Refine overloading and implicit disambiguation (#20084)

We sometimes have two alternatives a.m and b.m with the same symbol and
type but different prefixes. Previously these would always be ambiguous.
We now try to disambiguate this so that the alternative with the more
specific prefix wins. To determine this, we widen prefixes also going
from module classes to their parents and then compare the resulting
types.

This might fix a problem in ScalaTest that popped up after #20054.

Author: odersky

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

@@ -1615,11 +1615,12 @@ 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:
@@ -1643,17 +1644,22 @@ 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.
    *
@@ -1808,10 +1814,38 @@ trait Applications extends Compatibility {
         else tp
     }
 
-    def compareWithTypes(tp1: Type, tp2: Type) = {
+    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 =
+      val pre1 = widenPrefix(alt1)
+      val pre2 = widenPrefix(alt2)
+      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)
+
+      val winsType1 = isAsSpecific(alt1, tp1, alt2, tp2)
+      val winsType2 = isAsSpecific(alt2, tp2, alt1, tp1)
 
       overload.println(i"compare($alt1, $alt2)? $tp1 $tp2 $ownerScore $winsType1 $winsType2")
       if winsType1 && winsType2
@@ -1820,15 +1854,14 @@ 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 if ownerScore == 1 then
-        if winsType1 || !winsType2 then 1 else 0
-      else if ownerScore == -1 then
-        if winsType2 || !winsType1 then -1 else 0
-      else if winsType1 then
-        if winsType2 then 0 else 1
-      else
-        if winsType2 then -1 else 0
-    }
+      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
+    end compareWithTypes
 
     if alt1.symbol.is(ConstructorProxy) && !alt2.symbol.is(ConstructorProxy) then -1
     else if alt2.symbol.is(ConstructorProxy) && !alt1.symbol.is(ConstructorProxy) then 1

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 +1,17 @@
+
+class I[X]
+class J[X]
+
+trait A:
+  given I[B] = ???
+  given (using I[B]): J[B] = ???
+object A extends A
+
+trait B extends A
+object B extends B
+
+//import B.given, A.given
+
+def Test =
+  summon[I[B]]
+  summon[J[B]]