Revert "Compensate loss of transitivity"

This reverts commit 7c4bd67
See #21344 (comment)

Fixes #21320

[Cherry-picked 58f3407][modified]

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

@@ -1385,6 +1385,7 @@ trait Implicits:
       def disambiguate(alt1: SearchResult, alt2: SearchSuccess) = alt1 match
         case alt1: SearchSuccess =>
           var diff = compareAlternatives(alt1, alt2, disambiguate = true)
+          assert(diff <= 0 || isWarnPriorityChangeVersion)
             // 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/pos/i21320b.scala

@@ -0,0 +1,73 @@
+import scala.deriving.*
+import scala.compiletime.*
+
+trait ConfigMonoid[T]:
+  def zero: T
+  def orElse(main: T, defaults: T): T
+
+object ConfigMonoid:
+  given option[T]: ConfigMonoid[Option[T]] = ???
+
+  inline def zeroTuple[C <: Tuple]: Tuple =
+    inline erasedValue[C] match
+      case _: EmptyTuple => EmptyTuple
+      case _: (t *: ts) =>
+        summonInline[ConfigMonoid[t]].zero *: zeroTuple[ts]
+
+  inline def valueTuple[C <: Tuple, T](index: Int, main: T, defaults: T): Tuple =
+    inline erasedValue[C] match
+      case _: EmptyTuple => EmptyTuple
+      case _: (t *: ts) =>
+        def get(v: T) = v.asInstanceOf[Product].productElement(index).asInstanceOf[t]
+        summonInline[ConfigMonoid[t]].orElse(get(main), get(defaults)) *: valueTuple[ts, T](
+          index + 1,
+          main,
+          defaults
+        )
+
+  inline given derive[T](using m: Mirror.ProductOf[T]): ConfigMonoid[T] =
+    new ConfigMonoid[T]:
+      def zero: T = m.fromProduct(zeroTuple[m.MirroredElemTypes])
+      def orElse(main: T, defaults: T): T = m.fromProduct(valueTuple[m.MirroredElemTypes, T](0, main, defaults))
+
+
+
+final case class PublishOptions(
+  v1: Option[String] = None,
+  v2: Option[String] = None,
+  v3: Option[String] = None,
+  v4: Option[String] = None,
+  v5: Option[String] = None,
+  v6: Option[String] = None,
+  v7: Option[String] = None,
+  v8: Option[String] = None,
+  v9: Option[String] = None,
+  ci: PublishContextualOptions = PublishContextualOptions(),
+)
+object PublishOptions:
+  implicit val monoid: ConfigMonoid[PublishOptions] = ConfigMonoid.derive
+
+final case class PublishContextualOptions(
+  v1: Option[String] = None,
+  v2: Option[String] = None,
+  v3: Option[String] = None,
+  v4: Option[String] = None,
+  v5: Option[String] = None,
+  v6: Option[String] = None,
+  v7: Option[String] = None,
+  v8: Option[String] = None,
+  v9: Option[String] = None,
+  v10: Option[String] = None,
+  v11: Option[String] = None,
+  v12: Option[String] = None,
+  v13: Option[String] = None,
+  v14: Option[String] = None,
+  v15: Option[String] = None,
+  v16: Option[String] = None,
+  v17: Option[String] = None,
+  v18: Option[String] = None,
+  v19: Option[String] = None,
+  v20: Option[String] = None
+)
+object PublishContextualOptions:
+  implicit val monoid: ConfigMonoid[PublishContextualOptions] = ConfigMonoid.derive // was crash