Don't issue implicit errors if parameter has default value

If implicit parameters have default values then we try to re-type the
application with default values supplied as arguments. So far we
fell back to the original errors if retyping failed, which can lead
to confusing error messages. We now stick with the retyping in all cases.

Fixes #14842

Author: odersky

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

@@ -3539,14 +3539,10 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
             val namedArgs = wtp.paramNames.lazyZip(args).flatMap { (pname, arg) =>
               if (arg.tpe.isError) Nil else untpd.NamedArg(pname, untpd.TypedSplice(arg)) :: Nil
             }
-            tryEither {
-              val app = cpy.Apply(tree)(untpd.TypedSplice(tree), namedArgs)
-              if (wtp.isContextualMethod) app.setApplyKind(ApplyKind.Using)
-              typr.println(i"try with default implicit args $app")
-              typed(app, pt, locked)
-            } { (_, _) =>
-              issueErrors()
-            }
+            val app = cpy.Apply(tree)(untpd.TypedSplice(tree), namedArgs)
+            if (wtp.isContextualMethod) app.setApplyKind(ApplyKind.Using)
+            typr.println(i"try with default implicit args $app")
+            typed(app, pt, locked)
           else issueErrors()
         }
         else tree match {

tests/neg/i14842.check

@@ -0,0 +1,7 @@
+-- [E007] Type Mismatch Error: tests/neg/i14842.scala:11:35 ------------------------------------------------------------
+11 |  val x: Either[Int, Any] = node.as[Any] // error
+   |                            ^^^^^^^^^^^^
+   |                            Found:    Either[String, Any]
+   |                            Required: Either[Int, Any]
+   |
+   | longer explanation available when compiling with `-explain`

tests/neg/i14842.scala

@@ -0,0 +1,12 @@
+class Dummy
+object Dummy:
+  val empty = new Dummy
+
+sealed trait Node:
+  def as[T](using d: Dummy = Dummy.empty): Either[String, T] = ???
+
+object Sample extends App {
+  val node: Node = ???
+
+  val x: Either[Int, Any] = node.as[Any] // error
+}