Typeguards with destructuring parameters

Search Terms

Typeguard
type narrowing
destructuring

Suggestion

It'd be nice to be able to have typeguard functions with destructuring arguments.

Use Cases

When manipulating tuples or other anonymous structured types, whose typeguard relies only on a small part of the object, it would make the code much easier to read.
Currently, typeguards cannot be used with destructuring arguments, which makes it a weird special case.

Examples

const val: Array<[string, boolean]> = ([] as Array<[string | undefined, boolean]>)
    .filter(([foo, bar]: [string | undefined, boolean]): [foo, bar] is [string, boolean] => foo !== undefined);

const val2: Array<[string, boolean]> = ([] as Array<[string | undefined, boolean]>)
    .filter((val: [string | undefined, boolean]): val is [string, boolean] => val[0] !== undefined);

https://www.typescriptlang.org/play/index.html#src=const%20val%3A%20Array%3C%5Bstring%2C%20boolean%5D%3E%20%3D%20(%5B%5D%20as%20Array%3C%5Bstring%20%7C%20undefined%2C%20boolean%5D%3E)%0D%0A%20%20%20%20.filter((%5Bfoo%2C%20bar%5D%3A%20%5Bstring%20%7C%20undefined%2C%20boolean%5D)%3A%20%5Bfoo%2C%20bar%5D%20is%20%5Bstring%2C%20boolean%5D%20%3D%3E%20foo%20!%3D%3D%20undefined)%3B%0D%0A%0D%0Aconst%20val2%3A%20Array%3C%5Bstring%2C%20boolean%5D%3E%20%3D%20(%5B%5D%20as%20Array%3C%5Bstring%20%7C%20undefined%2C%20boolean%5D%3E)%0D%0A%20%20%20%20.filter((val%3A%20%5Bstring%20%7C%20undefined%2C%20boolean%5D)%3A%20val%20is%20%5Bstring%2C%20boolean%5D%20%3D%3E%20val%5B0%5D%20!%3D%3D%20undefined)%3B%0D%0A%0D%0Aconst%20val3%3A%20string%5B%5D%20%3D%20(%5B%5D%20as%20Array%3Cstring%20%7C%20undefined%3E)%0D%0A%20%20%20%20.filter((foo%3F%3A%20string)%20%3D%3E%20foo%20!%3D%3D%20undefined)%3B%0D%0A

Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript / JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. new expression-level syntax)

Start by reproducing the destructured and non-destructured examples in the linked TypeScript Playground. The issue names no source file or test; trace how type predicates and destructured parameters are handled. Done means the destructured predicate narrows the filtered tuple type without changing emitted JavaScript, with regression coverage.