Can't override trait contextual parameters

Compiler version

3.2.0-RC2

Minimized code

import cats.syntax.all.*
import cats.*

trait Newtype[Src](using
    val show: Show[Src]
):
  opaque type Type = Src
  given Show[Type] = show
  inline final def apply(x: Src): Type = x
  extension (self: Type) inline final def value: Src = self

transparent trait BigDecimalNewtype(using
    override val show: Show[BigDecimal] = Show.show(_.underlying.toPlainString)
) extends Newtype[BigDecimal]:
  extension (src: Type)
    def setPrecision(precision: Int): Type =
      apply(src.value.round(java.math.MathContext(precision)))

type Price = Price.Type
object Price extends BigDecimalNewtype

val a = Price(BigDecimal("12000")).setPrecision(3)

// bug? This should print 12000
Show[Price].show(a)

Show.show[BigDecimal](_.underlying.toPlainString).show(a.value)

https://scastie.scala-lang.org/P9LgB5vBS4OdaUVCaH8X2A

Output

1.20E+4: scala.Predef.String
12000: scala.Predef.String

Expectation

12000: scala.Predef.String
12000: scala.Predef.String

[mbovel]

Same example without cats dependency:

trait Show[T] extends Show.ContravariantShow[T]:
  def show(t: T): String

object Show:
  def apply[T](using instance: Show[T]): Show[T] = instance

  def show[A](f: A => String): Show[A] =
    new Show[A]:
      def show(a: A): String = f(a)

  trait ContravariantShow[-T] extends Serializable:
    def show(t: T): String

given catsStdShowForBigDecimal: Show[BigDecimal] with
  def show(t: BigDecimal) = t.toString()

trait Newtype[Src](using val show: Show[Src]):
  opaque type Type = Src
  given Show[Type] = show
  inline final def apply(x: Src): Type = x
  extension (self: Type) inline final def value: Src = self

transparent trait BigDecimalNewtype(using
    override val show: Show[BigDecimal] = Show.show(_.underlying.toPlainString)
) extends Newtype[BigDecimal]:
  extension (src: Type)
    def setPrecision(precision: Int): Type =
      apply(src.value.round(java.math.MathContext(precision)))

object Price extends BigDecimalNewtype
type Price = Price.Type

@main def main() =
  val a = Price(BigDecimal("12000")).setPrecision(3)

  // bug? This should print 12000
  println(Show[Price].show(a))

  println(Show.show[BigDecimal](_.underlying.toPlainString).show(a.value))

[odersky]

@Swoorup I think it would help to boil this down more and point out exactly what code is wrong. It would help to print the program with -Xprint:typer and say what does not match expectations.

The way I see it (the example is too complicated to say for sure without more digging) you think that the default argument of show in BigDecimalNewType should be used. But it isn’t because there is a given that can be supplied, namely the given Show[Type] in NewType. So this looks as expected to me.