Fix implicit search failure reporting

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

@@ -3707,7 +3707,6 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
 
   private def adapt1(tree: Tree, pt: Type, locked: TypeVars)(using Context): Tree = {
     assert(pt.exists && !pt.isInstanceOf[ExprType] || ctx.reporter.errorsReported, i"tree: $tree, pt: $pt")
-    def methodStr = err.refStr(methPart(tree).tpe)
 
     def readapt(tree: Tree)(using Context) = adapt(tree, pt, locked)
     def readaptSimplified(tree: Tree)(using Context) = readapt(simplify(tree, pt, locked))
@@ -3872,49 +3871,39 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
                 arg :: inferArgsAfter(arg)
         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)
-
-        def issueErrors(): Tree = {
-          def paramSymWithMethodTree(paramName: TermName) =
-            if tree.symbol.exists then
-              tree.symbol.paramSymss.flatten
-                .map(sym => sym.name -> sym)
-                .toMap
-                .get(paramName)
-                .map((_, tree))
-            else
-              None
-
-          wtp.paramNames.lazyZip(wtp.paramInfos).lazyZip(args).foreach { (paramName, formal, arg) =>
-            arg.tpe match {
+        /** Reports errors for arguments of `appTree` that have a
+          * `SearchFailureType`, recursively traversing arguments that are
+          * themselves applications. `mt` must be the type of `appTree.fun`.
+          */
+        def reportErrors(appTree: Apply, mt: MethodType): Unit =
+          val Apply(fun, args) = appTree
+          for (paramName, formal, arg) <- mt.paramNames.lazyZip(mt.paramInfos).lazyZip(args) do
+            arg.tpe match
               case failure: SearchFailureType =>
-                report.error(
-                  missingArgMsg(arg, formal, implicitParamString(paramName, methodStr, tree), paramSymWithMethodTree(paramName)),
-                  tree.srcPos.endPos
-                )
-              case _ =>
-            }
-          }
-          untpd.Apply(tree, args).withType(propFail)
-        }
+                arg match
+                  case childAppTree: Apply =>
+                    childAppTree.fun.tpe.widen match
+                      case childMt: MethodType => reportErrors(childAppTree, childMt)
+                      case _ => ()
+                  case _ => ()
+
+                val methodStr = err.refStr(methPart(fun).tpe)
+                val paramStr = implicitParamString(paramName, methodStr, fun)
+                val paramSymWithMethodCallTree =
+                  fun.symbol.paramSymss.flatten
+                    .find(_.name == paramName)
+                    .map((_, appTree))
+                val message = missingArgMsg(arg, formal, paramStr, paramSymWithMethodCallTree)
+                // Note: if the same error type appears on several trees, we
+                // might report it several times, but this is not a problem
+                // because only the first one will be displayed. We traverse in
+                // post-order, so that the most detailed message gets displayed.
+                report.error(message, fun.srcPos.endPos)
+              case _ => ()
 
-        if (propFail.exists) {
+        val args = implicitArgs(wtp.paramInfos, 0, pt)
+        val firstFailure = args.tpes.find(_.isInstanceOf[SearchFailureType])
+        if (firstFailure.isDefined) {
           // If there are several arguments, some arguments might already
           // have influenced the context, binding variables, but later ones
           // might fail. In that case the constraint and instantiated variables
@@ -3923,18 +3912,36 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
 
           // 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
-            val namedArgs = wtp.paramNames.lazyZip(args).flatMap { (pname, arg) =>
-              if (arg.tpe.isError) Nil else untpd.NamedArg(pname, untpd.TypedSplice(arg)) :: Nil
-            }
+          if hasDefaultParams then
+            // Only keep the arguments that don't have an error type, or that
+            // have an `AmbiguousImplicits` error type. The later ensures that a
+            // default argument can't override an ambiguous implicit. See tests
+            // `given-ambiguous-default*` and `19414*`.
+            val namedArgs =
+              wtp.paramNames.lazyZip(args)
+                .filter((_, arg) => !arg.tpe.isError || arg.tpe.isInstanceOf[AmbiguousImplicits])
+                .map((pname, arg) => untpd.NamedArg(pname, untpd.TypedSplice(arg)))
+
             val app = cpy.Apply(tree)(untpd.TypedSplice(tree), namedArgs)
             val needsUsing = wtp.isContextualMethod || wtp.match
               case MethodType(ContextBoundParamName(_) :: _) => sourceVersion.isAtLeast(`3.4`)
               case _ => false
             if needsUsing then app.setApplyKind(ApplyKind.Using)
             typr.println(i"try with default implicit args $app")
-            typed(app, pt, locked)
-          else issueErrors()
+            val retyped = typed(app, pt, locked)
+
+            // If the retyped tree still has an error type and is an `Apply`
+            // node, we can report the errors for each argument nicely.
+            // Otherwise, we don't report anything here.
+            retyped match
+              case retyped: Apply if retyped.tpe.isError => reportErrors(retyped, wtp)
+              case _ => ()
+
+            retyped
+          else
+            val res = untpd.Apply(tree, args).withType(firstFailure.get)
+            reportErrors(res, wtp)
+            res
         }
         else tree match {
           case tree: Block =>

tests/neg/19414-desugared.check

@@ -0,0 +1,4 @@
+-- [E172] Type Error: tests/neg/19414-desugared.scala:22:34 ------------------------------------------------------------
+22 |  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 writer of given instance given_BodySerializer_B

tests/neg/19414-desugared.scala

@@ -0,0 +1,22 @@
+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 it 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:15:34 ----------------------------------------------------------------------
+15 |  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 a context parameter of given instance given_BodySerializer_B

tests/neg/19414.scala

@@ -0,0 +1,15 @@
+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:18:23 --------------------------------------------------
+18 |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 a of given instance given_B

