Unable to match a lower-case constructor in quoted pattern #22616

Kalin-Rudnicki · 2025-02-17T05:11:23Z

Compiler version

3.6.3

Minimized code

final case class caseName(name: String) extends scala.annotation.Annotation
object caseName {

  given FromExpr[caseName] =
    new FromExpr[caseName] {
      override def unapply(x: Expr[caseName])(using Quotes): Option[caseName] =
        x match {
          case '{ caseName(${ Expr(name) }) }     => caseName(name).some
          // with/without the following line...
          case '{ new caseName(${ Expr(name) }) } => caseName(name).some
          case _                                  => println(x.show); None
        }
    }

}

sealed trait SealedTrait3[+A, +B] derives Show
object SealedTrait3 {
  final case class AB1[+B, +A](a: B, b: A) extends SealedTrait3[B, A]
  final case class AB2[+C, +D](a: C, b: D) extends SealedTrait3[D, C]
  final case class A[+A](a: A) extends SealedTrait3[A, Nothing]
  @caseName("_B_") final case class B[+B](b: B) extends SealedTrait3[Nothing, B]
  case object Neither extends SealedTrait3[Nothing, Nothing]
}

    def optionalAnnotation[Annot: Type]: Option[Expr[Annot]] = {
      val annotTpe = TypeRepr.of[Annot]
      val annotFlags = annotTpe.typeSymbol.flags

      if (annotFlags.is(Flags.Abstract) || annotFlags.is(Flags.Trait))
        report.errorAndAbort(s"Bad annotation type ${annotTpe.show} is abstract")

      this.getAnnotation(annotTpe.typeSymbol) match
        case Some(tree) if tree.tpe <:< annotTpe => tree.asExprOf[Annot].some
        case _                                   => None
    }

    def requiredAnnotation[Annot: Type]: Expr[Annot] =
      optionalAnnotation[Annot].getOrElse(report.errorAndAbort(s"Missing required annotation `${TypeRepr.of[Annot].show}` for `$this`"))

    def optionalAnnotationValue[Annot: {Type, FromExpr}]: Option[Annot] =
      optionalAnnotation[Annot].map { expr =>
        expr.value.getOrElse(report.errorAndAbort(s"Found annotation `${TypeRepr.of[Annot].show}` for `$this`, but are unable to extract Expr.value\n${expr.show}"))
      }

    def requiredAnnotationValue[Annot: {Type, FromExpr}]: Annot = {
      val expr = requiredAnnotation[Annot]
      expr.value.getOrElse(report.errorAndAbort(s"Found annotation `${TypeRepr.of[Annot].show}` for `$this`, but are unable to extract Expr.value\n${expr.show}"))
    }

Output

[error]    |Found annotation `oxygen.meta.example.caseName` for `oxygen.meta.example.SealedTrait3.B[B]`, but are unable to extract Expr.value
[error]    |new oxygen.meta.example.caseName("_B_")

then add a case for that in the FromExpr, and...

[error] 28 |          case '{ new caseName(${ Expr(name) }) } => caseName(name).some
[error]    |                      ^^^^^^^^
[error]    |                      caseName does not have a constructor

Expectation

That either:

case '{ caseName(${ Expr(name) }) } => caseName(name).some matches
case '{ new caseName(${ Expr(name) }) } => caseName(name).some is allowed, and matches

Quite difficult to handle when you see "we found this, but cant match on it", and then when you try to match on the exact expr.show, it says that expr is not valid?

The text was updated successfully, but these errors were encountered:

som-snytt · 2025-02-17T06:54:16Z

I don't know the details, but a recent conversation about annotation includes #22553

mbovel · 2025-02-17T15:12:14Z

I can have a look; @Gedochao can we assign this to me for this cycle?

Kalin-Rudnicki · 2025-02-17T17:16:03Z

Thanks @mbovel!
Also, reading the description for scala.annotation.ConstantAnnotation, it really seems like for any A <: ConstantAnnotation, the compiler should be providing a given FromExpr[A]. Im curious your thoughts on this, and whether that might be doable here as well?

Kalin-Rudnicki · 2025-02-17T17:24:57Z

I guess, generally it would be nice if the compiler provided a built-in mechanism for deriving FromExpr[A] for any product/sum type, but I believe it would be correct to say that a correctly written auto-derive for a ConstantAnnotation should always return Some[A]?

mbovel · 2025-04-23T12:33:12Z

The problem does not seem linked to annotations or classes extending annotations. It seems to be linked to classes with lower-case names.

