On Mon, 24 Apr 2023 12:05:35 GMT, Maurizio Cimadamore <mcimadam...@openjdk.org> wrote:
> > > I believe the exhaustiveness algorithm needs rules for union types, both > > > in javac and in the JLS: > > > ... > > > I would expect the above switch to be exhaustive, since it covers all > > > possible components of the union type. In other words, the union type > > > should act as a sealed hierarchy which permits Foo and Bar - and the two > > > cases should be merged together (e.g. `case Foo` + `case Bar` should > > > cover `Foo | Bar`). For the records, I get same error on JDK 20. > > > > > > Precise exception type, here, `Foo | Bar` inside a catch is not an union, > > it's a way to merge catches when the body of catches are the same. In term > > of typing, `Foo | Bar` behave as the lub of Foo and Bar, apart in a throw. > > So the switch is not exhaustive here because `lub(Foo, Bar) = Exception`. > > We may add a special case in JLS for supporting precise exception type in > > switch given that this issue have been reported several times, but this is > > not something that should be fixed by this PR. > > I see the dance in 14.20 between: > > " A catch clause whose exception parameter is denoted as a union of types is > called a multi-catch clause. " > > And > > "The declared type of an exception parameter that denotes its type as a union > with alternatives D1 | D2 | ... | Dn is lub(D1, D2, ..., Dn). " > > (Which is required to make membership (and other properties) work) > > The reason I'm bringing this up in the context of the new exhaustiveness > analysis is that this might add more ways to unify a pair of types in the > exhaustiveness algorithm, hence leading to a more complex implementation. In this case, as long as the union type could only be present at the top level (as opposed to e.g. a type of a record component), we could simply require all components to be covered for union types for exhaustiveness in `checkCovered`, no? We could do something different, if we had a good reason, but enhancing `checkCovered` should be enough, and shouldn't be too difficult (as long as the spec would say so). Do I miss something? ------------- PR Comment: https://git.openjdk.org/jdk/pull/13074#issuecomment-1520105514
