Improvements for implicit searches with top-level type variables (#16001)

Two improvements for implicit searches involving type variables.

1. We now always add a comment when an implicit search is rejected due
to the "too unspecific" criterion of #13886, commit
[Refine checking for underspecified implicit
queries](db5956b).

There have been quite a few regressions that hit that problem, so it is
good to know immediately what
   the issue is. 

2. There is now a better wildcard approximation of higher-kinded type
applications. This makes several programs (including original #15998)
compile, which were classified as not specific enough before.

Fixes #15998
Fixes #15820
Fixes #15670
Fixes #15160 
Fixes #13986

Author: smarter

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

@@ -125,25 +125,25 @@ object ErrorReporting {
     def typeMismatch(tree: Tree, pt: Type, implicitFailure: SearchFailureType = NoMatchingImplicits): Tree = {
       val normTp = normalize(tree.tpe, pt)
       val normPt = normalize(pt, pt)
-      
+
       def contextFunctionCount(tp: Type): Int = tp.stripped match
         case defn.ContextFunctionType(_, restp, _) => 1 + contextFunctionCount(restp)
         case _ => 0
       def strippedTpCount = contextFunctionCount(tree.tpe) - contextFunctionCount(normTp)
       def strippedPtCount = contextFunctionCount(pt) - contextFunctionCount(normPt)
-      
+
       val (treeTp, expectedTp) =
         if normTp <:< normPt || strippedTpCount != strippedPtCount
         then (tree.tpe, pt)
         else (normTp, normPt)
         // use normalized types if that also shows an error, and both sides stripped
         // the same number of context functions. Use original types otherwise.
-        
+
       def missingElse = tree match
         case If(_, _, elsep @ Literal(Constant(()))) if elsep.span.isSynthetic =>
           "\nMaybe you are missing an else part for the conditional?"
         case _ => ""
-        
+
       errorTree(tree, TypeMismatch(treeTp, expectedTp, Some(tree), implicitFailure.whyNoConversion, missingElse))
     }
 
@@ -262,6 +262,9 @@ class ImplicitSearchError(
         case _ =>
           defaultAmbiguousImplicitMsg(ambi)
       }
+    case ambi @ TooUnspecific(target) =>
+      ex"""No implicit search was attempted${location("for")}
+          |since the expected type $target is not specific enough"""
     case _ =>
       val shortMessage = userDefinedImplicitNotFoundParamMessage
         .orElse(userDefinedImplicitNotFoundTypeMessage)

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

@@ -411,14 +411,14 @@ object Implicits:
 
   /** A failed search */
   case class SearchFailure(tree: Tree) extends SearchResult {
-    final def isAmbiguous: Boolean = tree.tpe.isInstanceOf[AmbiguousImplicits]
+    final def isAmbiguous: Boolean = tree.tpe.isInstanceOf[AmbiguousImplicits | TooUnspecific]
     final def reason: SearchFailureType = tree.tpe.asInstanceOf[SearchFailureType]
   }
 
   object SearchFailure {
     def apply(tpe: SearchFailureType, span: Span)(using Context): SearchFailure = {
       val id = tpe match
-        case tpe: AmbiguousImplicits =>
+        case tpe: (AmbiguousImplicits | TooUnspecific) =>
           untpd.SearchFailureIdent(nme.AMBIGUOUS, s"/* ambiguous: ${tpe.explanation} */")
         case _ =>
           untpd.SearchFailureIdent(nme.MISSING, "/* missing */")
@@ -504,11 +504,14 @@ object Implicits:
     SearchFailure(ImplicitSearchTooLarge, NoSpan)(using NoContext)
 
