Type aliases and Focus are not friends in scala 3

[romainreuillon]

Dear Devs,

using monocle 3.0.0 and scala 3.0.1-RC2

Using Focus on type aliases with type parameter seem to pose some problems to monocle 3.0.0.

For instance, this code

import monocle._

object Test {

  case class CC[T](t: T, i: Int)

  type CCInt = CC[Int]

  val cc = CC(2, 3)
  Focus[CCInt](_.i).get(cc)
}

Raises the following error message:

[error] -- Error: /tmp/bugmonocle/Test.scala:11:2 --------------------------------------
[error] 11 |  Focus[CCInt](_.i).get(cc)
[error]    |  ^^^^^^^^^^^^^^^^^
[error]    |  method copy in class CC does not take parameters
[error]    | This location contains code that was inlined from Test.scala:11

And this code:

import monocle._

object Test {

  case class CC[T](t: T, i: Int)

  type CCInt = CC[Int]

  val cc = CC(2, 3)
  Focus[CCInt](_.t).get(cc)
}

Raises this error message:

[error] -- Error: /tmp/bugmonocle/Test.scala:11:14 -------------------------------------
[error] 11 |  Focus[CCInt](_.t).get(cc)
[error]    |  ^^^^^^^^^^^^^^^^^
[error]    |Exception occurred while executing macro expansion.
[error]    |java.lang.Exception: Expr cast exception: from.t
[error]    |of type: from.t
[error]    |did not conform to type: CC.this.T
[error]    |
[error]    |	at scala.quoted.runtime.impl.QuotesImpl.asExprOf(QuotesImpl.scala:71)
[error]    |	at scala.quoted.runtime.impl.QuotesImpl$reflect$TreeMethods$.asExprOf(QuotesImpl.scala:113)
[error]    |	at scala.quoted.runtime.impl.QuotesImpl$reflect$TreeMethods$.asExprOf(QuotesImpl.scala:112)
[error]    |	at monocle.internal.focus.features.selectfield.SelectFieldGenerator.generateSelectField$$anonfun$8(SelectFieldGenerator.scala:25)
[error]    |	at monocle.internal.focus.features.selectfield.SelectFieldGenerator.generateSelectField$$anonfun$adapted$2(SelectFieldGenerator.scala:26)

Best,
Romain

[kenbot]

Thanks for the report @romainreuillon! It looks like it shouldn't be too hard to track down. Interestingly, the pre-applied version works fine:

import monocle.Monocle._
cc.focus(_.i).get
// => 3

So it looks like the bit that is reading the explicitly supplied type is being a bit too literal when trying to discover the case class structure.