Fix how implicit candidates are combined

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

@@ -641,6 +641,13 @@ class PlainPrinter(_ctx: Context) extends Printer {
     else if (pos.source.exists) s"${pos.source.file.name}:${pos.line + 1}"
     else s"(no source file, offset = ${pos.span.point})"
 
+  def toText(cand: Candidate): Text =
+    "Cand("
+      ~ toTextRef(cand.ref)
+      ~ (if cand.isConversion then " conv" else "")
+      ~ (if cand.isExtension then " ext" else "")
+      ~ Str(" L" + cand.level) ~ ")"
+
   def toText(result: SearchResult): Text = result match {
     case result: SearchSuccess =>
       "SearchSuccess: " ~ toText(result.ref) ~ " via " ~ toText(result.tree)

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

@@ -7,7 +7,7 @@ import Texts._, ast.Trees._
 import Types.{Type, SingletonType, LambdaParam, NamedType},
        Symbols.Symbol, Scopes.Scope, Constants.Constant,
        Names.Name, Denotations._, Annotations.Annotation, Contexts.Context
-import typer.Implicits.SearchResult
+import typer.Implicits.*
 import util.SourcePosition
 import typer.ImportInfo
 
@@ -153,6 +153,9 @@ abstract class Printer {
   /** Textual representation of source position */
   def toText(pos: SourcePosition): Text
 
+  /** Textual representation of implicit candidates. */
+  def toText(cand: Candidate): Text
+
   /** Textual representation of implicit search result */
   def toText(result: SearchResult): Text
 

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

@@ -23,6 +23,7 @@ import ProtoTypes._
 import ErrorReporting._
 import Inferencing.{fullyDefinedType, isFullyDefined}
 import Scopes.newScope
+import Typer.BindingPrec, BindingPrec.*
 import transform.TypeUtils._
 import Hashable._
 import util.{EqHashMap, Stats}
@@ -49,17 +50,19 @@ object Implicits:
   }
 
   /** Both search candidates and successes are references with a specific nesting level. */
-  sealed trait RefAndLevel {
+  sealed trait RefAndLevel extends Showable {
     def ref: TermRef
     def level: Int
   }
 
   /** An eligible implicit candidate, consisting of an implicit reference and a nesting level */
-  case class Candidate(implicitRef: ImplicitRef, kind: Candidate.Kind, level: Int) extends RefAndLevel {
+  case class Candidate(implicitRef: ImplicitRef, kind: Candidate.Kind, level: Int) extends RefAndLevel with Showable {
     def ref: TermRef = implicitRef.underlyingRef
 
     def isExtension = (kind & Candidate.Extension) != 0
     def isConversion = (kind & Candidate.Conversion) != 0
+
+    def toText(printer: Printer): Text = printer.toText(this)
   }
   object Candidate {
     type Kind = Int
@@ -326,25 +329,28 @@ object Implicits:
       (this eq finalImplicits) || (outerImplicits eqn finalImplicits)
     }
 
