Could handling of Array pop() and shift() be made better?

[Dagur]

Proposal

In 0.239.x a change was made

Calling shift() and pop() on an Array will now return T | undefined. #8341

Which is great but I was wondering if it could be made a little bit smarter because it leads to checks and $FlowFixMe's that shouldn't be needed.

Use case

Example:

const a = someString.split(",").pop()
Here we know for certain that a is a string because split() will return a non-empty array of strings.

Example 2:

while (stringArray.length > 0) {
  const a = stringArray.shift();
 ...
}

We have made sure that the array is not empty but Flow still insists that a could be undefined.

[SamChou19815]

I don't think it can be improved, without hardcoding a lot of things just for this case. Then there is also the similar problem with the Map.has Map.get pair, and so on. We decided that it's better to accept the limitation and make the typing more sound, rather than keeping it unsound and hoping that one day we can come up with something that make most of the existing code still type check