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

TypeScript Version: typescript@3.7.0-dev.20190831

Search Terms: tagged disjoint union generic

This issue stems from a question I asked on StackOverflow that @jcalz generously researched for me. I adapted his example to be a closer parallel to the example given in #17110.

Code

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 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

まず、リンクされた Playground を使用し、期待される動作と実際の動作を比較して、typescript@3.7.0-dev.20190831 または TypeScript@next で汎用的なインデックス付けの例を再現します。既存の理由付けについては関連する issue #17110 を読んでください。報告されたアクセスが、関連する union のケースをリグレッションさせることなく受け入れられれば完了です。