Continuing #17110 - Given generic `W extends { x: A }` should be able to use `keyof W['x']` to index w.x

<!-- Please try to reproduce the issue with `typescript@next`. It may have already been fixed. --> **TypeScript Version:** `typescript@3.7.0-dev.20190831` <!-- Search terms you tried before logging this (so others can find this issue more easily) --> **Search Terms:** tagged disjoint union generic This issue stems from a question I asked on [StackOverflow](https://stackoverflow.com/questions/56718622/generic-function-operating-on-tagged-disjoint-union-why-is-type-specification?noredirect=1#comment100017317_56718622) that @jcalz generously researched for me. I adapted his example to be a closer parallel to the example given in #17110. **Code** ```ts interface A { a: string; } interface W { x: A } function foo<W1 extends W>(w: W1, k: keyof W1['x']) { w.x[k]; // Type 'keyof W1["x"]' cannot be used to index type 'A'. const x: W1['x'] = w.x; // Okay x[k] // Okay } ``` **Expected behavior:** Same as #17110. However, unlike #17110, `W['x']` is only `A` instead of `A | B | undefined`. The [explanation given](https://github.com/microsoft/TypeScript/issues/17110#issuecomment-459911913) as to why #17110 was not a bug was: > The constraint of `keyof W1["x"]` is `keyof (A | B)`, which simplifies to `keyof A & keyof B`, which is `never` (because there are no common properties). However, here `keyof W1['x']` is `keyof A` which is _not_ `never`. **Actual behavior:** Same as #17110. **Playground Link:** http://www.typescriptlang.org/play/#code/JYOwLgpgTgZghgYwgAgILIN7LgLmQZzClAHMBuZAXwChrRJZEUB1Ta5D5ADz1WpuowAriARhgAexDIYEiQB5mARmQQukEABN8yZgD4AFAHc8ygDTIA1nksQAnhJi6lAbQDkXNwF0AlGw5GAHRcLpZeFAD0EcgAKnYADihutg5Oyi4ARFwZXm7ICHAgIBJgyABGKEL4EJrIYBLIoJpqdQlJqG6BtBwIUoTcpq4e3sgAvMhBXJHRAPKWcHbs3KFeyFHIcwv8QA **Related Issues:** #17110