In JS, return type of type reference in @type tag is ignored on function declarations.

TypeScript Version: 3.0.0-dev.20180707

Code

/** @typedef {"a" | "b"} U */

/** @return {U} */
function f0() { return "a"; }

/** @type {() => U} */
function f1() { return "a"; }

/** @typedef {() => U} RetU */

/** @type {RetU} */
function f2() { return "a"; }

/** @type {RetU} */
const f3 = () => "a";

const x0 = f0();
const x1 = f1();
const x2 = f2();
const x3 = f3();

Expected behavior:

x0 through x3 are all of type "a" | "b".

Actual behavior:

x2 is of type string.

Noticed while reviewing #25486.

Technically, these aren't contextual types, but type annotations. That is the reason they don't work, actually: Typescript has no whole-type annotations for declarations so there was no easy analogue to translate the JSDoc to. Here's a smaller repro, which should not error, but currently does:

/** @typedef {(x: 'hi' | 'bye') => 0 | 1 | 2} Argle */
/** @type {Argle} */
function f(s) {
    return 0;
}

/** @type {0 | 1 | 2} */
var zeroonetwo = f('hi')

Get return type from `@type` tag

Get return type from `@type` tag (#25580)

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In JS, return type of type reference in `@type` tag is ignored on function declarations. #25525

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In JS, return type of type reference in @type tag is ignored on function declarations. #25525

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sandersn commented on Jul 10, 2018

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In JS, return type of type reference in `@type` tag is ignored on function declarations. #25525