Simplify Level Checking and Fix Errors

[odersky]

Drop the separate level number infrastructure (which was too coarse anyway) and base level checking exclsuively on owners.

[Linyxus]

Overall LGTM!

There are two tests that look like an expressiveness problem. I investigated a bit, and I think it may be due to Capability.visibility returns an owner that too narrow for local bindings. For instance for an object foo in a function main, its visibility will just be foo itself. But by principle, it should be visible in the entire main.

I tried to fix this by using levelOwner for visibility. This requires some tweaks to the tests. The commit for this change: dotty-staging@d8c44aa.

Most tweaks look reasonable to me, and it resolves the expressiveness problem. There are something in this change that looks suspicious to me. Like here a local c in function body shows up in the hidden set inside function result. But it does not look unsound to me. I guess a more general question is what does it mean for a capture set to have an owner? I have the intuition that for capture sets created under the same owner, there can be subtle differences for them too.

[Linyxus]

This looks suspicious to me. Seems like a expressiveness problem.

[Linyxus]

Also looks suspicious.

[odersky]

@Linyxus I don't think using levelOwner would work. The problem is when the element is inside some block, like here:

def foo = 
  val a = 
     val b: C^ = ...
     b

That will add the invisible b to the capture set of a, since levelOwner of b is foo.