Regression macro between M3 and RC1 - transparent inline

[julien-truffaut]

Compiler version

This is an error we encountered when migrating Monocle from Scala 3.0.0-M3 to 3.0.0-RC1.

Here is the link to the build: https://github.com/optics-dev/Monocle/runs/1917837206

Minimized code

object GenPrism {
  transparent inline def apply[Source, Target <: Source] = 
    Focus[Source](_.as[Target])
}

object Focus {

  def apply[S] = new MkFocus[S]

  class MkFocus[From] {
    transparent inline def apply[To](inline lambda: (From => To)): Any = 
      ${ FocusImpl('lambda) }
  }
}

Output

[error] -- Error: /Users/julien/project/Monocle/macro/src/main/scala-3.x/monocle/macros/GenPrism.scala:8:22
[error] 8 |    Focus[Source](_.as[Target])
[error]   |                  ^^^^^^^^^^^^
[error]   |              class Nothing in package scala does not take type parameters
[error] -- Error: /Users/julien/project/Monocle/macro/src/main/scala-3.x/monocle/macros/GenPrism.scala:7:25
[error] 7 |  transparent inline def apply[Source, Target <: Source] =
[error]   |                         ^
[error]   |                         `transparent` can only be used for inline methods

Expectation

[nicolasstucki]

The error is with the missing as operation. Then that courses the compiler to give a wrong error for the inline definition.

If we fix the as then the transparent error disapears.

[smarter]

(I recommend you add monocle to the Scala 3 build so this sort of things can be caught early: https://github.com/lampepfl/dotty/blob/master/community-build/README.md)

[smarter]

And as Nicolas said, your code snippet does not define as, so it seems that you minimized it too much (and found another issue with the compiler not reporting the missing definition)

[nicolasstucki]

Minimized

transparent inline def foo = error

-- [E006] Not Found Error: Foo.scala:1:29 --------------------------------------
1 |transparent inline def foo = error
  |                             ^^^^^
  |                             Not found: error

longer explanation available when compiling with `-explain`
-- Error: Foo.scala:1:23 -------------------------------------------------------
1 |transparent inline def foo = error
  |                       ^
  |                       `transparent` can only be used for inline methods
2 errors found

[julien-truffaut]

Thanks, I wasn't sure how to minimise it. Though I don't understand why it stopped working between M3 and RC1.

@smarter Thanks for pointing out the community build, I will look at adding Monocle

[smarter]

Though I don't understand why it stopped working between M3 and RC1.

Are you sure it worked before? It looks like the problematic code was added only a few days ago and without tests: https://github.com/optics-dev/Monocle/pull/1063/files, so it may have just been (incorrectly) compiling.

[kenbot]

Apologies for the lack of clarity in the snippet, the as extension method is brought in scope by import monocle.Focus._, and as the PR is just swapping out the implementation for existing functionality, the original tests are still in place and did not require modification.