Restrict implicit args to using

Typer#adaptNoArgsImplicitMethod populates implicit args
when an arg list is missing. To remedy missing implicits,
it tries a named application `using` args it did find.
Then Applications#tryDefault supplies a default arg if
available. A previous fix to allow tryDefault to supply
implicit args for `implicit` params is now restricted
to explicit `using`; typer now adds `using` for `implicit`
when it needs to try defaults.

This commit restores propagatedFailure and the previous
condition that default params are tried if there is an error
that is not an ambiguity. An additional restriction is that
default params must be useful: there must be a param which
has a default arg to be added (because it's not a named arg).

Author: som-snytt

compiler/src/dotty/tools/dotc/transform/CheckUnused.scala

@@ -525,7 +525,7 @@ object CheckUnused:
     def checkPrivate(sym: Symbol, pos: SrcPos) =
       if ctx.settings.WunusedHas.privates
         && !sym.isPrimaryConstructor
-        && sym.is(Private, butNot = SelfName | Synthetic | CaseAccessor)
+        && !sym.isOneOf(SelfName | Synthetic | CaseAccessor)
         && !sym.name.is(BodyRetainerName)
         && !sym.isSerializationSupport
         && !(sym.is(Mutable) && sym.isSetter && sym.owner.is(Trait)) // tracks sym.underlyingSymbol sibling getter

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

@@ -673,13 +673,14 @@ trait Applications extends Compatibility {
               }
               else defaultArgument(normalizedFun, n, testOnly)
 
-            def implicitArg = implicitArgTree(formal, appPos.span)
-
             if !defaultArg.isEmpty then
               defaultArg.tpe.widen match
                 case _: MethodOrPoly if testOnly => matchArgs(args1, formals1, n + 1)
                 case _ => matchArgs(args1, addTyped(treeToArg(defaultArg)), n + 1)
-            else if methodType.isImplicitMethod && ctx.mode.is(Mode.ImplicitsEnabled) then
+            else if (methodType.isContextualMethod || applyKind == ApplyKind.Using && methodType.isImplicitMethod)
+              && ctx.mode.is(Mode.ImplicitsEnabled)
+            then
+              val implicitArg = implicitArgTree(formal, appPos.span)
               matchArgs(args1, addTyped(treeToArg(implicitArg)), n + 1)
             else
               missingArg(n)

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

@@ -514,7 +514,7 @@ object Implicits:
     override def toString = s"TooUnspecific"
 
   /** An ambiguous implicits failure */
-  class AmbiguousImplicits(val alt1: SearchSuccess, val alt2: SearchSuccess, val expectedType: Type, val argument: Tree, val nested: Boolean = false) extends SearchFailureType {
+  class AmbiguousImplicits(val alt1: SearchSuccess, val alt2: SearchSuccess, val expectedType: Type, val argument: Tree, val nested: Boolean = false) extends SearchFailureType:
 
     def msg(using Context): Message =
       var str1 = err.refStr(alt1.ref)
@@ -531,7 +531,10 @@ object Implicits:
         i"""
            |Note that implicit $what cannot be applied because they are ambiguous;
            |$explanation"""
-  }
+
+    def asNested = if nested then this else AmbiguousImplicits(alt1, alt2, expectedType, argument, nested = true)
+
+  end AmbiguousImplicits
 
   class MismatchedImplicit(ref: TermRef,
                            val expectedType: Type,

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

@@ -36,7 +36,6 @@ import util.{Property, SimpleIdentityMap, SrcPos}
 import Applications.{tupleComponentTypes, wrapDefs, defaultArgument}
 
 import collection.mutable
-import annotation.tailrec
 import Implicits.*
 import util.Stats.record
 import config.Printers.{gadts, typr}
@@ -53,7 +52,9 @@ import config.Config
 import transform.CheckUnused.OriginalName
 
 import scala.annotation.constructorOnly
+import scala.annotation.{unchecked as _, *}
 import dotty.tools.dotc.rewrites.Rewrites
+import dotty.tools.dotc.util.chaining.*
 
