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strange error for missing using keyword in implicit argument #19680

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Dieken opened this issue Feb 13, 2024 · 7 comments · Fixed by #20023
Closed

strange error for missing using keyword in implicit argument #19680

Dieken opened this issue Feb 13, 2024 · 7 comments · Fixed by #20023
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@mbovel
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area:implicits related to implicits area:reporting Error reporting including formatting, implicit suggestions, etc better-errors Issues concerned with improving confusing/unhelpful diagnostic messages itype:enhancement
Milestone
3.5.0

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@Dieken
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Dieken commented Feb 13, 2024

Compiler version

3.3.1 and 3.4.0-RC4.

Minimized example

@main
def main(args: String*): Unit =
  println("hello")
  val config = Config(8080, "docs.scala-lang.org")
  given Config = config
  println(renderWebsite("/home")(config))

case class Config(port: Int, baseUrl: String)

def renderWebsite(path: String)(using config: Config): String =
  "<html>" + renderWidget(List("cart")) + "</html>"

def renderWidget(items: List[String])(using config: Config): String =
  "<div>" + items.mkString(
    ", "
  ) + s" ${config.port} ${config.baseUrl}" + "</div>"

Output Error/Warning message

-- [E007] Type Mismatch Error: /private/tmp/a.scala:6:33 -----------------------
6 |  println(renderWebsite("/home")(config))
  |                                 ^^^^^^
  |                                 Found:    (config : Config)
  |                                 Required: Int
  |
  | longer explanation available when compiling with `-explain`
1 error found
Errors encountered during compilation

Why this Error/Warning was not helpful

Obviously the error message is wrong.

Suggested improvement

It should remind me removing the whole argument or adding keyword using in line 6.
@Dieken Dieken added area:reporting Error reporting including formatting, implicit suggestions, etc better-errors Issues concerned with improving confusing/unhelpful diagnostic messages itype:enhancement stat:needs triage Every issue needs to have an "area" and "itype" label labels Feb 13, 2024
@Gedochao Gedochao added area:implicits related to implicits and removed stat:needs triage Every issue needs to have an "area" and "itype" label labels Feb 13, 2024
@som-snytt
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The message is not wrong, but it would be nice to use the Did you mean...? idiom to add that there was an implicit arg (in last position) in the application of the same type.

Did you mean `using config`? The application already includes a given Config.

It would also be nice for the more usual failure case where there is no given in scope.

Did you mean `using config`? The application requires a given Config.

Migrating a code base from implicit is likely to encounter this mistake.

@odersky
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odersky commented Feb 14, 2024 •
edited
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I think this is a very special case, and one that would likely detract from the true source of errors in 99.9% of the cases. So I am against making a special "did you mean" for this.

On the other hand, even without the implicit parameter complication, the error message is a bit baffling:

                                Found:    (config : Config)
                                Required: Int

Where does the Int come from? I think we should try to explain that it is the apply method of the result type of renderWebSite. Something like:

                                Found:    (config : Config)
                                Required: Int

                                The required type comes from a parameter of the automatically inserted `apply` method of `String`,
                                which is the result type of `renderWebSite`

Or alternatively, show the full expanded call (but this might become very large sometimes).

@mbovel mbovel self-assigned this Feb 14, 2024
@mbovel mbovel added the Spree Suitable for a future Spree label Feb 14, 2024
@som-snytt
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The challenge is to report something in brief (and not under -explain).

Maybe just for the case of "arg of correct type for some parameter in the expression (but perhaps lacks using)" then give the OP the suggested improvement (in some form, perhaps just the expansion).

The required type is just misdirection. (Reminiscent of errors from the days of postfixOps, a puzzler.)

I don't know if OP is in 0.1% of users.

@mbovel
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mbovel commented Mar 7, 2024

Further minimized:

class Config()
def renderWebsite(path: String)(using config: Config): String = ???
def renderWidget(using Config): Unit = renderWebsite("/tmp")(Config())
-- [E007] Type Mismatch Error: tests/pos/19680.scala:3:67 ----------------------
3 |def renderWidget(using Config): Unit = renderWebsite("/tmp")(Config())
  |                                                             ^^^^^^^^
  |                                                          Found:    Config
  |                                                          Required: Int
  |
  | longer explanation available when compiling with `-explain`

Note that if there is no Config given in scope, then we have an error message about the missing implicit:

class Config()
def renderWebsite(path: String)(using config: Config): String = ???
def renderWidget2: Unit = renderWebsite("/tmp")(Config())
-- [E172] Type Error: tests/pos/19680.scala:3:47 -------------------------------
3 |def renderWidget2: Unit = renderWebsite("/tmp")(Config())
  |                                               ^
  |No given instance of type Config was found for parameter config of method renderWebsite

