isolatedDeclarations should not have warning for functions that have no return statements

[Jack-Works]

🔎 Search Terms

isolatedDeclaration

🕗 Version & Regression Information

5.5.0

⏯ Playground Link

https://www.typescriptlang.org/play/?isolatedDeclarations=true&ts=5.5.0-dev.20240426#code/KYDwDg9gTgLgBAMwK4DsDGMCWEWIBQCUcA3gL5A

💻 Code

export function f() {}

🙁 Actual behavior

Function must have an explicit return type annotation with --isolatedDeclarations. (9007)

🙂 Expected behavior

No error

Additional information about the issue

Functions with no return statements can be easily inferred as type void, so there is no need to give this warning.

[jakebailey]

This error is intentional; a function without a return may not always be void.

declare function fail(): never;
export function f() {
    fail();
}
export const g = () => {
    fail();
}

Is:

export declare function f(): void;
export declare const g: () => never;

This can't be determined without type analysis, so is disallowed under isolatedDeclarations.

Playground Link

[RyanCavanaugh]

Correct error for function expressions, but not function declarations - declarations won't infer never

[jakebailey]

58331 is indiscriminate in what it affects; if we want to allow this for function declarations only, that may be possible, but it sure seems like an extra inconsistency just to avoid writing : void (and how often do people define a noop function?).

[Jack-Works]

hmm, I didn't think of never, so the function expression part is wrong. But I find it annoying to write : void for obviously clear function declarations.

[RyanCavanaugh]

Discussed and we don't want to make it untenable to, in the future, change the rules around function declaration return type inference. For example, it's maybe preferable that

function dofail() {
  Debug.fail("oops");
}

have an inferred return type of never (as it would if written as a function expression) rather than void.