Refine overloading and implicit disambiguation

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

@@ -1614,11 +1614,12 @@ trait Applications extends Compatibility {
     *  Module classes also inherit the relationship from their companions. This means,
     *  if no direct derivation exists between `sym1` and `sym2` also perform the following
     *  tests:
-    *   - If both sym1 and sym1 are module classes that have companion classes,
-    *     and sym2 does not inherit implicit members from a base class (#),
-    *     compare the companion classes.
-    *   - If sym1 is a module class with a companion, and sym2 is a normal class or trait,
-    *     compare the companion with sym2.
+    *   - If both sym1 and sym2 are module classes that have companion classes,
+    *     compare the companion classes. Return the result of that comparison,
+    *     provided the module class with the larger companion class does not itself
+    *     inherit implicit members from a base class (#),
+    *   - If one sym is a module class with a companion, and the other is a normal class or trait,
+    *     compare the companion with the other class or trait.
     *
     *  Condition (#) is necessary to make `compareOwner(_, _) > 0` a transitive relation.
     *  For instance:
@@ -1642,17 +1643,22 @@ trait Applications extends Compatibility {
     *  This means we get an ambiguity between `a` and `b` in all cases.
     */
   def compareOwner(sym1: Symbol, sym2: Symbol)(using Context): Int =
+    def cls1 = sym1.companionClass
+    def cls2 = sym2.companionClass
     if sym1 == sym2 then 0
     else if sym1.isSubClass(sym2) then 1
     else if sym2.isSubClass(sym1) then -1
-    else if sym1.is(Module) then
-      val cls1 = sym1.companionClass
-      if sym2.is(Module) then
-        if sym2.thisType.implicitMembers.forall(_.symbol.owner == sym2) then // test for (#)
-          compareOwner(cls1, sym2.companionClass)
-        else 0
-      else compareOwner(cls1, sym2)
-    else 0
+    else
+      if sym1.is(Module) && sym2.is(Module) then
+        val r = compareOwner(cls1, cls2)
+        if r == 0 then 0
+        else
+          val larger = if r < 0 then sym1 else sym2
+          if larger.thisType.implicitMembers.forall(_.symbol.owner == larger) then r
+          else 0
+      else if sym1.is(Module) then compareOwner(cls1, sym2)
+      else if sym2.is(Module) then compareOwner(sym1, cls2)
+      else 0
 
   /** Compare two alternatives of an overloaded call or an implicit search.
    *
@@ -1807,8 +1813,34 @@ trait Applications extends Compatibility {
         else tp
     }
 
+    def widenPrefix(alt: TermRef): Type = alt.prefix.widen match
+      case pre: (TypeRef | ThisType) if pre.typeSymbol.is(Module) =>
+        pre.parents.reduceLeft(TypeComparer.andType(_, _))
+      case wpre => wpre
+
+    /** If two alternatives have the same symbol, we pick the one with the most
+     *  specific prefix. To determine that, we widen the prefix types and also
+     *  widen module classes to the intersection of their parent classes. Then
+     *  if one of the resulting types is a more specific value type than the other,
+     *  it wins. Example:
+     *
+     *     trait A { given M = ... }
+     *     trait B extends A
+     *     object a extends A
+     *     object b extends B
+     *
+     *  In this case `b.M` would be regarded as more specific than `a.M`.
+     */
+    def comparePrefixes(pre1: Type, pre2: Type) =
+      val winsPrefix1 = isAsSpecificValueType(pre1, pre2)
+      val winsPrefix2 = isAsSpecificValueType(pre2, pre1)
+      if winsPrefix1 == winsPrefix2 then 0
+      else if winsPrefix1 then 1
+      else -1
+
     def compareWithTypes(tp1: Type, tp2: Type) = {
       val ownerScore = compareOwner(alt1.symbol.maybeOwner, alt2.symbol.maybeOwner)
+
       def winsType1 = isAsSpecific(alt1, tp1, alt2, tp2)
       def winsType2 = isAsSpecific(alt2, tp2, alt1, tp1)
 
@@ -1838,11 +1870,14 @@ trait Applications extends Compatibility {
       val strippedType2 = stripImplicit(fullType2)
 
       val result = compareWithTypes(strippedType1, strippedType2)
-      if (result != 0) result
-      else if (strippedType1 eq fullType1)
-        if (strippedType2 eq fullType2) 0         // no implicits either side: its' a draw
+      if result != 0 then result
+      else if strippedType1 eq fullType1 then
+        if strippedType2 eq fullType2 then
+          if alt1.symbol != alt2.symbol then 0    // no implicits either side: it's a draw ...
+          else comparePrefixes(                   // ... unless the symbol is the same, in which case
+            widenPrefix(alt1), widenPrefix(alt2)) // we compare prefixes
         else 1                                    // prefer 1st alternative with no implicits
-      else if (strippedType2 eq fullType2) -1     // prefer 2nd alternative with no implicits
+      else if strippedType2 eq fullType2 then -1  // prefer 2nd alternative with no implicits
       else compareWithTypes(fullType1, fullType2) // continue by comparing implicits parameters
   }
   end compare

compiler/src/dotty/tools/dotc/util/Signatures.scala

@@ -495,8 +495,8 @@ object Signatures {
         case res => List(tpe)
 
     def isSyntheticEvidence(name: String) =
-      if !name.startsWith(NameKinds.ContextBoundParamName.separator) then false else
-        symbol.paramSymss.flatten.find(_.name.show == name).exists(_.flags.is(Flags.Implicit))
+      name.startsWith(NameKinds.ContextBoundParamName.separator)
+      && symbol.paramSymss.flatten.find(_.name.show == name).exists(_.flags.is(Flags.Implicit))
 
     def toTypeParam(tpe: PolyType): List[Param] =
       val evidenceParams = (tpe.paramNamess.flatten zip tpe.paramInfoss.flatten).flatMap:

tests/pos/implicit-prefix-disambiguation.scala

@@ -0,0 +1,14 @@
+class I[X]
+
+trait A:
+  given I[B] = ???
+object A extends A
+
+trait B extends A
+object B extends B
+
+//import B.given, A.given
+
+def Test = summon[I[B]]
+
+