Fix #13197: Deskolemize lifted named arguments

[noti0na1]

Fix #13197

The lift named arguments contain skolem types, which causes checker in inliner to fail.

def foo(bar: Bar): Foo = {
  val b$1: (?1 : String | Int) & String = forceString[Int](bar.b)
  val a$1: String @uncheckedVariance = Foo.$lessinit$greater$default$1
  new Foo(a$1, b = b$1)
}

We add deskolemized to fix this issue.

[noti0na1]

This bug only happens when the function is inlined.

@smarter Could you have a look of the code? I'm not sure whether this is the correct fix.

[smarter]

I don't think widen should look inside types, this would be a big departure from its current behavior.
(?1 : String | Null) looks like a skolem and we have logic to widen skolem when ascribing a type to a val during typer: https://github.com/lampepfl/dotty/blob/aaac006a58c3f5d7fbd64d3a1a19261b51dfecf5/compiler/src/dotty/tools/dotc/typer/Namer.scala#L1707 It seems like the same logic should be used when we ascribe a type to a val during the inlining phase?

[noti0na1]

@smarter This could also be a bug from named parameters.

extension [T](x: T | String) inline def forceString: x.type & String =
  x.asInstanceOf

trait Bar:
  def b: String | Int

class Foo(a: String = "", b: String)

object Foo:
  def foo(bar: Bar) = Foo(b = bar.b.forceString)

In typer, we can see the function foo becomes:

def foo(bar: Bar): Foo = {
  val b$1: (?1 : String | Int) & String = forceString[Int](bar.b)
  val a$1: String @uncheckedVariance = Foo.$lessinit$greater$default$1
  new Foo(a$1, b = b$1)
}

I think we need to widen the skolem type for b$1 first?

[smarter]

Yes that looks like the root of the problem indeed.