Enable CFA / exhaustiveness-checking for exhaustive switch statements

[mattrberry]

The compiler complains about switchStatement, reporting that the switch statement doesn't exhaustively cover the cases.

String switchStatement() {
  switch ([]) {
    case [_, ...]:
      return 'at least 1';
    case []:
      return 'none';
  }
}

String switchExpression() => switch ([]) {
      [_, ...] => 'at least 1',
      [] => 'none',
    };

main.dart:1:8: Error: A non-null value must be returned since the return type 'String' doesn't allow null.
String switchStatement() {
       ^

This is obviously a fairly contrived case, but I've seen similar patterns come up a few times now. Every problem I've had is solvable, but not always in the nicest of ways.

It seems that the type-checker only does the full exhaustiveness checking for switch expressions or for switch statements with very obvious cases, e.g. if [...] was a case in the above example. I find this to be confusing. Some more discussion here.

What are the thoughts from the language team? Is this possible to support?

[srawlins]

Good question. Looks like a duplicate of #2977

[mattrberry]

I haven't had a chance to read all of the comments there, but that issue definitely seems to be a match. Sorry for the duplicate, I swear I tried to search the issues first 😅

[shayanzare-dev]

Good

[srawlins]

Yeah it took me a bit to find the issue too 😅