JSDocs @template does not work as expected on export

[mjbvz]

From Mike (@yo1dog) on March 15, 2018 20:10

• VSCode Version: 1.21.1
• OS Version: 10.10.5

When using JSDocs to describe a function using @template, Intellisence does not correctly infer the template type if the template type is used in a parameter in a function parameter description and is exported on the same line it is declared. Difficult to describe. See the cases bellow.

In the cases bellow, I will give the content of doSomething.js and the Intellisence output when hovering over doSomething(...) in the following code:

// otherFile.js
const doSomething = require('./doSomething');

doSomething('hello', (outItem => {}));

Case 1

Declaring the function then exporting after.

// doSomething.js
/**
 * @template T
 * @param {T} inItem
 * @param {(outItem:T) => void} cb
 */
function doSomething(inItem, cb) {
}

module.exports = doSomething;

const doSomething: <string>(inItem: string, cb: (outItem: string) => void) => void

Works as expected. Intellisence knows the template type is string.

Case 2

Declaring the function and exporting on the same line.

// doSomething.js
/**
 * @template T
 * @param {T} inItem
 * @param {(outItem:T) => void} cb
 */
module.exports = function doSomething(inItem, cb) {
};

const doSomething: <T>(inItem: T, cb: (outItem: T) => void) => void

Does not work as expected. Expected the same result as Case 1.

Case 3

Declaring the function and exporting on the same line, but not referencing T in the callback.

// doSomething.js
/**
 * @template T
 * @param {T} inItem
 * @param {(outItem:Buffer) => void} cb
 */
module.exports = function doSomething(inItem, cb) {
};

const doSomething: <string>(inItem: string, cb: (outItem: Buffer) => void) => void

Works as expected. Intellisence knows the template type is string.

Copied from original issue: microsoft/vscode#45909

[mjbvz]

Confirmed in TypeScript@2.8.0-dev.20180314