 object Typer {
 
@@ -3842,6 +3843,12 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
 
       def addImplicitArgs(using Context) =
         def hasDefaultParams = methPart(tree).symbol.hasDefaultParams
+        def findDefaultArgument(argIndex: Int): Tree =
+          def appPart(t: Tree): Tree = t match
+            case Block(_, expr)      => appPart(expr)
+            case Inlined(_, _, expr) => appPart(expr)
+            case t => t
+          defaultArgument(appPart(tree), n = argIndex, testOnly = false)
         def implicitArgs(formals: List[Type], argIndex: Int, pt: Type): List[Tree] = formals match
           case Nil => Nil
           case formal :: formals1 =>
@@ -3863,13 +3870,8 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
                 then implicitArgs(formals, argIndex, pt1)
                 else arg :: implicitArgs(formals1, argIndex + 1, pt1)
               case failed: SearchFailureType =>
-                lazy val defaultArg =
-                  def appPart(t: Tree): Tree = t match
-                    case Block(stats, expr) => appPart(expr)
-                    case Inlined(_, _, expr) => appPart(expr)
-                    case _ => t
-                  defaultArgument(appPart(tree), argIndex, testOnly = false)
-                    .showing(i"default argument: for $formal, $tree, $argIndex = $result", typr)
+                lazy val defaultArg = findDefaultArgument(argIndex)
+                  .showing(i"default argument: for $formal, $tree, $argIndex = $result", typr)
                 if !hasDefaultParams || defaultArg.isEmpty then
                   // no need to search further, the adapt fails in any case
                   // the reason why we continue inferring arguments in case of an AmbiguousImplicits
@@ -3891,44 +3893,44 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
                 arg :: inferArgsAfter(arg)
         end implicitArgs
 
-        /** Reports errors for arguments of `appTree` that have a
-          * `SearchFailureType`.
-          */
-        def issueErrors(fun: Tree, args: List[Tree]): Tree =
-          // Prefer other errors over ambiguities. If nested in outer searches a missing
-          // implicit can be healed by simply dropping this alternative and trying 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 =>
-                AmbiguousImplicits(tp.alt1, tp.alt2, tp.expectedType, tp.argument, nested = true)
-              case tp =>
-                tp
-          val res = untpd.Apply(fun, args).withType(errorType)
-
-          wtp.paramNames.lazyZip(wtp.paramInfos).lazyZip(args).foreach { (paramName, formal, arg) =>
-            arg.tpe match
-              case failure: SearchFailureType =>
-                val methodStr = err.refStr(methPart(fun).tpe)
-                val paramStr = implicitParamString(paramName, methodStr, fun)
-                val paramSym = fun.symbol.paramSymss.flatten.find(_.name == paramName)
-                val paramSymWithMethodCallTree = paramSym.map((_, res))
-                report.error(
-                   missingArgMsg(arg, formal, paramStr, paramSymWithMethodCallTree),
-                   tree.srcPos.endPos
-                 )
-              case _ =>
-          }
-
-          res
+        // Pick a failure type to propagate, if any.
+        // Prefer other errors over ambiguities. If nested in outer searches a missing
+        // implicit can be healed by simply dropping this alternative and trying 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.
+        def propagatedFailure(args: List[Tree]): Type = args match
+          case arg :: args => arg.tpe match
+            case ambi: AmbiguousImplicits => propagatedFailure(args) match
+              case NoType | (_: AmbiguousImplicits) => ambi
+              case failed => failed
+            case failed: SearchFailureType => failed
+            case _ => propagatedFailure(args)
+          case Nil => NoType
+
+        /** Reports errors for arguments of `appTree` that have a `SearchFailureType`.
+         */
+        def issueErrors(fun: Tree, args: List[Tree], failureType: Type): Tree =
+          val errorType = failureType match
+            case ai: AmbiguousImplicits => ai.asNested
+            case tp => tp
+          untpd.Apply(fun, args)
+            .withType(errorType)
+            .tap: res =>
+              wtp.paramNames.lazyZip(wtp.paramInfos).lazyZip(args).foreach: (paramName, formal, arg) =>
+                arg.tpe match
+                case failure: SearchFailureType =>
+                  val methodStr = err.refStr(methPart(fun).tpe)
+                  val paramStr = implicitParamString(paramName, methodStr, fun)
+                  val paramSym = fun.symbol.paramSymss.flatten.find(_.name == paramName)
+                  val paramSymWithMethodCallTree = paramSym.map((_, res))
+                  val msg = missingArgMsg(arg, formal, paramStr, paramSymWithMethodCallTree)
+                  report.error(msg, tree.srcPos.endPos)
+                case _ =>
 
