Add safe-init test to prevent adding cold elements to hot arrays

[Xavientois]

This adds a negative test to ensure that the initialization checker doesn't allow cold values to be inserted into hot arrays through a method call.

[Xavientois]

Test currently fails due to case not being handled correctly

[griggt]

@Xavientois FYI opening PR branches directly in the lampepfl/dotty repository causes CI to run twice for every push:

https://github.com/lampepfl/dotty/actions/runs/2116548617
https://github.com/lampepfl/dotty/actions/runs/2116555314

which is one of the reasons we don't normally do that.

[olhotak]

The usual thing to do is to push a branch to https://github.com/dotty-staging/dotty (or a fork in your own account).

[liufengyun]

Here i is not cold. You can use e.g. this instead. It's better to only leave the method foo in the global object A, and move other code to a class --- this way, we can be sure that the method call foo(...) will be A.foo(...) instead of A.this.foo(...).

[liufengyun]

I was not clear enough in the above. I think something like the following will serve the purpose:

class B {
  var a = new Array[B](2)
  A.foo(this, a)
  println(a(0).i)
  val i = 99
}

[Xavientois]

@Xavientois FYI opening PR branches directly in the lampepfl/dotty repository causes CI to run twice for every push:

Sorry about that! I'll be sure to open PRs from my fork in the future

[liufengyun]

We can remove this line.

[liufengyun]

There should be a space before error

[liufengyun]

LGTM

[som-snytt]

For the record, I think that with all the interesting dimensions of freedom, the precise formatting of // error ought not to be a hangup. IIRC I pushed back on // scalac: in test files. My point is that testing is already hard, and any cost in making it easier is amortized over the life of the project.