fix: support record in Object.keys

[christophehurpeau]

• There is an associated issue that is labeled
  'Bug' or 'help wanted' or is in the Community milestone
• Code is up-to-date with the master branch
• You've successfully run jake runtests locally
• You've signed the CLA
• There are new or updated unit tests validating the change

This PR fixes an issue when strictFunctionTypes: true. This code failed:

type K = 'foo' | 'bar'
const record: Record<K, boolean> = { foo: true, bar: false };
Object.keys(record).forEach((key: K) => {
}); 

with the error:

Argument of type '(key: K) => void' is not assignable to parameter of type '(value: string, index: number, array: string[]) => void'.
  Types of parameters 'key' and 'value' are incompatible.
    Type 'string' is not assignable to type 'K'.

[msftclas]

All CLA requirements met.

[calebsander]

Doesn't look like this is using the right overload. Your code is only compiling because you can assign a function (i.e. (key: K) => {}) whose argument is a subset of type A to a function whose argument is A. For example, this is valid:

const abc: (def: number | string) => void = (def: number) => {}

[christophehurpeau]

It appears that it was using src/lib/es5.d.ts, not lib/lib.es5.d.ts. Now it is Object.keys(record) : K[] !

[calebsander]

I think the change should only be in src. lib is auto-generated from that folder.

[christophehurpeau]

why did this changed to never ?

[DanielRosenwasser]

{} has no properties, so keyof will return the empty union (i.e. `never)

[DanielRosenwasser]

This won't work because the object could have keys the static type isn't describing, so you can get values at runtime which you didn't expect.

See other discussion about trying to tie in Object.keys and keyof together: #12870, #13971, #24243, #25832, #26901

[typescript-bot]

This issue has been marked as a duplicate and has seen no activity in the last day. It has been closed automatic house-keeping purposes.

[typescript-bot]

This PR doesn't have any linked issues. Please open an issue that references this PR. From there we can discuss and prioritise.