Need a flag to check if checking an expression for null or undefined has not that types

[cevek]

Often we have to check variables for empty values.
If variables has undefined or null types we check for that types.
But if we want to refactor our code, and replace nullable variables to always with value, we init its with empty values like false, 0, -1, ""
But checks that variables for null or undefined in other parts of program are still there, and make a lot of bugs

let a: string | undefined;
if (a !== undefined) {
   foo(a);
}

// and if we refactor

let a = '';
// In this check we potentially has a bug, cause undefined is not possible here, 
if (a !== undefined) {
   foo(a);
}

[ghost]

https://palantir.github.io/tslint/rules/strict-type-predicates/ attempts this. It could use #13502 to work even better, but should be capable of handling undefined checks already.

[cevek]

Another issue linked with this one

// no error
if (typeof v === null) {} 
if (typeof v === undefined) {} 

typeof v is always string, and check with raw null or undefined is incorrect

[RyanCavanaugh]

Covered by #14764?

[cevek]

@RyanCavanaugh yeah, it's the same what I want