unhelpful error message for ValueOf[N] when no singleton available for N (bounded by singleton)

[cayhorstmann]

Compiler version

3.2.0-RC1

Minimized code

import scala.compiletime.*

class Vec[N <: Int & Singleton] :
  private val xs: Array[Double] = Array.ofDim[Double](summon[ValueOf[N]].value)

Output

No singleton value available for N.

Expectation

My expectation is that the Singleton bound makes ValueOf realize that N is a singleton. But I am no expert, and the fact that #8257 was closed makes me wonder. But then my expectation would be to get an error message that is clearer.

A bit more detail at https://users.scala-lang.org/t/how-do-i-compute-type-level-m-n/8715/2

[WojciechMazur]

The message we see here is a customized error for the missing implicit value of ValueOf[N]. As far as I'm aware, Singleton bound does not automatically add the implicit value needed for ValueOf to the scope and is probably correct by design.
As you've observed in the discussion from the link above, adding using ValueOf[N] fixes the issue, it can be written in a shorter form

class Vec[N <: Int: ValueOf] :
  private val xs: Array[Double] = Array.ofDim[Double](valueOf[N])

[bishabosha]

The problem is that ValueOf[N] is only implicitly available for concrete types, not abstract types. In the definition of xs, N is still abstract. N is only known when Vec[N] is instantiated. So then you add the (using ValueOf[N]) to the class constructor (or context bound as in #15829 (comment))

[bishabosha]

It seems from the forum discussion and the nature of this issue being opened that we should do more to signal to the user what to do when the implicit search fails - so let's keep this open to improve the error message presented.

However, the error message for the missing implicit is defined in the Scala 2 library - so not much we can do here to change the basic message - however we have the possibility to add an addendum to that message to explain why synthesis of ValueOf failed: https://github.com/lampepfl/dotty/blob/e560c2d6970c864ceea5607597d74cf49c443965/compiler/src/dotty/tools/dotc/typer/Synthesizer.scala#L216-L233

so for the case where a constant type, unit type, or singleton term-ref cant be extracted, we can add an error message with a recommendation/explanation of the problem

[SethTisue]

the fact that #8257 was closed makes me wonder

@mbovel you say that 8257 actually eventually got fixed...?

[mbovel]

@SethTisue Yes, added a comment in #8257.

[mbovel]

Trying to summon ValueOf:

def f[N <: Int & Singleton](n: N) =
  println(summon[ValueOf[N]].value)

yields the following error:

-- Error: /Users/mbovel/dotty/test.scala:3:28 ----------------------------------
3 |  println(summon[ValueOf[N]].value)
  |                            ^
  |          No singleton value available for N
  |
  |          where:    N is a type in method f with bounds <: Int & Singleton
  |          .

And calling valueOf:

def f[N <: Int & Singleton](n: N) =
  println(valueOf[N])

yields the following error:

-- Error: /Users/mbovel/dotty/test.scala:3:17 ----------------------------------
 3 |  println(valueOf[N])
   |          ^^^^^^^^^^
   |          cannot reduce summonFrom with
   |           patterns :  case given ev @ _:ValueOf[N]
   |----------------------------------------------------------------------------
   |Inline stack trace
   |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
   |This location contains code that was inlined from Predef.scala:25
25 |  inline def valueOf[T]: T = summonFrom {
   |                                        ^
26 |    case ev: ValueOf[T] => ev.value
27 |  }
    ----------------------------------------------------------------------------