Macro type class derivation (inline definition)

[nicolasstucki]

Compiler version

3.0.0.RC-1

Minimized example

There is no way to derive automatically an inline given definition.

enum Opt[+T] derives Show:
  case Sm[+T](x: T) extends Opt[T]
  case Nn extends Opt[Nothing]

class Show[T]
object Show:
  inline def derived[T](using deriving.Mirror.Of[T]): Show[T] = ???

the previous code derives

object Opt:
  given derived$Show[T](implicit x$0: Show[T]): Show[Opt[T]] = Show.derived[Opt[T]](...)

but in some cases we want do derive

object Opt:
  inline given derived$Show[T](implicit x$0: Show[T]): Show[Opt[T]] = Show.derived[Opt[T]](...)

Why?

• Summon extra implicits using summonInline or Expr.summon
• Genrerate shorter/faster code

Workarround

Write the given definition manually and not use derives

object Opt:
  inline given show[T](implicit x$0: Show[T]): Show[Opt[T]] = Show.derived[Opt[T]](...)

Example: see #11647

Expectation

We should have a way to easily derive such definitions