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:

首先，在启用 strictNullChecks 的情况下复现 issue 的 TypeScript 代码，并比较 if 和 switch 分支。跟踪 compiler 对错误函数的控制流缩窄和缺少返回分析。当该案例中报告的 diagnostic 得到解决，且回归覆盖确认了该行为时，即视为完成。