Fix ownerscore disambiguation

Author: odersky

Diff for: compiler/src/dotty/tools/dotc/typer/Applications.scala

@@ -1975,13 +1975,19 @@ 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 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
+      else
+        // For implicit resolution, take ownerscore as more significant than type resolution
+        // Reason: People use owner hierarchies to explicitly prioritize, we should not
+        // break that by changing implicit priority of types. On the other hand we do want
+        // to comparePrefixes if there is a draw; StringFormaterTest breaks if we don't do that.
+        def drawOrOwner = if preferGeneral then ownerScore else 0
+        ownerScore match
+          case  1 => if winsType1 || !winsType2 then  1 else drawOrOwner
+          case -1 => if winsType2 || !winsType1 then -1 else drawOrOwner
+          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

Diff for: tests/pos/given-priority.scala

@@ -0,0 +1,24 @@
+/* These tests show various mechanisms available for implicit prioritization.
+ */
+import language.`3.6`
+
+class A  // The type for which we infer terms below
+class B extends A
+
+/* First, two schemes that require a pre-planned architecture for how and
+ * where given instances are defined.
+ *
+ * Traditional scheme: prioritize with location in class hierarchy
+ */
+class LowPriorityImplicits:
+  given g1: A()
+
+object NormalImplicits extends LowPriorityImplicits:
+  given g2: B()
+
+def test1 =
+  import NormalImplicits.given
+  val x = summon[A]
+  val _: B = x
+  val y = summon[B]
+  val _: B = y