Compiles successfully:

import scala.quoted.{Expr, Quotes}

class Foo(name: String) extends scala.annotation.StaticAnnotation

object Macro:
  inline def myMacro3(): Unit =
    ${ myMacroImpl3('{new Foo("hello")}) }

  def myMacroImpl3(x: Expr[Foo])(using q: Quotes): Expr[Unit] =
    x match
      case '{new Foo(${Expr(name)})} =>
        println(name)
      case _ =>
        println("not a Foo")

    '{()}

$ scala compile --server=false -S 3.6.3 22616
hello

Errors out:

import scala.quoted.{FromExpr, Expr, Quotes}

class foo(name: String)

object Macro:
  inline def myMacro3(): Unit =
    ${ myMacroImpl3('{new foo("hello")}) }

  def myMacroImpl3(x: Expr[foo])(using q: Quotes): Expr[Unit] =
    x match
      case '{new foo(${Expr(name)})} =>
        println(name)
      case _ =>
        println("not a foo")

    '{()}

$ scala compile --server=false -S 3.6.3 22616
-- Error: /localhome/bovel/scala3/22616/defs.scala:11:17 -------------------------------------------------------------------------------------------------------------------------
11 |      case '{new foo(${Expr(name)})} =>
   |                 ^^^
   |                 foo does not have a constructor
-- Error: /localhome/bovel/scala3/22616/defs.scala:11:27 -------------------------------------------------------------------------------------------------------------------------
11 |      case '{new foo(${Expr(name)})} =>
   |                       ^^^^^^^^^^
   |                       Type must be fully defined.
   |                       Consider annotating the splice using a type ascription:
   |                         (${Expr(name)}: XYZ).
-- [E006] Not Found Error: /localhome/bovel/scala3/22616/defs.scala:12:16 --------------------------------------------------------------------------------------------------------
12 |        println(name)
   |                ^^^^
   |                Not found: name - did you mean Some?
   |
   | longer explanation available when compiling with `-explain`
3 errors found
Compilation failed

$ scala compile -Ydebug-error --server=false -S 3.6.3 22616
-- Error: /localhome/bovel/scala3/22616/defs.scala:11:17 -------------------------------------------------------------------------------------------------------------------------
11 |      case '{new foo(${Expr(name)})} =>
   |                 ^^^
   |                 foo does not have a constructor
java.lang.Exception: Stack trace
        at java.base/java.lang.Thread.dumpStack(Thread.java:1389)
        at dotty.tools.dotc.report$.error(report.scala:70)
        at dotty.tools.dotc.typer.ErrorReporting$.errorType(ErrorReporting.scala:32)
        at dotty.tools.dotc.typer.TypeAssigner.notAMemberErrorType(TypeAssigner.scala:186)
        at dotty.tools.dotc.typer.TypeAssigner.notAMemberErrorType$(TypeAssigner.scala:16)
        at dotty.tools.dotc.typer.Typer.notAMemberErrorType(Typer.scala:148)
        at dotty.tools.dotc.typer.Typer.reportAnError$1(Typer.scala:897)
        at dotty.tools.dotc.typer.Typer.typedSelectWithAdapt(Typer.scala:911)
        at dotty.tools.dotc.typer.Typer.typeSelectOnTerm$1(Typer.scala:996)
        at dotty.tools.dotc.typer.Typer.typedSelect(Typer.scala:1034)
        at dotty.tools.dotc.typer.Typer.typedNamed$1(Typer.scala:3475)
        at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:3584)
        at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3662)
        at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3666)
        at dotty.tools.dotc.typer.Typer.typedExpr(Typer.scala:3777)
        at dotty.tools.dotc.typer.Applications.realApply$1(Applications.scala:1051)
        at dotty.tools.dotc.typer.Applications.typedApply(Applications.scala:1244)
        at dotty.tools.dotc.typer.Applications.typedApply$(Applications.scala:434)
        at dotty.tools.dotc.typer.Typer.typedApply(Typer.scala:148)
        at dotty.tools.dotc.typer.Typer.typedUnnamed$1(Typer.scala:3500)
        at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:3585)
        at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3662)
        at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3666)
        at dotty.tools.dotc.typer.Typer.typedExpr(Typer.scala:3777)
        at dotty.tools.dotc.typer.QuotesAndSplices.typedQuotePattern(QuotesAndSplices.scala:286)
        at dotty.tools.dotc.typer.QuotesAndSplices.typedQuote(QuotesAndSplices.scala:56)
        at dotty.tools.dotc.typer.QuotesAndSplices.typedQuote$(QuotesAndSplices.scala:32)

mbovel · 2025-04-23T14:24:02Z

Adding backticks enables the lower case version to compile as expected:

  def myMacroImpl3(x: Expr[foo])(using q: Quotes): Expr[Unit] =
    x match
      case '{new `foo`($name)} =>
        println(name)
      case _ =>
        println("not a foo")

mbovel · 2025-04-23T14:31:54Z

I am not sure if the current behavior is expected or is a bug; I can't find a definitive answer in the documentation. In general, what do lower case identifiers in term positions in quoted patterns mean?

