Fix semantics of inline override

[nicolasstucki]

The current semantics of inline override is inconsistent and confusing.

Current state

class Greeting {
  def greet(name: String): String = "Greetings " + name
}

class Hello extends Greeting {
  inline override def greet(name: String): String = "Hello " + name
}

object App {
  val greeting: Greeting = new Greeting
  val hello1: Hello = new Hello
  val hello2: Greeting = hello1


  println(greeting.greet("World")) // prints: Greetings World

  println(hello1.greet("World")) // prints: Hello World
  // Inlined Hello.greet directly

  println(hello2.greet("World")) // prints: Greetings World
  // Not inlined as it is statically calling Greeting.greet, so far so good
  // It should have dynamically call Hello.greet, but inline method are erased!
}

Semantically coherent version

object App {
  val greeting: Greeting = new Greeting
  val hello1: Hello = new Hello
  val hello2: Greeting = hello1

  println(greeting.greet("World")) // prints: Greetings World

  println(hello1.greet("World")) // prints: Hello World
  // Inlined as we call Hello.greet directly

  println(hello2.greet("World")) // prints: Hello World
  // Not inlined as it is statiacally calling Greeting.greet.
  // Dynamically calls Hello.greet
}

How to make this happen

class Hello extends Greeting {
  inline override def greet(name: String): String = "Hello " + name
}

should become

class Hello extends Greeting {
  override def greet(name: String): String = "Hello " + name
}

and

class Hello extends Greeting {
  inline override def greet(name: String): String = ${ myHello('name) }
}

would become

class Hello extends Greeting {
  override def greet(name: String): String = ${ myHello('name) } // where macro is evaluated in place
}

[bishabosha]

this appears fixed by #8543