narrowing to "never" by "if" statement does not exclude "undefined" return type

TypeScript Version: 2.7.1

Search Terms: narrowing types never switch case if undefined strictNullChecks

Code

// Build this code with "--strictNullChecks" option.

function assertNever(x: never): never {
    throw new Error("not reached");
}

enum A {
    Foo, Bar
}

// There are no errors on this function.
function good(a: A): number {
    switch (a) {
        case A.Foo: return 0;
        case A.Bar: return 1;
    }
}

// This function causes the following error though "a" is "never" at the end of the function.
// error TS2366: Function lacks ending return statement and return type does not include 'undefined'.
function bad(a: A): number {
    if (a === A.Foo) return 0;
    if (a === A.Bar) return 1;
}

// No error if explicit return of "never" is added.
function ok(a: A): number {
    if (a === A.Foo) return 0;
    if (a === A.Bar) return 1;
    return assertNever(a);
}

Expected behavior: The bad() function should not cause error so that if statement works like switch.

Actual behavior: As commented in the code, only bad() function causes an error.

Playground Link: (please check "strictNullChecks" option.)

http://www.typescriptlang.org/play/#src=function%20assertNever(x%3A%20never)%3A%20never%20%7B%0D%0A%20%20%20%20throw%20new%20Error(%22unexpected%22)%3B%0D%0A%7D%0D%0A%0D%0Aenum%20A%20%7B%0D%0A%20%20%20%20Foo%2C%20Bar%0D%0A%7D%0D%0A%0D%0Afunction%20good(a%3A%20A)%3A%20number%20%7B%0D%0A%20%20%20%20switch%20(a)%20%7B%0D%0A%20%20%20%20%20%20%20%20case%20A.Foo%3A%20return%200%3B%0D%0A%20%20%20%20%20%20%20%20case%20A.Bar%3A%20return%201%3B%0D%0A%20%20%20%20%7D%0D%0A%7D%0D%0A%0D%0Afunction%20bad(a%3A%20A)%3A%20number%20%7B%0D%0A%20%20%20%20if%20(a%20%3D%3D%3D%20A.Foo)%20return%200%3B%0D%0A%20%20%20%20if%20(a%20%3D%3D%3D%20A.Bar)%20return%201%3B%0D%0A%7D%0D%0A%0D%0Afunction%20ok(a%3A%20A)%3A%20number%20%7B%0D%0A%20%20%20%20if%20(a%20%3D%3D%3D%20A.Foo)%20return%200%3B%0D%0A%20%20%20%20if%20(a%20%3D%3D%3D%20A.Bar)%20return%201%3B%0D%0A%20%20%20%20return%20assertNever(a)%3B%0D%0A%7D

Related Issues:

Commencez par reproduire le code TypeScript de l'issue avec strictNullChecks activé et comparez les cas if et switch. Suivez le narrowing du control-flow du compilateur ainsi que son analyse des retours manquants pour la mauvaise fonction. Le travail est terminé lorsque le diagnostic signalé est résolu pour ce cas et que la couverture de régression confirme le comportement.