"Compiler bug" when using constValue inside an inline method

[adamw]

Compiler version

3.0.0-RC3

Minimized code

import scala.deriving.Mirror
import scala.compiletime.{constValue, error}
import scala.quoted.*

object TestMacro {
  inline def test1[CASE_CLASS <: Product](using m: Mirror.ProductOf[CASE_CLASS]): Unit = {
    error(s"Error: ${constValue[m.MirroredElemLabels]}.")
  }
}

object Test extends App {
  case class A(x: String, y: Int)
  TestMacro.test1[A]
}

This comes from an attempt to provide user-friendly error messages for inline derivations which use mirrors.

Output

[error] -- Error: Test.scala:14:2 
[error] 14 |  TestMacro.test1[A]
[error]    |  ^^^^^^^^^^^^^^^^^^
[error]    |  Compiler bug: `constValue` was not evaluated by the compiler
[error] -- Error: Test.scala:14:2 
[error] 14 |  TestMacro.test1[A]
[error]    |  ^^^^^^^^^^^^^^^^^^
[error]    |not a constant type: m$proxy1.MirroredElemLabels; cannot take constValue

Expectation

[adamw]

Sorry wrong key, adding details :)

[adamw]

Ready :)

[nicolasstucki]

m.MirroredElemLabels is a Tuple. constValue expects values of type Int, Boolean, ..., String.

This use case is too advances for compiletime.error. In this case a macro should be used. Something like:

object TestMacro {
  inline def test1[CASE_CLASS <: Product](using m: Mirror.ProductOf[CASE_CLASS]): Unit =
    ${ code('m) }

  def code[CASE_CLASS <: Product: Type](m: Expr[Mirror.ProductOf[CASE_CLASS]])(using Quotes): Expr[Unit] =
    m match
      case '{ $_ : Mirror { type MirroredElemLabels = t } } =>
        quotes.reflect.report.error(s"Error: ${ Type.show[t] }", m)
    '{}
}

We have Type.valueOfConstant that is similar to compiletime.constValue. But we are missing a Type.valueOfTuple which would be similar to compiletime.constValueTuple

[adamw]

Ah, I though tuples of primitive types are allowed as well. I suppose the issue is then about the error message :)

So to report user-friendly errors, which include the type/ label information, I have to resort to a macro, instead of using inline+Mirror?

[adamw]

Hm reading your answer again, shouldn't constValueTuple solve the problem?

inline def test1[CASE_CLASS <: Product](using m: Mirror.ProductOf[CASE_CLASS]): Unit = {
  error(s"Error: ${constValueTuple[m.MirroredElemLabels]}.")
}

I'm getting a

[error] 14 |  TestMacro.test1[A]
[error]    |  ^^^^^^^^^^^^^^^^^^
[error]    |A literal string is expected as an argument to `compiletime.error`. Got _root_.scala.StringContext.apply(["Error: ","." : String]*).s(

[nicolasstucki]

That is #10315

[adamw]

Ah :) Indeed - so I suppose construction of an error message which is anything above a constant string means going to a macro. Though ... maybe I can call the macro from the inline branch? 🤔 I'll have to try that.

Would you like me to close this issue as a duplicate, or keep it open to improve the error message?

[nicolasstucki]

What could the error message have said that would have helped you understand the issue?

[adamw]

E.g. "Not a constant: s"Error: ${constValue[m.MirroredElemLabels]}."

[adamw]

Although, maybe this is already covered by the second error, not a constant type: m$proxy1.MirroredElemLabels; cannot take constValue. I guess I got focused on the "Compiler bug" part and didn't look much further ;) That's the misleading part - it's not a compiler bug, it's my incorrect usage of inlines.

[nicolasstucki]

Oh, I missed that part. We should not emit the first error.

[nicolasstucki]

Ah, I missed it because it is already fixed. I remember that issue.

Now it fails with

13 |  TestMacro.test1[A]
   |  ^^^^^^^^^^^^^^^^^^
   |not a constant type: m$proxy1.MirroredElemLabels; cannot take constValue
   | This location contains code that was inlined from Foo.scala:7

[adamw]

Awesome, thank you! :)