Allow non returning functions to have contextually required `undefined` return type

Search Terms

#36239

Suggestion

Allow non returning functions to have contextually required undefined return type.

Use Cases

• Callbacks

Examples

declare function f(a: () => undefined): void;
f(() => { }); // error -> ok
f((): undefined => { }); // error -> ok
const g1: () => undefined = () => { }; // error -> ok
const g2 = (): undefined => { }; // error -> ok if possible
function h1() {
}
f(h1); // error -> error
function h2(): undefined { // error -> ok if possible
}

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.

[RyanCavanaugh]

I like this idea 👍

[RyanCavanaugh]

@ljwagerfield it sounds like you should turn on the noImplicitReturns flag, which does exactly that?