Allow constraints for non-primitive types to be evaluable at compile-time

[Iltotore]

This is not possible at the moment because Constraint#apply's parameter is not inline. Making it inline would force all children to propagate make it inline too, breaking backward compatibility.

For example, this:

val x: List[String] :| FixedLength[3] = List("a", "b", "c")

is not verifiable at compile-time but this will be easily doable once value in Constraint#apply is inline.

[KaranAhlawat]

If I understand correctly, right now the snippet you shared does not evaluate at compile-time.
So, if we were to use it as an IronType, we would necessarily have to write List(...).refine or some equivalent method, yes?

[Iltotore]

Yes. It is currently not possible to use the compile-time conversion with such type.

The fix is simple but would introduce a pretty big compatibility break. Therefore, it will only appear when 3.0.0 will be released.

[Iltotore]

While working on 2.6.0, I noticed that Dotty might not fully reduce non-primitive expressions. This might be due to the missing inline keyword in Constraint#apply or not. It needs further investigation.

[Iltotore]

The solution I am currently implementing seems to work 🎉. The compatibility break should be minimal and I'll try to make a Scalafix rule to mitigate it.