constValueOpt doesn't work for class parameters

[doofin]

Compiler version

3.2.2

Minimized code

import scala.compiletime.*

class c2[i <: Int] { println("constValueOpt:" + constValueOpt[i]) }
new c2[2]

Output

constValueOpt:None

Expectation

should be constValueOpt:Some(2)

[soronpo]

This is not a bug. When the class is being compiled the compiler has no knowledge what i type is, and that is when the class body, along with constValueOpt is being compiled as well.

[soronpo]

Here is a better way:

import scala.compiletime.*

class c2[i <: Int]:
  inline def printme: Unit = println("constValueOpt:" + constValueOpt[i])
new c2[2].printme

[doofin]

thanks for the reply, I know inline def will work,but feel that this limitation is not necessary. could you point me to the LOC in the compiler ?

[soronpo]

https://github.com/lampepfl/dotty/blob/f8cb74ea7a05fc5c9d873b73b2811fdb5f5c2f2d/compiler/src/dotty/tools/dotc/inlines/Inlines.scala#L421

[doofin]

so according to the code, constValueOpt is only supported at inlinedMethod and it would be tricky to extend it ?

[soronpo]

Not part of the compiler team, but from what I understand, this is not something that is possible.

[doofin]

@soronpo Just have an idea: why not get the value before compile time (maybe at macro expansion time ) and inline it when compiling ?

[smarter]

Another solution is to use the ValueOf typeclass:

class c2[i <: Int: ValueOf] { println("valueOf:" + valueOf[i]) }

[doofin]

@smarter thanks! this looks interesting and almost same as what I think, but it will give errors:

No singleton value available for Int; eligible singleton types for ValueOf synthesis include literals and stable paths.

[smarter]

What does the use-site look like? If it's new c2[2] it should work, if it's a method parameter then you'll need to propagate the typeclass constraint:

def foo[i <: Int: ValueOf] = new c2[i]

[doofin]

@smarter thanks! this actually works, it's just my typo