No auto-completion on typescript interface with classes implement

Redirected by https://github.com/microsoft/vscode-cpptools/issues/3789

Type: LanguageService
Describe the bug

• macOS Mojave 10.14.5
• VSCODE : 1.35.1
• TS 3.5.1

To Reproduce

Create a file with the following :

import { Strategy } from "passport-local";

class SSOPlugin implements Plugin {

  constructor() {}

  init() {}
}

interface Plugin {
  authenticators?: Record<string, any>;

  strategies: { [key: string]: KuzzleStrategy };
}

interface KuzzleStrategy {
  config: {
    authenticator: string;
    fields: string[];
  };
  methods: {
    afterRegister?: () => any;
    create: () => any;
    delete: () => any;
    exists: () => any;
    getById?: () => any;
    getInfo?: () => any;
    update: () => any;
    validate: () => any;
    verify: () => any;
  };
}

export = SSOPlugin;

In constructor or any method using this. and auto-completion doesn't give anything.

But if I give the wrong types, the compiler complain (also red line on IDE).

Expected behavior

I should get auto-completion.

Start by reproducing the reported TypeScript language-service behavior with the interface and implementing class shown in the issue, then compare completions inside the constructor and methods with the compiler's type errors. Trace the completion entry point for this in an implementing class and add a regression test covering the interface members. Done means the expected members appear in auto-completion without changing type checking.