Improved union type guards #1657
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Optimizing AST node visitor
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👍 |
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Note that (x instanceof Array) !== Array.isArray(x) in cross-window situations, though not that common these days. |
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If I modify the code as follows and add an class Message {
value: string;
}
interface MessageHandler {
handle(message: Message): void;
}
function saySize(message: Message | Message[], handler: MessageHandler) {
if (message instanceof Array) {
message.forEach(m => handler.handle(m));
}
else {
handler.handle(message); // Argument of type 'Message | Message[]' is not assignable to parameter of type 'Message'.
}
} |
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Fixes #1587.
This PR improves handing of union types in type guards. Specifically, when a type guard of the form
x instanceof Cis true, the type ofxis narrowed as follows:Cis a subtype of the type ofx, the type ofxis narrowed toC.xis of a union type, all types that are not subtypes ofCare removed from the type ofx.In the example
the
Messagetype is removed under the type guard because it is not a subtype ofArray.