Suggestion: Discriminate between union types based on extracted property

[inversion]

Search Terms

Discriminated union type guard destructuring destructured extracted assigned property

Suggestion

If the discriminating property of an interface is assigned to a const via destructuring or directly it is not usable as a type guard.

Use Cases

• Code brevity

Examples

In the example I would expect bool to be usable as a type guard in functions f and g but it is not. Is there something I am missing here that would make this unsound?

interface A {

	bool: false;
}

interface B {

	bool: true;

	data: string;
}

type C = A | B;

function f(x: C) {

	const { bool } = x;

	if (bool) { // Error: Property 'data' does not exist on type 'C'. Property 'data' does not exist on type 'A'.

		return x.data;
	}
}

function g(x: C) {

	const bool = x.bool;

	if (bool) { // Error: Property 'data' does not exist on type 'C'. Property 'data' does not exist on type 'A'.

		return x.data;
	}
}

function h(x: C) {

	if (x.bool) {

		return x.data;
	}
}

https://www.typescriptlang.org/play/#src=interface%20A%20%7B%0D%0A%0D%0A%09bool%3A%20false%3B%0D%0A%7D%0D%0A%0D%0Ainterface%20B%20%7B%0D%0A%0D%0A%09bool%3A%20true%3B%0D%0A%0D%0A%09data%3A%20string%3B%0D%0A%7D%0D%0A%0D%0Atype%20C%20%3D%20A%20%7C%20B%3B%0D%0A%0D%0A%0D%0Afunction%20f(x%3A%20C)%20%7B%0D%0A%0D%0A%09const%20%7B%20bool%20%7D%20%3D%20x%3B%0D%0A%0D%0A%09if%20(bool)%20%7B%0D%0A%0D%0A%09%09return%20x.data%3B%0D%0A%09%7D%0D%0A%7D%0D%0A%0D%0A%0D%0Afunction%20g(x%3A%20C)%20%7B%0D%0A%0D%0A%09if%20(x.bool)%20%7B%0D%0A%0D%0A%09%09return%20x.data%3B%0D%0A%09%7D%0D%0A%7D

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. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

[jack-williams]

Tracked here #12184

[fatcerberus]

Is there something I am missing here that would make this unsound?

No, but this in general is difficult from an implementation standpoint - the compiler would have to track interactions between different variables. Basically right now the control-flow analyzer only considers what the expression inside if (...) implies for the variables directly involved in that expression. At that point it doesn't really know where, e.g. your bool variable actually originated from.

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.