Internal "Wrong number of arguments" error when reading a Scala 2 annotation in a macro

[jchyb]

Compiler version

Scala 2: 2.13.14
Scala 3: 3.3.4

Minimized code

Compiled with Scala 2:

class annot(str: List[String]) extends scala.annotation.StaticAnnotation
@annot(List("test", "test2")) class TestClass

Compiled with Scala 3, with results of the above on the classpath:
Macro.scala:

import scala.quoted._

object Macro {
  inline def call: Unit = ${ callImpl }
  def callImpl(using Quotes): Expr[Unit] = {
    import quotes.reflect._
    val sym = TypeRepr.of[TestClass].typeSymbol
    val annot = TypeRepr.of[annot].typeSymbol

    println(sym.getAnnotation(annot)) // causes error

    '{()}
  }
}

Main.scala:

@main def main() = Macro.call

Output

[error] -- Error: /Users/jchyb/workspace/mix-macros-scala-2-and-3/app3/src/main/scala/Main.scala:4:25 
[error] 4 |@main def main() = Macro.call
[error]   |                   ^^^^^^^^^^
[error]   |wrong number of arguments at <no phase> for (elems: String*): CC[String]: (elems: String*): CC[String], expected: 1, found: 2
Some(Apply(Select(New(TypeTree[TypeRef(ThisType(TypeRef(NoPrefix,module class <empty>)),class annot)]),<init>),List(Apply(TypeApply(Select(Select(Select(Ident(scala),package),List),apply),List(TypeTree[TypeRef(ThisType(TypeRef(NoPrefix,module class lang)),class String)])),List(Literal(Constant(test)), Literal(Constant(test2)))))))

Expectation

No error.
An interesting observation: this bug does not happen when the annotation argument is not a list. Also we do not return from the macro instantly as we see the error, instead still printing out the tree and ending compilation when we leave the macro.

[jchyb]

The issue seems to lie with how we read the Apply trees in Scala2Unpickler (

scala3/compiler/src/dotty/tools/dotc/core/unpickleScala2/Scala2Unpickler.scala

Line 1283 in 6b9f9f7

Apply(fun, args) // note: can't deal with overloaded syms yet

). Before constructing the Apply List.apply gets typed as:

MethodType(
  List(elems),
  List(AppliedType(TypeRef(ThisType(TypeRef(NoPrefix,module class scala)),class <repeated>),List(TypeRef(ThisType(TypeRef(NoPrefix,module class lang)),class String)))), 
  AppliedType(TypeRef(ThisType(TypeRef(ThisType(TypeRef(NoPrefix,module class collection)),trait IterableFactory)),type CC),List(TypeRef(ThisType(TypeRef(NoPrefix,module class lang)),class String))))

and the arguments are read as:

List(Literal(Constant(test)), Literal(Constant(test2))

The issue lies with the varargs application to the List, as shown by the presence of scala.<repeated>. In Tasty we also have a Repeated token to tell us about varargs application. Manually checking for scala.<repeated> and wrapping the arguments with SeqLiteral helps here, but we soon run into another issue, shown by another error:

[error] -- [E007] Type Mismatch Error: /Users/jchyb/workspace/mix-macros-scala-2-and-3/app3/src/main/scala/Main.scala:4:25 
[error] 4 |@main def main() = Macro.call
[error]   |                   ^^^^^^^^^^
[error]   |                   Found:    CC[String]
[error]   |                   Required: List[String]
[error]   |
[error]   | longer explanation available when compiling with `-explain`

This one, I do not know how to approach.

[jchyb]

Also, if we skip the List/Seq in the annotation and use varargs directly, it works