Multitypes

function foo( x : Int|String ) {
    switch(x) {
    case Int: // x is Int
    case String: // x is String
    }
}

class Foo<T: Int|String> {
    var data : T;
}

A multitype is a new type that consists of several subtypes : these subtypes need to be runtime-checkable (no structures, no type-parameters, no Dynamic).

The semantics of switch is changed when used on a multitype. It requires a case for each subtype (or "default"). In each case expression, the variable is retyped accordingly to the matched subtype.