         val args = implicitArgs(wtp.paramInfos, 0, pt)
-        if (args.tpes.exists(_.isInstanceOf[SearchFailureType])) {
+        val failureType = propagatedFailure(args)
+        if failureType.exists then
           // 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
@@ -3937,29 +3939,36 @@ 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 then
+          if hasDefaultParams && !failureType.isInstanceOf[AmbiguousImplicits] then
             // Only keep the arguments that don't have an error type, or that
-            // have an `AmbiguousImplicits` error type. The later ensures that a
+            // have an `AmbiguousImplicits` error type. The latter 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)
-            if (wtp.isContextualMethod) app.setApplyKind(ApplyKind.Using)
-            typr.println(i"try with default implicit args $app")
-            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 Apply(tree, args) if retyped.tpe.isError => issueErrors(tree, args)
-              case _ => retyped
-          else issueErrors(tree, args)
-        }
+              wtp.paramNames.lazyZip(args).collect:
+                case (pname, arg) if !arg.tpe.isError || arg.tpe.isInstanceOf[AmbiguousImplicits] =>
+                  untpd.NamedArg(pname, untpd.TypedSplice(arg))
+              .toList
+            val usingDefaultArgs =
+              wtp.paramNames.zipWithIndex
+                .exists((n, i) => !namedArgs.exists(_.name == n) && !findDefaultArgument(i).isEmpty)
+
+            if !usingDefaultArgs then
+              issueErrors(tree, args, failureType)
+            else
+              val app = cpy.Apply(tree)(untpd.TypedSplice(tree), namedArgs)
+              // old-style implicit needs to be marked using so that implicits are searched
+              typr.println(i"try with default implicit args $app")
+              // 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.
+              typed(app, pt, locked) match
+                case retyped @ Apply(tree, args) if retyped.tpe.isError =>
+                  propagatedFailure(args) match
+                  case sft: SearchFailureType => issueErrors(tree, args, sft)
+                  case _ => issueErrors(tree, args, retyped.tpe)
+                case retyped => retyped
+          else issueErrors(tree, args, failureType)
         else
           inContext(origCtx):
             // Reset context in case it was set to a supercall context before.

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

@@ -1,9 +1,9 @@
 -- [E172] Type Error: tests/neg/given-ambiguous-default-1.scala:18:23 --------------------------------------------------
 18 |def f: Unit = summon[B] // error: Ambiguous given instances
    |                       ^
-   |            No best given instance of type B was found for parameter x of method summon in object Predef.
-   |            I found:
+   |                No best given instance of type B was found for parameter x of method summon in object Predef.
+   |                I found:
    |
-   |                given_B(a = /* ambiguous: both given instance a1 and given instance a2 match type A */summon[A])
+   |                    given_B(/* ambiguous: both given instance a1 and given instance a2 match type A */summon[A])
    |
-   |            But both given instance a1 and given instance a2 match type A.
+   |                But both given instance a1 and given instance a2 match type A.

tests/neg/i19594.check

@@ -1,8 +1,20 @@
--- [E172] Type Error: tests/neg/i19594.scala:12:14 ---------------------------------------------------------------------
-12 |  assertEquals(true, 1, "values are not the same") // error
-   |  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   |  Can you see me?!
--- [E172] Type Error: tests/neg/i19594.scala:13:14 ---------------------------------------------------------------------
-13 |  assertEquals(true, 1) // error
-   |  ^^^^^^^^^^^^^^^^^^^^^
-   |  Can you see me?!
+-- [E172] Type Error: tests/neg/i19594.scala:15:50 ---------------------------------------------------------------------
+15 |  assertEquals(true, 1, "values are not the same") // error
+   |                                                  ^
+   |                                                  Can you see me?!
+-- [E172] Type Error: tests/neg/i19594.scala:16:23 ---------------------------------------------------------------------
+16 |  assertEquals(true, 1) // error
+   |                       ^
+   |                       Can you see me?!
+-- [E172] Type Error: tests/neg/i19594.scala:20:12 ---------------------------------------------------------------------
+20 |  f(true, 1) // error
+   |            ^
+   |            Can you see me?!
+-- [E172] Type Error: tests/neg/i19594.scala:23:39 ---------------------------------------------------------------------
+23 |  g(true, 1, "values are not the same") // error
+   |                                       ^
+   |                                       Can you see me?!
+-- [E172] Type Error: tests/neg/i19594.scala:26:3 ----------------------------------------------------------------------
+26 |  h(true, 1) // error
+   |  ^^^^^^^^^^
+   |  No given instance of type String was found

tests/neg/i19594.scala

@@ -1,7 +1,10 @@
 import scala.annotation.implicitNotFound
 
 @implicitNotFound("Can you see me?!")
-trait Compare[A, B]
+trait Compare[-A, -B]
+
+object Compare:
+  val any: Compare[Any, Any] = new Compare {}
 
 object example extends App:
 