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

@@ -0,0 +1,18 @@
+/** This test checks that provided ambiguous 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:18:23 --------------------------------------------------
+18 |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 a of given instance given_C

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

@@ -0,0 +1,18 @@
+/** 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

tests/neg/i8827a.check

@@ -1,12 +1,7 @@
 -- [E172] Type Error: tests/neg/i8827a.scala:16:26 ---------------------------------------------------------------------
 16 |  summon[Order[List[Foo]]] // error
    |                          ^
-   | No given instance of type pkg.Order[List[pkg.Foo]] was found for parameter x of method summon in object Predef.
-   | I found:
-   |
-   |     pkg.Order.orderList[pkg.Foo](/* missing */summon[pkg.Order[pkg.Foo]])
-   |
-   | But no implicit values were found that match type pkg.Order[pkg.Foo].
+   | No given instance of type pkg.Order[pkg.Foo] was found for parameter orderA of method orderList in object Order
    |
    | The following import might fix the problem:
    |

tests/neg/i8827b.check

@@ -1,12 +1,7 @@
 -- [E172] Type Error: tests/neg/i8827b.scala:16:28 ---------------------------------------------------------------------
 16 |  summon[Order[Option[Foo]]] // error
    |                            ^
-   |No given instance of type pkg.Order[Option[pkg.Foo]] was found for parameter x of method summon in object Predef.
-   |I found:
-   |
-   |    pkg.Order.given_Order_Option[pkg.Foo](/* missing */summon[pkg.Order[pkg.Foo]])
-   |
-   |But no implicit values were found that match type pkg.Order[pkg.Foo].
+   |No given instance of type pkg.Order[pkg.Foo] was found for parameter orderA of given instance given_Order_Option in object Order
    |
    |The following import might fix the problem:
    |

tests/neg/i9568.check

@@ -4,13 +4,10 @@
    |          No given instance of type => Monad[F] was found for parameter ev of method blaMonad in object Test.
    |          I found:
    |
-   |              Test.blaMonad[F², S](Test.blaMonad[F³, S²])
+   |              Test.blaMonad[F², S]
    |
-   |          But method blaMonad in object Test does not match type => Monad[F²]
+   |          But method blaMonad in object Test does not match type => Monad[F]
    |
    |          where:    F  is a type variable with constraint <: [_] =>> Any
    |                    F² is a type variable with constraint <: [_] =>> Any
-   |                    F³ is a type variable with constraint <: [_] =>> Any
-   |                    S  is a type variable
-   |                    S² is a type variable
    |          .

tests/neg/implicitSearch.check

@@ -1,12 +1,7 @@
 -- [E172] Type Error: tests/neg/implicitSearch.scala:13:12 -------------------------------------------------------------
 13 |    sort(xs)  // error (with a partially constructed implicit argument shown)
    |            ^
-   |      No given instance of type Test.Ord[List[List[T]]] was found for parameter o of method sort in object Test.
-   |      I found:
-   |
-   |          Test.listOrd[List[T]](Test.listOrd[T](/* missing */summon[Test.Ord[T]]))
-   |
-   |      But no implicit values were found that match type Test.Ord[T].
+   |            No given instance of type Test.Ord[T] was found for parameter o of method listOrd in object Test
 -- [E172] Type Error: tests/neg/implicitSearch.scala:15:38 -------------------------------------------------------------
 15 |    listOrd(listOrd(implicitly[Ord[T]] /*not found*/)) // error
    |                                      ^

tests/neg/missing-implicit3.check

@@ -1,14 +1,9 @@
 -- [E172] Type Error: tests/neg/missing-implicit3.scala:13:36 ----------------------------------------------------------
 13 |val sortedFoos = sort(List(new Foo)) // error
    |                                    ^
-   |     No given instance of type ord.Ord[ord.Foo] was found for a context parameter of method sort in package ord.
-   |     I found:
+   |No given instance of type ord.Foo => Comparable[? >: ord.Foo] was found for parameter x$1 of given instance ordered in object Ord
    |
-   |         ord.Ord.ordered[ord.Foo](/* missing */summon[ord.Foo => Comparable[? >: ord.Foo]])
+   |The following import might make progress towards fixing the problem:
    |
-   |     But no implicit values were found that match type ord.Foo => Comparable[? >: ord.Foo].
-   |
-   |     The following import might make progress towards fixing the problem:
-   |
-   |       import scala.math.Ordered.orderingToOrdered
+   |  import scala.math.Ordered.orderingToOrdered
    |