Only report changes in given priority when disambiguating

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

@@ -1320,14 +1320,14 @@ trait Implicits:
        *  3.6 and higher: compare with preferGeneral = true
        *
        */
-      def compareAlternatives(alt1: RefAndLevel, alt2: RefAndLevel): Int =
+      def compareAlternatives(alt1: RefAndLevel, alt2: RefAndLevel, reportChanges: Boolean = false): Int =
         def comp(using Context) = explore(compare(alt1.ref, alt2.ref, preferGeneral = true))
         def warn(msg: Message) =
-          priorityChangeWarnings += ((alt1.ref, alt2.ref, msg))
+          if reportChanges then priorityChangeWarnings += ((alt1.ref, alt2.ref, msg))
         if alt1.ref eq alt2.ref then 0
         else if alt1.level != alt2.level then alt1.level - alt2.level
         else
-          var cmp = comp(using searchContext())
+          val cmp = comp(using searchContext())
           val sv = Feature.sourceVersion
           if sv.stable == SourceVersion.`3.5` || sv == SourceVersion.`3.6-migration` then
             val prev = comp(using searchContext().addMode(Mode.OldImplicitResolution))
@@ -1358,7 +1358,7 @@ trait Implicits:
        */
       def disambiguate(alt1: SearchResult, alt2: SearchSuccess) = alt1 match
         case alt1: SearchSuccess =>
-          var diff = compareAlternatives(alt1, alt2)
+          var diff = compareAlternatives(alt1, alt2, reportChanges = true)
           assert(diff <= 0)   // diff > 0 candidates should already have been eliminated in `rank`
           if diff == 0 && alt1.ref =:= alt2.ref then
             diff = 1 // See i12951 for a test where this happens

tests/neg/given-triangle.check

@@ -0,0 +1,4 @@
+-- [E172] Type Error: tests/neg/given-triangle.scala:14:18 -------------------------------------------------------------
+14 |@main def Test = f  // error
+   |                  ^
+   |Ambiguous given instances: both given instance given_B and given instance given_C match type A of parameter a of method f

tests/warn/given-triangle.scala → tests/neg/given-triangle.scala

@@ -1,5 +1,3 @@
-//> using options -source 3.6-migration
-
 class A
 class B extends A
 class C extends A
@@ -13,4 +11,4 @@ def f(using a: A, b: B, c: C) =
   println(b.getClass)
   println(c.getClass)
 
-@main def Test = f  // warn
+@main def Test = f  // error

tests/pos/i20572.scala

@@ -0,0 +1,7 @@
+//> using options -Werror
+trait Writes[T]
+trait Format[T] extends Writes[T]
+given [T: List]: Writes[T] = null
+given [T]: Format[T] = null
+
+val _ = summon[Writes[Int]]

tests/pos/i21036.scala

@@ -0,0 +1,16 @@
+//> using options -source 3.5 -Werror
+trait SameRuntime[A, B]
+trait BSONWriter[T]
+trait BSONHandler[T] extends BSONWriter[T]
+
+opaque type Id = String
+object Id:
+  given SameRuntime[Id, String] = ???
+
+given BSONHandler[String]                    = ???
+given [T: BSONHandler]: BSONHandler[List[T]] = ???
+
+given opaqueWriter[T, A](using rs: SameRuntime[T, A], writer: BSONWriter[A]): BSONWriter[T] = ???
+
+val x = summon[BSONHandler[List[Id]]] // this doesn't emit warning
+val y = summon[BSONWriter[List[Id]]] // this did emit warning

tests/run/given-triangle.scala

@@ -1,4 +1,4 @@
-import language.future
+import language.`3.6`
 
 class A
 class B extends A