Union types

Language Design
98

here is another possible use-case for union types that is currently not possible:

return when(union) {
    is A,
    is B ->  when(union) {
        // smart-cast `union` to A|B union type
        // do something common for A & B
        when(union) {
            is A -> // do A-specific
            is B -> // do B-specific
            // don't handle `is C`
        }
    }
    is C -> // do C-specific
}
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