On 8/19/2019 11:52 AM, Brian Goetz wrote:
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.)
OK.
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.
To be clear, infer `final` on A. We were already inferring `sealed` last
week; the new thing this week is to infer A's `permits` list as empty
(giving A an overall score of implicitly `final`) rather than "any
co-declared subtypes of A". Given the code above, it is a compile-time
error for any class in any compilation unit to attempt to subclass A.
Where this is all headed is that we never infer `sealed` with a
non-empty `permits`. As you said, no need for `permits none`. It is good
to draw a clear distinction between
`sealed`-with-at-least-one-permitted-subtype versus
`final`-with-zero-subtypes.
Alex