@@ -11,3 +14,13 @@ object example extends App:
 
   assertEquals(true, 1, "values are not the same") // error
   assertEquals(true, 1) // error
+
+object updated:
+  def f[A, B](a: A, b: B)(using Compare[A, B]) = ()
+  f(true, 1) // error
+
+  def g[A, B](a: A, b: B, clue: => Any)(implicit comp: Compare[A, B]) = ()
+  g(true, 1, "values are not the same") // error
+
+  def h[A, B](a: A, b: B)(using c: Compare[A, B] = Compare.any, s: String) = ()
+  h(true, 1) // error

tests/neg/i22439.check

@@ -0,0 +1,26 @@
+-- [E171] Type Error: tests/neg/i22439.scala:7:3 -----------------------------------------------------------------------
+7 |  f() // error f() missing arg
+  |  ^^^
+  |  missing argument for parameter i of method f: (implicit i: Int, j: Int): Int
+-- [E050] Type Error: tests/neg/i22439.scala:8:2 -----------------------------------------------------------------------
+8 |  g() // error g(given_Int, given_Int)() doesn't take more params
+  |  ^
+  |  method g does not take more parameters
+  |
+  | longer explanation available when compiling with `-explain`
+-- [E171] Type Error: tests/neg/i22439.scala:11:3 ----------------------------------------------------------------------
+11 |  f(j = 27) // error missing argument for parameter i of method f
+   |  ^^^^^^^^^
+   |  missing argument for parameter i of method f: (implicit i: Int, j: Int): Int
+-- [E172] Type Error: tests/neg/i22439.scala:16:3 ----------------------------------------------------------------------
+16 |  h // error
+   |   ^
+   |   No given instance of type String was found for parameter s of method h
+-- [E172] Type Error: tests/neg/i22439.scala:17:3 ----------------------------------------------------------------------
+17 |  h(using i = 17) // error
+   |  ^^^^^^^^^^^^^^^
+   |  No given instance of type String was found
+-- [E171] Type Error: tests/neg/i22439.scala:21:25 ---------------------------------------------------------------------
+21 |  val (ws, zs) = vs.unzip() // error!
+   |                 ^^^^^^^^^^
+   |missing argument for parameter asPair of method unzip in trait StrictOptimizedIterableOps: (implicit asPair: ((Int, Int)) => (A1, A2)): (List[A1], List[A2])

tests/neg/i22439.scala

@@ -0,0 +1,21 @@
+
+@main def test() = println:
+  given Int = 42
+  def f(implicit i: Int, j: Int) = i + j
+  def g(using i: Int, j: Int) = i + j
+  val x: Int = f
+  f() // error f() missing arg
+  g() // error g(given_Int, given_Int)() doesn't take more params
+  f // ok implicits
+  g // ok implicits
+  f(j = 27) // error missing argument for parameter i of method f
+  f(using j = 27) // ok, explicit supplemented by implicit
+  g(using j = 27) // ok, explicit supplemented by implicit
+
+  def h(using i: Int, s: String) = s * i
+  h // error
+  h(using i = 17) // error
+
+  val vs = List((42, 27))
+  val (xs, ys) = vs.unzip
+  val (ws, zs) = vs.unzip() // error!

tests/run/extra-implicits.scala

@@ -1,18 +1,18 @@
 
 case class A(x: String)
 case class B(x: String)
-given a1: A("default")
-given b1: B("default")
-val a2 = A("explicit")
-val b2 = B("explicit")
+given A("default")
+given B("default")
+val a = A("explicit")
+val b = B("explicit")
 
 def f(using a: A, b: B): Unit =
   println(a)
   println(b)
 
 @main def Test =
-  f(using a2)
-  f(using a = a2)
-  f(using b = b2)
-  f(using b = b2, a = a2)
+  f(using a)
+  f(using a = a)
+  f(using b = b)
+  f(using b = b, a = a)