Bug diagnosing error "Object is possibly 'undefined'" #31344

TypeScript Version: 3.4.5

Search Terms: Object is possibly 'undefined'

Code

function Sum(A?: number) {
    const AisUndefined: boolean = typeof A === "undefined";

    if (AisUndefined) {
        return 'is undefined';
    }

    const sum: number = A + 10; //Error here: "Object is possibly 'undefined'"
    return sum;
}

Expected behavior: As I assign the condition typeof A === "undefined" in the constant AisUndefined and make a if statement to return a string in case of A is definitely undefined, in the constant sum, there should be no problem making a sum of A with a number assuming that A is definitely not undefined as the code reach there, after the if statement.

Actual behavior: When I try to make a sum with A and a number, and probably any other operation that takes on that A is not undefined, typescript identifies this as an error, saying that Object is possibly 'undefined', basically ignoring the return inside the if statement.

Note: If I replace the constant AisUndefined by their own condition, like:

if (typeof A === "undefined") {
    return 'is undefined';
}

Therefrom, the error disappears.

Playground Link: https://www.typescriptlang.org/play//#src=function%20Sum(A%3F%3A%20number)%20%7B%0D%0A%20%20%20%20const%20AisUndefined%3A%20boolean%20%3D%20typeof%20A%20%3D%3D%3D%20%22undefined%22%3B%0D%0A%0D%0A%20%20%20%20if%20(AisUndefined)%20%7B%0D%0A%20%20%20%20%20%20%20%20return%20'is%20undefined'%3B%0D%0A%20%20%20%20%7D%0D%0A%0D%0A%20%20%20%20const%20sum%3A%20number%20%3D%20A%20%2B%2010%3B%0D%0A%20%20%20%20return%20sum%3B%0D%0A%7D (You will have to check the strictNullChecks option to get this error)

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