noImplicitReturns control flow errors with explicit type guard checks

[tomrav]

TypeScript Version: 2.8.1 & 2.9.0-dev.20180409

Search Terms: noImplicitReturns throw type guard

Code

interface A { type: 'A' }
interface B { type: 'B' }

function isA(val: any): val is A { return val && val.type === 'A'}
function isB(val: any): val is B { return val && val.type === 'B'}

function getAOrThrow(val: A | B): A {
    if (isA(val)) { 
        return val; // val inferred to A
    } else if (isB(val)){ // val inferred to B
        throw new Error(); 
    }
    val // type inferred to never
}

Expected behavior:
Type check should pass without error.

Actual behavior:
Happens only with noImplicitReturns turned on.
getAOrThrow errors on its return value with Function lacks ending return statement and return type does not include 'undefined'..
If I remove the second type guard call (isB) and just leave it as a catch-all else, there is no error.

Playground Link:
Cannot reproduce in playground due to noImplicitReturns being turned off.

Related Issues:
Couldn't find any.

[tomrav]

Seems similar to #23281

[sledorze]

@tomrav Here's a playground with you example and 'noImplicitReturns' activated (you can switch options in the 'Options' menu)

http://www.typescriptlang.org/play/#src=interface%20A%20%7B%20type%3A%20'A'%20%7D%0D%0Ainterface%20B%20%7B%20type%3A%20'B'%20%7D%0D%0A%0D%0Afunction%20isA(val%3A%20any)%3A%20val%20is%20A%20%7B%20return%20val%20%26%26%20val.type%20%3D%3D%3D%20'A'%7D%0D%0Afunction%20isB(val%3A%20any)%3A%20val%20is%20B%20%7B%20return%20val%20%26%26%20val.type%20%3D%3D%3D%20'B'%7D%0D%0A%0D%0Afunction%20getAOrThrow(val%3A%20A%20%7C%20B)%3A%20A%20%7B%0D%0A%20%20%20%20if%20(isA(val))%20%7B%20%0D%0A%20%20%20%20%20%20%20%20return%20val%3B%20%2F%2F%20val%20inferred%20to%20A%0D%0A%20%20%20%20%7D%20else%20if%20(isB(val))%7B%20%2F%2F%20val%20inferred%20to%20B%0D%0A%20%20%20%20%20%20%20%20throw%20new%20Error()%3B%20%0D%0A%20%20%20%20%7D%0D%0A%20%20%20%20val%20%2F%2F%20type%20inferred%20to%20never%0D%0A%7D

[tomrav]

Thanks for the tip, didn't know you could do that! 👍

[mhegazy]

Please see #17358, #20409, #10470 and #9655

[tomrav]

I see how you're approaching it now, I'll close the issue.
Thanks 👍