Bring back ambiguity filter when report implicit not found

This reverts one part of scala#20261. When we fail with both an ambiguity on one implicit
argument and another error on another we prefer the other error. I added a comment
why this is needed.

Fixes scala#20354

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

@@ -4113,7 +4113,14 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
           * `SearchFailureType`.
           */
         def issueErrors(fun: Tree, args: List[Tree]): Tree =
-          def firstFailure = args.tpes.find(_.isInstanceOf[SearchFailureType]).getOrElse(NoType)
+          // Prefer other errors over ambiguities. If nested in outer searches a missing
+          // implicit can be healed by simply dropping this alternative and tryng something
+          // else. But an ambiguity is sticky and propagates outwards. If we have both
+          // a missing implicit on one argument and an ambiguity on another the whole
+          // branch should be classified as a missing implicit.
+          val firstNonAmbiguous = args.tpes.find(tp => tp.isError && !tp.isInstanceOf[AmbiguousImplicits])
+          def firstError = args.tpes.find(_.isInstanceOf[SearchFailureType]).getOrElse(NoType)
+          def firstFailure = firstNonAmbiguous.getOrElse(firstError)
           val errorType =
             firstFailure match
               case tp: AmbiguousImplicits =>

tests/run/i20354.scala

@@ -0,0 +1,40 @@
+trait CmdLineParser { outer =>
+
+  trait Opt[+T] {
+    val default: T
+    val names: Set[String]
+    val help: String
+  }
+
+  trait IntOpt extends Opt[Int] {
+    val parser = outer                          //       <=== comment out this line, we get "true true"
+  }
+}
+
+object FirstParser extends CmdLineParser {
+  object OptMinSuccess extends IntOpt {
+    val default = 100
+    val names = Set("bla")
+    val help = "bla"
+  }
+
+  val opts = List(OptMinSuccess)
+}
+
+object SecondParser extends CmdLineParser {
+  object OptMinSuccess extends IntOpt {
+    val default = 50
+    val names = Set("bla")
+    val help = "help"
+  }
+}
+@main def Test =
+
+  val a = SecondParser.OptMinSuccess.isInstanceOf[FirstParser.IntOpt]
+
+  println(a)
+
+  (SecondParser.OptMinSuccess: SecondParser.IntOpt)  match {
+    case _: FirstParser.IntOpt => println("true")
+    case _ => println("false")
+  }