inline delegate not working with opaque type

[japgolly]

minimized code

object X {
  opaque type LineNo = Int
  object LineNo {
    inline def unsafe(i: Int): LineNo = i

    inline delegate hello for LineNo = ${impl}

    private def impl given (r: Reflection): Expr[LineNo] = {
      import r._
      '{unsafe(${rootPosition.startLine})}
    }
  }

  val x1 = the[LineNo] // nope

  import delegate LineNo._
  val x2 = the[LineNo] // nope

  val x3 = LineNo.hello
}

error message

[error] -- Error: X.scala:67:22 
[error] 67 |  val x1 = the[LineNo] // nope
[error]    |                      ^
[error]    |no implicit argument of type X.LineNo was found for parameter x of method the in object DottyPredef
[error] -- Error: X.scala:70:22 
[error] 70 |  val x2 = the[LineNo] // nope
[error]    |                      ^
[error]    |no implicit argument of type X.LineNo was found for parameter x of method the in object DottyPredef.
[error]    |I found:
[error]    |
[error]    |    {
[error]    |      <empty> :X.LineNo
[error]    |    }
[error]    |
[error]    |But method hello in object LineNo does not match type X.LineNo.
[error] two errors found

expectation

• x1 should work because the implicit is in the companion object
• x2 should work because we've explicitly imported delegates

[OlivierBlanvillain]

The x1 issue seams to be a duplicate of #6716, but I would expect x2 to compile.

[odersky]

We decided that inline and opaque cannot be mixed. The example now produces this as first error:

4 |    inline def unsafe(i: Int): LineNo = i
  |               ^
  |Implementation restriction: No inline methods allowed where opaque type aliases are in scope
``

[benhutchison]

This is a pity as it's likely people, (ie me), will want to combine opaque types and macros, for compile-time checked literal values.

I hit the same issue today, and FWIW this illustrates a reasonable workaround I came to:
https://github.com/benhutchison/compose-scala3-workshop/blob/master/nat/src/main/scala/scala3workshop/nat/Nat.scala#L37

[johnynek]

Just hit this also. It is a shame these features don't compose.

Is there documentation of which features don't compose? Like a matrix which shows which are prohibited?

[japgolly]

@odersky @nicolasstucki How long should we expect the limitation that opaque types and inline can't be combined, to last? Is it something that will be addressed in after 3.0? 3.1? I'm finding that I never use opaque types because of this limitation.

[bblfish]

@japgolly did you test the difference to see how much slower things are? I wonder if final would help.

In banana-rdf we have built a framework allowing us to write code independently of the various implementations of RDF, so that we can switch between them with one line of code. Opaque types should help for there to be no leakage of the underlying implementations. But ideally we'd like to also use the relevant function calls from the underlying libraries directly. The lib covers Java, JS and should also come handy with native libs too. There are WASM RDF libs in the works.

[odersky]

Note:opaque and inline can now work together.

[bblfish]

Ah latest beta? I'll switch.

[odersky]

It was merged on June 17, so it should be in 3.02 RC-1. 3.02 final should be released later this week. #12815

…

On Mon, Aug 30, 2021 at 10:36 AM Henry Story ***@***.***> wrote: Ah latest beta? I'll switch. — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub <#6802 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AAGCKVRQVOKABTYIMMCQV33T7M7JFANCNFSM4H5OWG5A> . Triage notifications on the go with GitHub Mobile for iOS <https://apps.apple.com/app/apple-store/id1477376905?ct=notification-email&mt=8&pt=524675> or Android <https://play.google.com/store/apps/details?id=com.github.android&referrer=utm_campaign%3Dnotification-email%26utm_medium%3Demail%26utm_source%3Dgithub>.

-- Martin Odersky Professor, Programming Methods Group (LAMP) Faculty IC, EPFL Station 14, Lausanne, Switzerland