Object literals should have a `this` type too

<!-- 🚨 STOP 🚨 𝗦𝗧𝗢𝗣 🚨 𝑺𝑻𝑶𝑷 🚨 Half of all issues filed here are duplicates, answered in the FAQ, or not appropriate for the bug tracker. Even if you think you've found a *bug*, please read the FAQ first, especially the Common "Bugs" That Aren't Bugs section! Please help us by doing the following steps before logging an issue: * Search: https://github.com/Microsoft/TypeScript/search?type=Issues * Read the FAQ: https://github.com/Microsoft/TypeScript/wiki/FAQ Please fill in the *entire* template below. --> <!-- Please try to reproduce the issue with `typescript@next`. It may have already been fixed. --> **TypeScript Version:** 3.7.0-dev.20190831 <!-- Search terms you tried before logging this (so others can find this issue more easily) --> **Search Terms:** `this` return type object composing composition **Code** I tried writing some self-contained parts of objects as a kind of multiple-inheritance or object composition, which I would combine later, and ran into this case. ```ts const a = { Clone() { return this; }, x: 1, }; const b = { Clone: a.Clone, y: 2, }; console.log(b.Clone().x); // number, bad! Should be an error! console.log(b.Clone().y); // compiler error, bad! This should be acceptable! ``` **Expected behavior:** `this`, the return type of `Clone`, should evaluate to `typeof b` in `b.Clone()` **Actual behavior:** It evaluates to `typeof a`, since it was defined in `a` This can be circumvented by defining `this` as a type parameter like so: ```ts const a = { Clone<T>(this: T) { return this; }, x: 1, } const b = { Clone: a.Clone, y: 2 }; console.log(b.Clone().x); // compiler error, good! console.log(b.Clone().y); // works! Also good! ``` Another interesting problem at play here though, is that using a type parameter for `this` also means that TS doesn't know what could be inside of `this`. This means you have to manually specify whatever members you hope to have access to. It would be nice if you could do `T extends typeof a` and…