JSDoc: support @implements tag on class

[darky]

TypeScript Version: nightly (2.5.0-dev.20170728)

Code

index.d.ts

class AClass {
    method(a: number): number
}

some.js

/** @type {AClass} */
class A {
    method(a) {
        return false;
    }
}

Expected behavior:

Type 'false' is not assignable to type 'number'.

Actual behavior:

Not working. Impossible to set type on class.

[ghost]

The problem with @type {AClass} is that that would declare the type to be an instance of AClass.
Maybe you want something like this? (won't work today):

/** @import { I } from "foo.d.ts" */

/** @implements {I} */
class AClass { ... }

However, in TS code we don't allow you to omit the parameter types on an override, even if they should be inferrable due to already being declared in the parent; the best link I have on that is #2000 (comment).

[darky]

Yeah, if it resolve problem, would be cool!

[que-etc]

The Lighthouse team uses a neat workaround to assert implementation of an interface.

Here is the gist:

// @file: interface.d.ts
export interface MyInterface {
    foo(): string
}

// @file: implementation.js

/** @typedef {import('./interface').MyInterface} MyInterface */

export class MyClass {
    constructor() {
        /**
         * Assert that MyClass correctly implements MyInterface.
         * @type {MyInterface}
         */
        const instance = this;
    }

    /**
     * @returns {number}
     */
    foo() {
        return 42;
    }
}

[sandersn]

Duplicate of #30156, which has a real code example.