   /** A failure value indicating that an implicit search for a conversion was not tried */
-  class TooUnspecific(target: Type) extends NoMatchingImplicits(NoType, EmptyTree, OrderingConstraint.empty):
+  case class TooUnspecific(target: Type) extends NoMatchingImplicits(NoType, EmptyTree, OrderingConstraint.empty):
     override def whyNoConversion(using Context): String =
       i"""
          |Note that implicit conversions were not tried because the result of an implicit conversion
          |must be more specific than $target"""
+    override def explanation(using Context) =
+      i"""${super.explanation}.
+         |The expected type $target is not specific enough, so no search was attempted"""
     override def toString = s"TooUnspecific"
 
   /** An ambiguous implicits failure */
@@ -1484,7 +1487,7 @@ trait Implicits:
 
     private def searchImplicit(contextual: Boolean): SearchResult =
       if isUnderspecified(wildProto) then
-        NoMatchingImplicitsFailure
+        SearchFailure(TooUnspecific(pt), span)
       else
         val eligible =
           if contextual then

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

@@ -822,17 +822,23 @@ object ProtoTypes {
   /** Approximate occurrences of parameter types and uninstantiated typevars
    *  by wildcard types.
    */
-  private def wildApprox(tp: Type, theMap: WildApproxMap | Null, seen: Set[TypeParamRef], internal: Set[TypeLambda])(using Context): Type = tp match {
+  private def wildApprox(tp: Type, theMap: WildApproxMap | Null, seen: Set[TypeParamRef], internal: Set[TypeLambda])(using Context): Type =
+    tp match {
     case tp: NamedType => // default case, inlined for speed
       val isPatternBoundTypeRef = tp.isInstanceOf[TypeRef] && tp.symbol.isPatternBound
       if (isPatternBoundTypeRef) WildcardType(tp.underlying.bounds)
       else if (tp.symbol.isStatic || (tp.prefix `eq` NoPrefix)) tp
       else tp.derivedSelect(wildApprox(tp.prefix, theMap, seen, internal))
     case tp @ AppliedType(tycon, args) =>
+      def wildArgs = args.mapConserve(arg => wildApprox(arg, theMap, seen, internal))
       wildApprox(tycon, theMap, seen, internal) match {
-        case _: WildcardType => WildcardType // this ensures we get a * type
-        case tycon1 => tp.derivedAppliedType(tycon1,
-          args.mapConserve(arg => wildApprox(arg, theMap, seen, internal)))
+        case WildcardType(TypeBounds(lo, hi)) if hi.typeParams.hasSameLengthAs(args) =>
+          val args1 = wildArgs
+          val lo1 = if lo.typeParams.hasSameLengthAs(args) then lo.appliedTo(args1) else lo
+          WildcardType(TypeBounds(lo1, hi.appliedTo(args1)))
+        case WildcardType(_) =>
+          WildcardType
+        case tycon1 => tp.derivedAppliedType(tycon1, wildArgs)
       }
     case tp: RefinedType => // default case, inlined for speed
       tp.derivedRefinedType(

tests/neg/i15998.check

@@ -0,0 +1,20 @@
+-- [E007] Type Mismatch Error: tests/neg/i15998.scala:6:12 -------------------------------------------------------------
+6 |val _ = foo(1) // error
+  |            ^
+  |            Found:    (1 : Int)
+  |            Required: CC[A]
+  |
+  |            where:    A  is a type variable
+  |                      CC is a type variable with constraint <: [B] =>> Any
+  |
+  |            Note that implicit conversions were not tried because the result of an implicit conversion
+  |            must be more specific than CC[A]
+  |
+  | longer explanation available when compiling with `-explain`
+-- Error: tests/neg/i15998.scala:11:11 ---------------------------------------------------------------------------------
+11 |val _ = bar  // error
+   |           ^
+   |           No implicit search was attempted for parameter x of method bar
+   |           since the expected type X is not specific enough
+   |
+   |           where:    X is a type variable

tests/neg/i15998.scala

@@ -0,0 +1,11 @@
+
+given split: Conversion[Int, List[Int]] = ???
+
+def foo[A, CC[B]](ring: CC[A]): Unit = ()
+
+val _ = foo(1) // error
+
+
+def bar[X](using x: X): X = x
+
+val _ = bar  // error

