3.6 regression: unions of callable types involving `this` types

<!-- 🚨 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:** Version 3.6.3, Version 3.7.0-dev.20190911 <!-- Search terms you tried before logging this (so others can find this issue more easily) --> **Search Terms:** intersect function **Code** ```ts class A { public uniqueToA = "a"; public getString(this: A): string { return "a"; } } class B { public uniqueToB = "b"; public getString(this: B): string { return "b"; } } function f(model: A | B): string { return model.getString(); } ``` **Expected behavior:** In Typescript 3.5.3, the `model.getString()` in `function f` has no error. **Actual behavior:** In Typescript 3.6.3 and 3.7.0-dev.20190911, the code above gets the following errors: ``` index.ts:18:12 - error TS2684: The 'this' context of type 'A | B' is not assignable to method's 'this' of type 'A & B'. Type 'A' is not assignable to type 'A & B'. Property 'uniqueToB' is missing in type 'A' but required in type 'B'. 18 return model.getString(); ~~~~~ index.ts:10:12 10 public uniqueToB = "b"; ~~~~~~~~~ 'uniqueToB' is declared here. ``` **Playground Link:** <!-- A link to a TypeScript Playground "Share" link which demonstrates this behavior --> http://www.typescriptlang.org/play/#code/MYGwhgzhAECC0G8BQ1XQA4FcBGICWw0mAdngI6YCmAKgPbwC80ARGMwNxIppa4HQBzSgBcAysIBOeYgIAUwgBZ4IALjgBKNREnSBibmjQSR…