Support generic typedef

TypeScript Version: 2.7.0-dev.20171220

Code

/** @typedef {T | null | undefined} Maybe<T> */

/** @type Maybe<number> */
const x = 0;

Expected behavior:

No error.

Actual behavior:

a.js(1,15): error TS2304: Cannot find name 'T'.
a.js(3,11): error TS2315: Type 'Maybe' is not generic.

[weswigham]

@Andy-MS In your example, I'm pretty sure you're supposed to introduce the T in the @typedef with an @template tag.

[weswigham]

Scratch that, I don't think closure supports generic typedefs at all. But if we're talking about enabling it in a jsdocy way, I'm pretty sure the template tag is where you'd introduce your type argument list.

[brendankenny]

I believe this is now fixed (by #24600?). @weswigham's closure example

/**
 * @template T
 * @typedef {T | null | undefined} Maybe
 */

/** @type Maybe<number> */
const x = 0;

now works in latest.

[mhegazy]

@brendankenny's comment in #20831 (comment) is the expected format to declare a generic type parameter in a type def.