Irregular behavior of generic non arrow function that is typed via JSDoc import.

[lillallol]

Bug Report

🔎 Search Terms

allowJs, checkJs, type imports via JSDoc, generic functions, no compile, inconsistent behavior, inline type, arrow

💻 Code

• ./privateApi.ts

export type IFn = <T>(m : T[][]) => T[][];

const inTsArrow = <T>(m : T[][]) : T[][] => {
    return m;
}
inTsArrow([[1]]); // no error gets linted as expected

function inTs<T>(m : T[][]) : T[][] {
    return m;
}
inTs([[1]]); // no error gets linted as expected

• ./index.js

/**@type {import("./privateApi").IFn}*/
export function inJs(m) {
    return m;
}
inJs([[1]]); // lints error. Why?

/**@type {import("./privateApi").IFn}*/
const inJsArrow = (m) => {
    return m;
}
inJsArrow([[1]]); // no error gets linted as expected

• ./tsconfig.json

{
    "compilerOptions": {
        "checkJs": true,
        "allowJs": true,
        "target": "ESNext",
        "strict": true,
        "noEmit": true,
        "rootDir": "./",
        
        "skipDefaultLibCheck": true,
        
        "noUnusedLocals": true,
        "noUnusedParameters": true,
    },
    "include": [
        "./**/*.ts",
        "./**/*.js"
    ],
    "exclude": [
        "node_modules",
        "TBD"
    ]
}

[sandersn]

Looks like the code that applies @type to signatures doesn't recognise that the signature for inJs should be generic. inJsArrow uses the normal type annotation machinery and doesn't have this problem.

Notably, for uncalled identifiers, quickinfo gives function inJs(m: T[][]): T[][], but const inJsArrow: IFn. In the former, T is unbound, so there's no way for anything to be assignable to it.

[lillallol]

@sandersn

Here is another bug, where optional function parameter is treated differently for arrow and now arrow functions:

• ./privateApi.ts

export type IAdd = (a:number,b?:number) => number;

• ./index.js

/**@type {import("./privateApi").IAdd}*/
function add(a,b) {
    if (b === undefined) return a;
    return a + b;
}

add(1);//lints error because it wrongly requires the second optional parameter

/**@type {import("./privateApi").IAdd}*/
const addArrow = (a,b) => {
    if (b === undefined) return a;
    return a + b;
};

addArrow(1);//works correct

should I open a new issue about that?

Edit : Tested with TypeScript 4.7.3

[sandersn]

No, it's basically the same problem.