tests/pos/i15160.scala

@@ -0,0 +1,17 @@
+trait Eq[A] {
+  def eqv(a1: A, a2: A): Boolean
+}
+
+given stringEq: Eq[String] with {
+  def eqv(a1: String, a2: String) = a1 == a2
+}
+
+abstract class Newtype[Src] {
+  opaque type Type = Src
+
+  protected final def derive[F[_]](using ev: F[Src]): F[Type] = ev
+}
+
+object Sample extends Newtype[String] {
+  given eq: Eq[Type] = derive
+}

tests/pos/i15670.scala

@@ -0,0 +1,29 @@
+trait JsonRowEntry {
+  def readAs[E](implicit c: Read[E]): Option[E] = ???
+}
+trait Read[T]
+trait Codec[T] extends Read[T]
+trait CodecTypeProjection[C[_]]
+object JsonTransform  {
+  given SetCodec[T, C[_]: CodecTypeProjection]: scala.Conversion[C[T], C[Set[T]]] = ???
+  given SetCodecExp[T, C[_]: CodecTypeProjection](using codec: C[T]): C[Set[T]] = codec
+  given Codec[String] = ???
+  given CodecTypeProjection[Read] = ???
+}
+
+@main def Test() = {
+  import JsonTransform.given
+  val tree = new JsonRowEntry {}
+  tree.readAs[Set[String]]
+}
+
+trait Box[E]
+
+trait Domain
+
+def fun[E, D[_] <: Domain](box: Box[E])(implicit domain: D[E]): Unit = {
+
+  val newBox: Box[E] =  ???
+
+  fun(newBox)
+}

tests/pos/i15820.scala

@@ -0,0 +1,8 @@
+sealed trait Domain[E]
+
+final def splitBounds[E, D[X] <: Domain[X]](
+    bounds: Seq[E],
+  )( using domain: D[E]): Seq[E] =
+      val newBounds: Seq[E] = ???
+      splitBounds(newBounds) // does not compile
+      splitBounds[E,D](newBounds) // does compile

tests/run/i13986.check

@@ -0,0 +1 @@
+mu

tests/run/i13986.scala

@@ -0,0 +1,15 @@
+sealed trait Xa[T]
+sealed trait Mu[T] extends Xa[T]
+object Xa {
+  implicit def convertMu[X[x] <: Xa[x], A, B](implicit t: X[A]): X[B] = t.asInstanceOf[X[B]]
+}
+object Mu {
+  implicit def mu: Mu[Int] = new Mu[Int] {
+    override def toString = "mu"
+  }
+}
+
+object Test extends App {
+  def constrain(a: Mu[Long]): Unit = println(a)
+  constrain(Xa.convertMu)
+}

tests/neg/i13987.scala renamed to tests/run/i13987.scala

@@ -10,7 +10,7 @@ object Mu {
   implicit def mu: Mu[Int] = new Mu[Int] {}
 }
 
-object App extends App {
-  def constrain(a: Mu[Long]): Unit = println(a)
+object Test extends App {
+  def constrain(a: Mu[Long]): Unit = ()
   constrain(Xa.convertMu)  // error
 }

tests/run/i15998.scala

@@ -0,0 +1,17 @@
+import scala.collection.SeqOps
+
+trait ComparingOps:
+  extension[A, CC[B] <: SeqOps[B, CC, CC[B]]](ring: CC[A])
+    def isRotationOf(that: CC[A]): Boolean = true
+
+object RingSeq extends ComparingOps
+import RingSeq.*
+
+@main def Test =
+  RingSeq.isRotationOf("DAB") // error
+  "ABCD".isRotationOf("DAB")  // error
+
+  // workaround
+  RingSeq.isRotationOf[Char, IndexedSeq]("DAB")
+  RingSeq.isRotationOf(wrapString("DAB"))
+  wrapString("ABCD").isRotationOf("DAB")