How do you know from `sealed class X {}` and the rest of its
compilation unit that all X's subtypes are co-declared?
By co-declared, I mean "in the same compilation unit."
The emphasis should be on the word "all", not "co-declared". How do
you know that ALL of X's subtypes are declared in the same compilation
unit?
Here's what I'm suggesting.
If you say
sealed interface X { ... }
with no permits clause, then the permits clause is inferred from the
contents of the compilation unit, which is _by definition_ all the
permitted subtypes. (If there are no subtypes in the current
compilation unit, a warning may be in order.)
Similarly, if you have a subtype of X:
sealed interface X {
class A implements X { }
}
that is _in the same compilation unit_, then we will infer `sealed` on A
unless you say otherwise.
In all other cases, subtypes of sealed types must either say "sealed" or
"non-sealed" (or "final").
