Can not infer not null / undefined when use enum as the optional key

[taoalpha]

TypeScript Version: 3.4.0-dev.201xxxxx

Search Terms:
enum as key
Code

enum A {
  a,
  b,
  c
}

type b = A.a|A.b

const a: {
  [key in A]?: {
    [key in A]?: string
  }
} = {
  [A.a]: {
    [A.a]: "a"
  }
}

function t() {
  // below line will have error on a[A.a][A.a] for possibly `undefined` when enabled `strictNullChecks`
  if (a[A.a] && a[A.a][A.a]) {
    return a[A.a][A.a];
    }
    return
}

Expected behavior:
no err when enable strictNullChecks, as the undefined should be ruled out after first a[A.a] check.

Actual behavior:

Object is possibly 'undefined'. for a[A.a][A.a]

Playground Link: https://www.typescriptlang.org/play/#src=enum%20A%20%7B%0D%0A%20%20a%2C%0D%0A%20%20b%2C%0D%0A%20%20c%0D%0A%7D%0D%0A%0D%0Atype%20b%20%3D%20A.a%7CA.b%0D%0A%0D%0Aconst%20a%3A%20%7B%0D%0A%20%20%5Bkey%20in%20A%5D%3F%3A%20%7B%0D%0A%20%20%20%20%5Bkey%20in%20A%5D%3F%3A%20string%0D%0A%20%20%7D%0D%0A%7D%20%3D%20%7B%0D%0A%20%20%5BA.a%5D%3A%20%7B%0D%0A%20%20%20%20%5BA.a%5D%3A%20%22a%22%0D%0A%20%20%7D%0D%0A%7D%0D%0A%0D%0Afunction%20t()%20%7B%0D%0A%20%20if%20(a%5BA.a%5D%20%26%26%20a%5BA.a%5D%5BA.a%5D)%20%7B%0D%0A%20%20%20%20return%20a%5BA.a%5D%5BA.a%5D%3B%0D%0A%20%20%20%20%7D%0D%0A%20%20%20%20return%0D%0A%7D

[RyanCavanaugh]

Duplicate #28081

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.

[jcalz]

The fix for #28081, namely #31478, did not address enum keys; should this be reopened or should a new issue be filed?