Need to return value in a "never" path

[dingfengquek]

TypeScript Version: 2.1.4

Code

type X = string | number;
function isString1(x: X): boolean {
    if      (typeof x === 'string') { return true ; }
    else if (typeof x === 'number') { return false; }
    else {
        x; // Is of never type
        return false; // However, need a return to compile
    }
}
function isString2(x: X): boolean {
    if      (typeof x === 'string') { return true ; }
    else if (typeof x === 'number') { return false; }
    else { // x is of never type here
        ((a:never)=>{})(x); // Validate
        return false; // However, need a return to compile        
    }
}
function isString3(x: X): boolean {
    if      (typeof x === 'string') { return true ; }
    else if (typeof x === 'number') { return false; }
    else { // x is of never type here
        ((a:never)=>{ throw new Error();})(x); // Validate
        // Does not need a return to compile, is good
    }
}
const compileTimeAssertNever = (x:never) => { throw new Error('unreachable'); };
function isString4(x: X): boolean {
    if      (typeof x === 'string') { return true ; }
    else if (typeof x === 'number') { return false; }
    else { // x is of never type here
        compileTimeAssertNever(x); // Validate
        return false; // However, need a return to compile
        // Shouldn't this be like isString3?
        // It would be nice if it was
    }
}

It would be even better if an error doesn't need to be thrown, since once "x" is known to be type "never", the code block can never be reached.

[zpdDG4gta8XKpMCd]

Just do this, it will work:

return compileTimeAsserNever (x);

[mhegazy]

Please see #10470