tasty reader, selection of top level definition from package does not work in an annotation

[bishabosha]

originally found in typelevel/cats#4304

Reproduction steps

Scala version: 2.13.9

//> using scala "2.13.9"
//> using options "-Ytasty-reader", "-Ydebug-tasty"
//> using lib "org.typelevel:cats-core_3:2.8.0"

import cats.syntax.all._

object Main extends App

Problem

this code makes the error

./bar.scala:7:21: can't find type required by method catsInstancesForIdCompat2_6_1 in object cats.Invariant: compat.targetName; perhaps it is missing from the classpath.

The cause? annotations of the Invariant.catsInstancesForIdCompat2_6_1 method get forced, one of them being @cats.compat.targetName(...).

targetName is a type alias defined in cats.compat.compat$package but it seems that is never looked at because top level definition wrappers are not considered as part of the package scope in scala 2.

in the -Ydebug-tasty trace, we see that tasty reader tries to force the cats.compat.package package object which is of course empty.

#[6266, Invariant]: [8) <<< forcing annotations of symbol: `(#11713) method catsInstancesForIdCompat2_6_1`
#[6266, Invariant]: [8 0) <<< read within owner: `(#11713) method catsInstancesForIdCompat2_6_1` with modes `ReadAnnotation | ReadAnnotationCtor` at Addr(1118)
#[6266, Invariant]: [8 0 0) <<< reading annotation tree: `(#1738) class targetName` at Addr(1118)
#[6266, Invariant]: [8 0 0 0) <<< force symbol info: `(#7525) package class compat` in context `(#11713) method catsInstancesForIdCompat2_6_1` [space,deep]
#[6266, Invariant]: [8 0 0 0) >>> `(#7525) package class compat` was forced [space,deep]
[error] ./bar.scala:7:21: can't find type required by method catsInstancesForIdCompat2_6_1 in object cats.Invariant: compat.targetName; perhaps it is missing from the classpath.
[error] object Main extends App
[error]                     ^
Error compiling project (Scala 2.13.9, JVM)
Compilation failed

[SethTisue]

@bishabosha if you want to get a fix into 2.13.10, you might need to prepare one soon, since bug 12650 may force us to do 2.13.10 promptly

[bishabosha]

this is more complicated to "fix" because there is also no support for targetName either - so fixing this resolution error will still have the incorrect method call at runtime - but if the method is not used directly from scala 2 then fine

[bishabosha]

@sjrd opinion is that top level definitions are too complex to put effort to support, in typelevel/cats#4304 I suggest to move cats.compat.targetName to a normal package object, then it is an open question of can we implement support for targetName annotation.

[SethTisue]

@bishabosha shall we "Backlog" this, then, or did you want to make an attempt for 2.13.11? I'll assume Backlog, but we can flip it back.

[bishabosha]

Backlog is fine