+    def bindingPrec: BindingPrec =
+      if isImport then if ctx.importInfo.uncheckedNN.isWildcardImport then WildImport else NamedImport else Definition
+
     private def combineEligibles(ownEligible: List[Candidate], outerEligible: List[Candidate]): List[Candidate] =
       if ownEligible.isEmpty then outerEligible
       else if outerEligible.isEmpty then ownEligible
       else
-        def filter(xs: List[Candidate], remove: List[Candidate]) =
-          val shadowed = remove.map(_.ref.implicitName).toSet
-          xs.filterConserve(cand => !shadowed.contains(cand.ref.implicitName))
-
+        val ownNames = mutable.Set(ownEligible.map(_.ref.implicitName)*)
         val outer = outerImplicits.uncheckedNN
-        def isWildcardImport(using Context) = ctx.importInfo.nn.isWildcardImport
-        def preferDefinitions = isImport && !outer.isImport
-        def preferNamedImport = isWildcardImport && !isWildcardImport(using outer.irefCtx)
-
-        if !migrateTo3(using irefCtx) && level == outer.level && (preferDefinitions || preferNamedImport) then
-          // special cases: definitions beat imports, and named imports beat
-          // wildcard imports, provided both are in contexts with same scope
-          filter(ownEligible, outerEligible) ::: outerEligible
+        if !migrateTo3(using irefCtx) && level == outer.level && outer.bindingPrec.beats(bindingPrec) then
+          val keptOuters = outerEligible.filterConserve: cand =>
+            if ownNames.contains(cand.ref.implicitName) then
+              val keepOuter = cand.level == level
+              if keepOuter then ownNames -= cand.ref.implicitName
+              keepOuter
+            else true
+          val keptOwn = ownEligible.filterConserve: cand =>
+            ownNames.contains(cand.ref.implicitName)
+          keptOwn ::: keptOuters
         else
-          ownEligible ::: filter(outerEligible, ownEligible)
+          ownEligible ::: outerEligible.filterConserve: cand =>
+            !ownNames.contains(cand.ref.implicitName)
 
     def uncachedEligible(tp: Type)(using Context): List[Candidate] =
       Stats.record("uncached eligible")

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

@@ -65,6 +65,11 @@ object Typer {
     case NothingBound, PackageClause, WildImport, NamedImport, Inheritance, Definition
 
     def isImportPrec = this == NamedImport || this == WildImport
+
+    /** special cases: definitions beat imports, and named imports beat
+     *  wildcard imports, provided both are in contexts with same scope */
+    def beats(prevPrec: BindingPrec): Boolean =
+      this == Definition || this == NamedImport && prevPrec == WildImport
   }
 
   /** Assert tree has a position, unless it is empty or a typed splice */
@@ -226,9 +231,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
       def checkNewOrShadowed(found: Type, newPrec: BindingPrec, scala2pkg: Boolean = false)(using Context): Type =
         if !previous.exists || TypeComparer.isSameRef(previous, found) then
            found
-        else if (prevCtx.scope eq ctx.scope)
-                && (newPrec == Definition || newPrec == NamedImport && prevPrec == WildImport)
-        then
+        else if (prevCtx.scope eq ctx.scope) && newPrec.beats(prevPrec) then
           // special cases: definitions beat imports, and named imports beat
           // wildcard imports, provided both are in contexts with same scope
           found

tests/pos/i18316.orig.scala

@@ -0,0 +1,24 @@
+import scala.language.implicitConversions
+object squerel {
+  trait EqualityExpression
+  object PrimitiveTypeMode:
+    implicit def intToTE(f: Int): TypedExpression[Int] = ???
+
+  trait TypedExpression[A1]:
+    def ===[A2](b: TypedExpression[A2]): EqualityExpression = ???
+}
+
+object scalactic {
+  trait TripleEqualsSupport:
+    class Equalizer[L](val leftSide: L):
+      def ===(rightSide: Any): Boolean = ???
+
+  trait TripleEquals extends TripleEqualsSupport:
+    implicit def convertToEqualizer[T](left: T): Equalizer[T] = ???
+}
+
+import squerel.PrimitiveTypeMode._ // remove to make code compile
+object Test extends scalactic.TripleEquals {
+  import squerel.PrimitiveTypeMode._
+  val fails: squerel.EqualityExpression = 1 === 1
+}

tests/pos/i18316.scala

@@ -0,0 +1,13 @@
+class R1
+class R2
+
+class Foo { def meth(x: Int): R1 = null }
+class Bar { def meth(x: Int): R2 = null }
+
+object Impl { implicit def mkFoo(i: Int): Foo = null }
+trait Trait { implicit def mkBar(i: Int): Bar = null }
+
+import Impl.mkFoo // remove to make code compile
+object Test extends Trait:
+  import Impl.mkFoo
+  val fails: R1 = 1.meth(1)