TypeScript Version: 3.5.1

Search Terms: void typeof undefined narrow use before assign

Code

// with strictNullChecks
const x2: void = undefined;
const x3: void = undefined;

if(typeof x2 === "undefined") {
    x2 // Error: Variable 'x2' is used before being assigned.
}

if(typeof x3 !== "undefined") {
    x3 // never. x3 should probably be void
}

// without strictNullChecks
const x2: void = undefined;

if(typeof x2 === "undefined") {
    x2 // x2 remains void here, not undefined.
}

if(typeof x2 !== "undefined" && x2 !== null) {
    x2 // x2 remains void here, rather than never.
}

Expected behavior:

Using x2 inside the first conditional in the strictNullChecks example should not error, as the variable absolutely was assigned to before use (whatever its type should be).

Also, the semantics around narrowing void should be consistent when strictNullChecks is on and when it's off. And, based on some of the comments I've seen about void's purpose, it seems like the semantics with strictNullChecks off are the right ones.

Actual behavior:

The use of x2 triggers an error, as mentioned.

And narrowing void is a no-op when strictNullChecks is disabled (i.e., it always results in void), whereas narrowing void can produce never when strictNullChecks is enabled.

Playground Link:
With strictNullChecks: https://www.typescriptlang.org/play/index.html#code/PTAEHcEsBcAtQM7QE6QMbQHIFcA2uBhWAUzQGsEAoNAewDslQAPAJgC5QA3GyAE1AC8obHV7EAZpDrFeAbmr1GTAMwdufQcNESpM+ZUjiAFNACeAB2I1xzFoIFCARCLGTpvRwEpQAb0qgA21AQUABRZGQaZA4ANQBDVDiAI1xiUAByVnTQSARhBBlQJIkotOKpAHNQOIQESAr3ADpKAF9KA2MzS2tmZVAAQgdQZ203GS9ff0CVYLBpTmJkVqA

Without strictNullChecks: https://www.typescriptlang.org/play/index.html?strictNullChecks=false#code/PTAEHcEsBcAsHsCu1QGdoCdIGNoDlEAbQgYVgFNsBrVAKG3gDt1QAPAJgC5QA3eSACagAvKESMB5AGaRG5AQG5akKQApoATwAO5eFLbsRw0QCJxkmXIEmAlKADetUM4OgQrjOQC2AQ1mpefiEKTwAaUEZ4FHNpWXkAOloAX1plNU0dPVcAQmNQMwlYqxNQADJSnLzGIkI7RxdXdw5QT19-QMFQEPJwjB84cgxQOB9GCPIeQcSkoA