Insert traits with implicit parameters as extra parents of classes

[odersky]

If a trait A extends some other trait B which takes an implicit parameter, and
a class C extends A but not B, we need to insert B into the parents of C, so
that the implicit argument can be generated.

This required a generalization of the syncing between parent trees and parent types.
Namer will augment the parent types of a class with possibly other types.
Previously the only such change was a possibly leading class type, but we now
also generate trait parents that take context parameters. The new method parentTrees
reflects any such changes in the parent trees in Typer.

Fixes #7613

[smarter]

Nice! Would you mind documenting this in http://dotty.epfl.ch/docs/reference/other-new-features/trait-parameters.html ?

[therealcisse]

Will the following code work as a result of this change ?

trait Semigroup[A]:
  extension (a: A) def combine(b: A): A

trait Monoid[A: Semigroup]:
  def unit: A

given Semigroup[Int] with
  extension (a: Int) def combine(b: Int): Int = a + b

given Monoid[Int] with
  def unit: Int = 0

def sumList[A: Monoid](ls: List[A]): A = ls.foldLeft(summon[Monoid[A]].unit)(_.combine(_))

@main def Test() = 
  val ls = List(1,2,3)
  println(sumList(ls))
   

[odersky]

@amsayk The change would help, but your example does not compile since combine is not defined on Monoid.

[therealcisse]

Yeah that makes sense. Maybe an export would help to fix that then.

trait Semigroup[A]:
  extension (a: A) def combine(b: A): A

trait Monoid[A](using S: Semigroup[A]):
  export S.combine
  def unit: A

given Semigroup[Int] with
  extension (a: Int) def combine(b: Int): Int = a + b

given Monoid[Int] with
  def unit: Int = 0

def sumList[A: Monoid](ls: List[A]): A = ls.foldLeft(summon[Monoid[A]].unit)(_.combine(_))

@main def Test() = 
  val ls = List(1,2,3)
  println(sumList(ls))

I think this example showcases the line between OOP inheritance and addhoc polymorphism. One would intuitively think that since A is a Monoid, it is also a Semigroup (because a given of Semigroup exists) and thus should have the combine method.

[therealcisse]

On a somewhat related note, I want to ask:

The above code uses the syntax _.combine(_) because the extension method was desugared using two parameter groups. This syntax looks a little overloaded to be especially now that Scala is trying to reduces use cases for the underscore.

If extension methods with desugared to a single parameter group, we could simple write:

  def sumList[A: Monoid](ls: List[A]): A = ls.foldLeft(summon[Monoid[A]].unit)(combine)

So my question is: Does this even make sense in Scala and if so what do you think about changing the encoding to single parameter group ?

[odersky]

So my question is: Does this even make sense in Scala and if so what do you think about changing the encoding to single parameter group ?

That would be ambiguous. But you can write

(_ `combine` _)

if you want to be more symmetric.