Regression macro matching with Varargs fails: Sequence argument type annotation * cannot be used here

[hmf]

Compiler version

Was working with version 3.2.2. Now fails with 3.3.1. Basis for is code was the example in this page.

EDIT: This seems to be working in 3.3.0.

Minimized code

        case '{ List[String].apply(${Varargs(Exprs(elems))}: _*) } =>  
          Some(elems.toList.asInstanceOf[List[T]])

Output

error] -- [E178] Type Error: /home/hmf/VSCodeProjects/sploty/meta/src/icollection/IMap.scala:1753:20 
[error] 1753 |        case '{ List[String].apply(${Varargs(Exprs(elems))}: _*) } =>  
[error]      |                ^^^^^^^^^^^^
[error]      |     missing argument list for method apply in trait IterableFactory
[error]      |
[error]      |       def apply[A](elems: A*): CC[A]
[error]      |--------------------------------------------------------------------------
[error]      | Explanation (enabled by `-explain`)
[error]      |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
[error]      | Unapplied methods are only converted to functions when a function type is expected.
[error]       --------------------------------------------------------------------------
[error] Explanation
[error] ===========
[error] Unapplied methods are only converted to functions when a function type is expected.
[error] -- Error: /home/hmf/VSCodeProjects/sploty/meta/src/icollection/IMap.scala:1753:35 
[error] 1753 |        case '{ List[String].apply(${Varargs(Exprs(elems))}: _*) } =>  
[error]      |                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
[error]      |Sequence argument type annotation `*` cannot be used here:
[error]      |the corresponding parameter has type Int which is not a repeated parameter type
[error] -- [E006] Not Found Error: /home/hmf/VSCodeProjects/sploty/meta/src/icollection/IMap.scala:1755:15 
[error] 1755 |          Some(elems.toList.asInstanceOf[List[T]])
[error]      |               ^^^^^
[error]      |               Not found: elems
[error]      |--------------------------------------------------------------------------
[error]      | Explanation (enabled by `-explain`)
[error]      |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
[error]      | The identifier for `elems` is not bound, that is,
[error]      | no declaration for this identifier can be found.
[error]      | That can happen, for example, if `elems` or its declaration has either been
[error]      | misspelt or if an import is missing.
[error]       --------------------------------------------------------------------------
[error] Explanation
[error] ===========
[error] The identifier for `elems` is not bound, that is,
[error] no declaration for this identifier can be found.
[error] That can happen, for example, if `elems` or its declaration has either been
[error] misspelt or if an import is missing.

Expectation

Was hoping no changes were needed for the upgrade.

[som-snytt]

Edit: sorry, I missed that the ticket is for 3.3, I assumed 3.4.

I see -source is required to induce messaging in 3.3.x.

So -source:future-migration warns and -source:future errors:

Welcome to Scala 3.3.1 (21.0.2, Java OpenJDK 64-Bit Server VM).
Type in expressions for evaluation. Or try :help.

scala> val xs = List("a", "b", "c")
val xs: List[String] = List(a, b, c)

scala> List(xs: _*)
1 warning found
-- Migration Warning: --------------------------------------------------------------------------------------------------
1 |List(xs: _*)
  |         ^
  |         The syntax `x: _*` is no longer supported for vararg splices; use `x*` instead
val res0: List[String] = List(a, b, c)

That is different from my normal expectation about -deprecation and syntax changes. (-help actually announces that deprecation is for APIs, not syntax.) Scala 2 had various policies for deprecation, warning under -Xsource:3 and elevated errors under -Xsource:3-cross. That is because 3 is always the future in Scala 2.

[nicolasstucki]

This error is correct

[error] 1753 |        case '{ List[String].apply(${Varargs(Exprs(elems))}: _*) } =>  
[error]      |                ^^^^^^^^^^^^
[error]      |     missing argument list for method apply in trait IterableFactory
[error]      |
[error]      |       def apply[A](elems: A*): CC[A]

The type argument of the apply should not be added directly on the list List[String](....) is equivalent to List.apply[String](....) and not List[String].apply(....).

The following version work:

    case '{ List.apply[String](${Varargs(Exprs(elems))}: _*) } => ... // derprecated `: _*` syntax

    case '{ List[String](${Varargs(Exprs(elems))}: _*) } => ... // derprecated `: _*` syntax

    case '{ List.apply[String](${Varargs(Exprs(elems))}*) } => ...

    case '{ List[String](${Varargs(Exprs(elems))}*) } => ... // probably the best version

[nicolasstucki]

We do need to update the documentation to avoid using the deprecated : _* syntax.

[nicolasstucki]

Example update in scala/docs.scala-lang#2980