Implement applied constructor types

[KacperFKorban]

closes #22542

Introduce new syntax to make classes with tracked parameters
easier to use. The new syntax is essentially the ability to use an application
of a class constructor as a type, we call such types applied constructor types.

With this new feature the following example compiles correctly and the type in
the comment is the resulting type of the applied constructor types.

import scala.language.experimental.modularity

class C(tracked val v: Any)

val c: C(42) /* C { val v: 42 } */ = C(42)

Syntax change

SimpleType        ::=  SimpleLiteral
                    |  ‘?’ TypeBounds
---                 |  SimpleType1
+++                 |  SimpleType1 {ParArgumentExprs}

A SimpleType can now optionally be followed by ParArgumentExprs.

[bishabosha]

What about constructors with mixed tracked and non-tracked parameters?

[KacperFKorban]

Added some tests for those, but the current approach is: you have to write all parameters, but the non-tracked params won't influence the resulting type.

[bishabosha]

that sounds strange, so a bit like an unused function parameter? maybe there should be a placeholder like erasedValue or ? for now?

[KacperFKorban]

It doesn't have to be unused, though one might argue that it makes it worse 😅 BUT, the following works:

class Generic[A](val a: A)
val _: Generic(compiletime.erasedValue[Int]) = Generic(42)
val _: Generic(??? : Int) = Generic(42)
val _: Generic(43) = Generic(42)

[KacperFKorban]

The CI failure seems to be the same as on main and looks unrelated.

[mbovel]

I've noticed during your presentation that it also works for using parameters:

import language.experimental.modularity

class Box[T](using tracked val value: Int)

def main =
  given Int = 42
  val b: Box() = Box()

after typer:

def main: Unit =
  final lazy given val given_Int: Int = 42
  val b: Box[Any]{val value: (given_Int : Int)} = new Box[Any](given_Int)()