Don't try default args when there are ambiguous implicits

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

@@ -3849,22 +3849,9 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
         end implicitArgs
 
         val args = implicitArgs(wtp.paramInfos, 0, pt)
-
-        def propagatedFailure(args: List[Tree]): Type = args match {
-          case arg :: args1 =>
-            arg.tpe match {
-              case ambi: AmbiguousImplicits =>
-                propagatedFailure(args1) match {
-                  case NoType | (_: AmbiguousImplicits) => ambi
-                  case failed => failed
-                }
-              case failed: SearchFailureType => failed
-              case _ => propagatedFailure(args1)
-            }
-          case Nil => NoType
-        }
-
-        val propFail = propagatedFailure(args)
+        val firstAmbiguous = args.tpes.find(_.isInstanceOf[AmbiguousImplicits])
+        def firstError = args.tpes.find(_.isError)
+        val propFail = firstAmbiguous.orElse(firstError).getOrElse(NoType)
 
         def issueErrors(): Tree = {
           def paramSymWithMethodTree(paramName: TermName) =
@@ -3897,9 +3884,12 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
           // need to be reset.
           ctx.typerState.resetTo(saved)
 
-          // If method has default params, fall back to regular application
-          // where all inferred implicits are passed as named args.
-          if hasDefaultParams && !propFail.isInstanceOf[AmbiguousImplicits] then
+          // If method has default params and there are no "Ambiguous implicits"
+          // error, fall back to regular application where all inferred
+          // implicits are passed as named args.
+          // If there are "Ambiguous implicits" errors, these take precedence
+          // over the default params (see issue #19414 and related tests).
+          if hasDefaultParams && firstAmbiguous.isEmpty then
             val namedArgs = wtp.paramNames.lazyZip(args).flatMap { (pname, arg) =>
               if (arg.tpe.isError) Nil else untpd.NamedArg(pname, untpd.TypedSplice(arg)) :: Nil
             }

tests/neg/19414-desugared.check

@@ -0,0 +1,4 @@
+-- [E172] Type Error: tests/neg/19414-desugared.scala:34:34 ------------------------------------------------------------
+34 |  summon[BodySerializer[JsObject]] // error: Ambiguous given instances
+   |                                  ^
+   |Ambiguous given instances: both given instance given_Writer_JsValue and given instance given_Writer_JsObject match type Writer[B] of parameter x of method summon in object Predef

tests/neg/19414-desugared.scala

@@ -0,0 +1,34 @@
+/** This test checks that provided given instances take precedence over default
+  * given arguments, even when there are multiple default arguments.
+  *
+  * Before the fix for issue #19414, this code would fail with a "No given
+  * instance of type BodySerializer[JsObject]".
+  *
+  * See also:
+  *   - tests/neg/19414.scala
+  *   - tests/neg/given-ambiguous-default-1.scala
+  *   - tests/neg/given-ambiguous-default-2.scala
+  */
+
+trait JsValue
+trait JsObject extends JsValue
+
+trait Writer[T]
+trait BodySerializer[-B]
+
+class Printer
+
+given Writer[JsValue] = ???
+given Writer[JsObject] = ???
+
+// This is not an exact desugaring of the original code: currently the compiler
+// actually changes the modifier of the parameter list from `using` to
+// `implicit` when desugaring the context-bound `B: Writer` to `implicit writer:
+// Writer[B]`, but we can't write in user code as this is not valid syntax.
+given [B](using
+    writer: Writer[B],
+    printer: Printer = new Printer
+): BodySerializer[B] = ???
+
+def f: Unit =
+  summon[BodySerializer[JsObject]] // error: Ambiguous given instances

tests/neg/19414.check

@@ -0,0 +1,4 @@
+-- [E172] Type Error: tests/neg/19414.scala:27:34 ----------------------------------------------------------------------
+27 |  summon[BodySerializer[JsObject]] // error: Ambiguous given instances
+   |                                  ^
+   |Ambiguous given instances: both given instance given_Writer_JsValue and given instance given_Writer_JsObject match type Writer[B] of parameter x of method summon in object Predef

tests/neg/19414.scala

@@ -0,0 +1,27 @@
+/** This test checks that provided given instances take precedence over default
+  * given arguments, even when there are multiple default arguments.
+  *
+  * Before the fix for issue #19414, this code would fail with a "No given
+  * instance of type BodySerializer[JsObject]".
+  *
+  * See also:
+  *   - tests/neg/19414-desugared.scala
+  *   - tests/neg/given-ambiguous-default-1.scala
+  *   - tests/neg/given-ambiguous-default-2.scala
+  */
+
+trait JsValue
+trait JsObject extends JsValue
+
+trait Writer[T]
+trait BodySerializer[-B]
+
+class Printer
+
+given Writer[JsValue] = ???
+given Writer[JsObject] = ???
+
+given [B: Writer](using printer: Printer = new Printer): BodySerializer[B] = ???
+
+def f: Unit =
+  summon[BodySerializer[JsObject]] // error: Ambiguous given instances

tests/neg/given-ambiguous-default-1.check

@@ -0,0 +1,4 @@
+-- [E172] Type Error: tests/neg/given-ambiguous-default-1.scala:19:23 --------------------------------------------------
+19 |def f: Unit = summon[B] // error: Ambiguous given instances
+   |                       ^
+   |Ambiguous given instances: both given instance a1 and given instance a2 match type A of parameter x of method summon in object Predef

tests/neg/given-ambiguous-default-1.scala

@@ -0,0 +1,19 @@
+/** This test checks that provided given instances take precedence over default
+  * given arguments.
+  *
+  * In the following code, the compiler must report an "Ambiguous implicits"
+  * error for the parameter `a`, and must not use its default value.
+  *
+  * See also:
+  *   - tests/neg/19414.scala
+  *   - tests/neg/19414-desugared.scala
+  *   - tests/neg/given-ambiguous-default-2.scala
+  */
+
+class A
+class B
+given a1: A = ???
+given a2: A = ???
+given (using a: A = A()): B = ???
+
+def f: Unit = summon[B] // error: Ambiguous given instances

tests/neg/given-ambiguous-default-2.check

@@ -0,0 +1,4 @@
+-- [E172] Type Error: tests/neg/given-ambiguous-default-2.scala:19:23 --------------------------------------------------
+19 |def f: Unit = summon[C] // error: Ambiguous given instances
+   |                       ^
+   |Ambiguous given instances: both given instance a1 and given instance a2 match type A of parameter x of method summon in object Predef

tests/neg/given-ambiguous-default-2.scala

@@ -0,0 +1,19 @@
+/** This test checks that provided given instances take precedence over default
+  * given arguments, even when there are multiple default arguments.
+  *
+  * Before the fix for issue #19414, this code would compile without errors.
+  *
+  * See also:
+  *   - tests/neg/given-ambiguous-default-1.scala
+  *   - tests/neg/19414.scala
+  *   - tests/neg/19414-desugared.scala
+  */
+
+class A
+class B
+class C
+given a1: A = ???
+given a2: A = ???
+given (using a: A = A(), b: B = B()): C = ???
+
+def f: Unit = summon[C] // error: Ambiguous given instances