Constructor proxy is restricted if class is protected

[som-snytt]

Calling a constructor proxy should be at least as restricted as using new.

Add Protected to a constructor companion if the class is protected.

If the companion exists but is not protected, then make the constructor proxy protected.

Explicit new already errored for access.

Fixes #22560

[som-snytt]

Silly how-I-fixed-it parody:

How I fixed it: I searched github for something similar Prof Odersky had fixed, and I tried to copy that. I saw some bit flags, and since I miss coding in C, I thought I'd try that. It didn't work but threw an exception. It's way past midnight because I had an extra coffee this afternoon. I've already done the word puzzles for tomorrow. I said I better use the class symbol to check if it's protected, so I did that and the test passed, so that's when I stopped. The test is just the OP, plus a line to show something else that doesn't work and something else that does.

Edit: just because it's a parody doesn't mean it's not true.

The follow-up to add a pos test was not a quickie and took some time, per the next comment. I would have liked visibility ("instrumentation") into operations on symbols and trees, but lacking that, excellent naming in the code base was very helpful.

[som-snytt]

Refresher that qualified protected does not mean access is granted to subclasses of the privateWIthin boundary symbol.

The expectation is that this works:

class Enumeration:
  protected class Val(i: Int):
    def this() = this(42)
  object Val:
    protected[Enumeration] def apply(i: Int) = new Val(i)

class Test extends Enumeration:
  val Hearts = Val(27)
  val Diamonds = Val()

Scala 2 has the same behavior.

An alternative is to require the companion to have the same access modifiers (that is, also be protected for this use case).

[som-snytt]

The ❌ is an unrelated server failure.

[som-snytt]

This delta was due to a println. Philosophically, tryInsert is the important part, and putting it right of the arrow makes it less visible, and unduly highlights errorTree. The edit with new syntax is clearer.

Usual idiom where "thing to the right" is less important for understanding because it doesn't carry much information:

def f(x: X) = x match
  case _: Y =>

[odersky]

LGTM