New incompatibility: case class companions don’t extend Function anymore

[julienrf]

In Scala 2, a case class companion was extending Function.

case class Foo(x: Int, b: Boolean)

// The Scala 2 compiler synthesizes the following companion
object Foo extends Function2[Int, Boolean, Foo] {
  def apply(x: Int, b: Boolean) = new Foo(x, b)
}

This is not the case anymore, which means that companions don’t inherit function methods such as apply, tupled, andThen, and compose.

One work-around to write code that cross-compiles is to manually write a function that calls the constructor:

val fooConstructor: (Int, Boolean) => Foo = (x, b) => Foo(x, b)
// Then, `fooConstructor` has all the methods of Function

[adpi2]

As a work-around, we can also use the apply method because it can be eta-expanded to a Function2

Foo.apply.curried(1)(true) // compiles in Scala 2.13 and Scala 3.0.0-M3

What do you think @julienrf ?

[adpi2]

Actually it doesn't compile in Scala 2.13:

[error] Unapplied methods are only converted to functions when a function type is expected.
[error] You can make this conversion explicit by writing `apply _` or `apply(_,_)` instead of `apply`.
[error]   val foo1 = Foo.apply.curried(1)(true)

But this one cross-compiles:

(Foo.apply _).curried(1)(true)