // only t1 is available
+
+  { import implied Override._
+    import implied Impl3._
+    assert(the[E[String]].str == "hi") // `over` takes priority since its result type is a subtype of t1's.
+  }
+}
+
+/* Now consider the dual problem: How to install a fallback with lower priority than existing
+ * implied instances that kicks in when none of the other instances are applicable.
+ * We get there in two stages. The first stage is by defining an explicit `withFallback` method
+ * that takes the right implicit and returns it. This can be achieved using an implicit parameter
+ * with a default argument.
+ */
+object Impl4 {
+  implied t1 for E[String]("string")
+  implied t2[T] given Arg[T] for E[T]("generic")
+}
+
+object fallback4 {
+  def withFallback[T] given (ev: E[T] = new E[T]("fallback")): E[T] = ev
+}
+
+def test4 = {
+  import implied Impl4._
+  import fallback4._
+  assert(withFallback[String].str == "string")  // t1 is applicable
+  assert(withFallback[Int].str == "fallback")   // No applicable instances, pick the default
+
+  { implied for Arg[Int]
+    assert(withFallback[Int].str == "generic")  // t2 is applicable
+  }
+}
+
+/* The final setup considers the problem how to define a fallback with lower priority than existing
+ * implicits that exists as an implicit instance alongside the others. This can be achieved
+ * by combining the implicit parameter with default technique for getting an existing impplicit
+ * or a fallback with the result refinement technique for overriding all existing implicit instances.
+ *
+ * It employs a more re-usable version of the result refinement trick.
+ */
+opaque type HigherPriority = Any
+object HigherPriority {
+  def inject[T](x: T): T & HigherPriority = x
+}
+
+object fallback5 {
+  implied [T] given (ev: E[T] = new E[T]("fallback")) for (E[T] & HigherPriority) = HigherPriority.inject(ev)
+}
+
+def test5 = {
+  import implied Impl4._
+  import implied fallback5._
+
+  // All inferred terms go through the implied instance in fallback5.
+  // They differ in what implicit argument is synthesized for that instance.
+  assert(the[E[String]].str == "string")  // t1 is applicable
+  assert(the[E[Int]].str == "fallback")   // No applicable instances, pick the default
+
+  { implied for Arg[Int]
+    assert(the[E[Int]].str == "generic")  // t2 is applicable
+  }
+}
+
+object Test extends App {
+  test1
+  test2
+  test2a
+  test3
+  test4
+  test5
+}
+