Only allow sound inline parameters in overrides

[nicolasstucki]

Inline overrides can only use inline parameters if the overriden parameter is inline.

[nicolasstucki]

Ping @sjrd

[smarter]

What about the reverse? Is it ok for a non-inline parameter to override an inline parameter?

[sjrd]

It's debatable whether it's OK or desirable. But at least it is sound, because, as @nicolasstucki observed, you can always emulate the semantics of a non-inline param with an inline param that you immediately assign to a val in the body of the method.

However, personally I think we should not allow it either. In Scala, typically overrides must have exactly the same parameters. I think we should also apply that to the quality of being inline. And if someone really needs that capability, well they can always do the emulation themselves.

[sjrd]

Well, actually I think they're unsound. If we have a method with two parameters, the first one being either inline+val or by-value, we can end up with different evaluation order for the actual arguments:

def foo(inline x: Int, y: Int): Int = {
  val x2 = x
  x2 * x2 + y
}

foo({ println(1); 3 }, { println(2); 4 })
// prints 2
// prints 1
// returns 13

but

def foo(x: Int, y: Int): Int = {
  x * x + y
}

foo({ println(1); 3 }, { println(2); 4 })
// prints 1
// prints 2
// returns 13

So overriding an inline parameter with a by-value parameter is unsound.

[nicolasstucki]

I already added a commit disallowing them.

Typos fixed