And here is the error stack trace, for the first case, with the Config given:

        at dotty.tools.dotc.report$.error(report.scala:70)
        at dotty.tools.dotc.typer.ErrorReporting$.errorType(ErrorReporting.scala:32)
        at dotty.tools.dotc.typer.ErrorReporting$.errorTree(ErrorReporting.scala:23)
        at dotty.tools.dotc.typer.ErrorReporting$.errorTree(ErrorReporting.scala:26)
        at dotty.tools.dotc.typer.ErrorReporting$Errors.typeMismatch(ErrorReporting.scala:197)
        at dotty.tools.dotc.typer.Typer.recover$1(Typer.scala:4246)
        at dotty.tools.dotc.typer.Typer.adaptToSubType$1(Typer.scala:4270)
        at dotty.tools.dotc.typer.Typer.adaptNoArgsOther$1(Typer.scala:4079)
        at dotty.tools.dotc.typer.Typer.adaptNoArgs$1(Typer.scala:4159)
        at dotty.tools.dotc.typer.Typer.adapt1(Typer.scala:4386)
        at dotty.tools.dotc.typer.Typer.adapt(Typer.scala:3682)
        at dotty.tools.dotc.typer.ProtoTypes$FunProto.typedArg(ProtoTypes.scala:513)
        at dotty.tools.dotc.typer.Applications$ApplyToUntyped.typedArg(Applications.scala:913)
        at dotty.tools.dotc.typer.Applications$ApplyToUntyped.typedArg(Applications.scala:913)
        at dotty.tools.dotc.typer.Applications$Application.addTyped$1(Applications.scala:605)
        at dotty.tools.dotc.typer.Applications$Application.matchArgs(Applications.scala:669)
        at dotty.tools.dotc.typer.Applications$Application.init(Applications.scala:491)
        at dotty.tools.dotc.typer.Applications$TypedApply.<init>(Applications.scala:795)
        at dotty.tools.dotc.typer.Applications$ApplyToUntyped.<init>(Applications.scala:912)
        at dotty.tools.dotc.typer.Applications.ApplyTo(Applications.scala:1142)
        at dotty.tools.dotc.typer.Applications.ApplyTo$(Applications.scala:351)
        at dotty.tools.dotc.typer.Typer.ApplyTo(Typer.scala:120)
        at dotty.tools.dotc.typer.Applications.simpleApply$1(Applications.scala:985)
        at dotty.tools.dotc.typer.Applications.realApply$1$$anonfun$2(Applications.scala:1068)
        at dotty.tools.dotc.typer.Typer.tryEither(Typer.scala:3418)
        at dotty.tools.dotc.typer.Applications.realApply$1(Applications.scala:1079)
        at dotty.tools.dotc.typer.Applications.typedApply(Applications.scala:1117)
        at dotty.tools.dotc.typer.Applications.typedApply$(Applications.scala:351)
        at dotty.tools.dotc.typer.Typer.typedApply(Typer.scala:120)
        at dotty.tools.dotc.typer.Typer.typedUnnamed$1(Typer.scala:3118)
        at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:3202)
        at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3279)
        at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3283)
        at dotty.tools.dotc.typer.Typer.typedExpr(Typer.scala:3394)
        at dotty.tools.dotc.typer.Typer.$anonfun$62(Typer.scala:2608)
        at dotty.tools.dotc.inlines.PrepareInlineable$.dropInlineIfError(PrepareInlineable.scala:256)
        at dotty.tools.dotc.typer.Typer.typedDefDef(Typer.scala:2608)
        at dotty.tools.dotc.typer.Typer.typedNamed$1(Typer.scala:3100)
        at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:3201)
        at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3279)
        at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3283)
        at dotty.tools.dotc.typer.Typer.traverse$1(Typer.scala:3305)
        at dotty.tools.dotc.typer.Typer.typedStats(Typer.scala:3351)
        at dotty.tools.dotc.typer.Typer.typedClassDef(Typer.scala:2795)
        at dotty.tools.dotc.typer.Typer.typedTypeOrClassDef$1(Typer.scala:3106)
        at dotty.tools.dotc.typer.Typer.typedNamed$1(Typer.scala:3110)
        at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:3201)
        ...

@mbovel
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mbovel commented Mar 15, 2024

This issue was picked for the Issue Spree of March 19th, 2024. @mbovel, @iusildra, @jan-pieter will be working on it. If you have any insight into the issue or guidance on how to fix it, please leave it here.

@mbovel
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mbovel commented Mar 19, 2024

It's not clear to me how the info about the expected type should flow from FunProto.typedArg to the point where we throw the message, as it seems we don't have this info starting from Typer.adapt.

@mbovel mbovel removed the Spree Suitable for a future Spree label Apr 8, 2024
@mbovel mbovel mentioned this issue Apr 8, 2024
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@mbovel
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mbovel commented Apr 29, 2024

The challenge is to report something in brief (and not under -explain).

My PR unfortunately currently only adds the message under -explain because I am afraid it could show it too many cases otherwise. I hope this can still be useful.

odersky added a commit that referenced this issue May 1, 2024
@odersky
Add note about type mismatch in automatically inserted apply argument (…
084ab1a 
…#20023)

Started during the last spree with @jan-pieter and @iusildra.

Fixes #19680.
@Kordyjan Kordyjan added this to the 3.5.0 milestone May 10, 2024
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Labels
area:implicits related to implicits area:reporting Error reporting including formatting, implicit suggestions, etc better-errors Issues concerned with improving confusing/unhelpful diagnostic messages itype:enhancement
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Add note about type mismatch in automatically inserted apply argument mbovel/dotty
6 participants
@Dieken @som-snytt @mbovel @odersky @Kordyjan @Gedochao