I see I can match case '{true} or case '{bar($name)}. So maybe I should also be able to match new foo($name)?

Somehow related: this page says that lower case identifiers in type positions are binders: “Just as in a normal pattern, the type variables are written using lower case names.”

cc @hamzaremmal @jchyb @nicolasstucki

mbovel · 2025-04-23T14:44:21Z

Further thinking about it, I think this is a bug, and that the culprit is this condition in typedIdent:

scala3/compiler/src/dotty/tools/dotc/typer/Typer.scala

Line 614 in 05b102a

if untpd.isVarPattern(tree) && name.isTypeName then

The condition is true when typing foo in new foo, while it probably shouldn't.

jchyb · 2025-04-23T14:55:11Z

I agree it looks like a bug (especially with that error message). I also believe the type positions part in lower case identifiers in type positions are binders only relates to Typed() and TypeApply() trees (but I can't say that for sure, as I have not really delved to that part of the code yet).

mbovel · 2025-04-23T14:58:23Z

I also believe the type positions part in lower case identifiers in type positions are binders only relates to Typed() and TypeApply() trees

That would make sense to me. Unfortunately, I don't think we can easily know if we are in one of these nodes when typing the nested identifier. At least, mode and prototype do not help:

ctx.mode: Mode(Type,ImplicitsEnabled,QuotedExprPattern)
pt: WildcardType(NoType)

mbovel · 2025-04-28T13:52:59Z

I also believe the type positions part in lower case identifiers in type positions are binders only relates to Typed() and TypeApply() trees (but I can't say that for sure, as I have not really delved to that part of the code yet).

It might actually only be inside Typed trees, given what I observe in practice, and this sentence from the documentation

The pattern $x: t will match an expression of any type and t will be bound to the type of the pattern.

And that even seems to be too general; for example, this doesn't compile:

import scala.quoted.{FromExpr, Expr, Quotes}

case class Foo[T](x: String)

object Macro:
  inline def myMacro3(): Unit =
    ${ myMacroImpl3('{Foo[Any]("hello")}) }

  def myMacroImpl3(x: Expr[Foo[Any]])(using Quotes): Expr[Unit] =
    x match
      case '{ Foo($y: t) } =>
        '{Foo[t]($y); ()}
      case _ =>
        println("not a foo")

    '{()}

-- [E007] Type Mismatch Error: test.scala:11:18 --------------------------------
11 |      case '{ Foo($y: t) } =>
   |                  ^^^^^
   |                  Found:    t
   |                  Required: String
   |
   | longer explanation available when compiling with `-explain`
Set()
-- [E006] Not Found Error: test.scala:12:14 ------------------------------------
12 |        '{Foo[t]($y); ()}
   |              ^
   |              Not found: type t
   |
   | longer explanation available when compiling with `-explain`
-- [E006] Not Found Error: test.scala:12:18 ------------------------------------
12 |        '{Foo[t]($y); ()}
   |                  ^
   |                  Not found: y
   |
   | longer explanation available when compiling with `-explain`
3 errors found

But it does if the field x of the class Foo has type T: case class Foo[T](x: T) 🤔

Kalin-Rudnicki added itype:bug stat:needs triage Every issue needs to have an "area" and "itype" label labels Feb 17, 2025

Gedochao added area:annotations area:metaprogramming:quotes Issues related to quotes and splices and removed stat:needs triage Every issue needs to have an "area" and "itype" label labels Feb 17, 2025

mbovel self-assigned this Feb 17, 2025

prolativ mentioned this issue Mar 11, 2025

Cannot extract annotation parameter from annotated type in a macro #22774

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mbovel changed the title ~~Unable to create unapply for annotation~~ Unable to match a lower-case constructor in quoted pattern Apr 23, 2025

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Unable to match a lower-case constructor in quoted pattern #22616

Unable to match a lower-case constructor in quoted pattern #22616

Kalin-Rudnicki commented Feb 17, 2025

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mbovel commented Apr 28, 2025

Unable to match a lower-case constructor in quoted pattern #22616

Unable to match a lower-case constructor in quoted pattern #22616

Comments

Kalin-Rudnicki commented Feb 17, 2025

Compiler version

Minimized code

Output

Expectation

som-snytt commented Feb 17, 2025

mbovel commented Feb 17, 2025

Kalin-Rudnicki commented Feb 17, 2025

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