Union not narrowed with `typeof x.y` as a discriminant

TypeScript Version: 3.6.0-dev.20190713

Search Terms: TS2339, union type object, union type property does not exist And github suggestions while I was writing the title.

Code

type Test = {
    someKey: false
} | {
    someKey: {
        someKeyInside?: number
    },
    someKeyThatExistOnlyIfOtherKeyIsNotFalse: string;
};

declare const test: Test;

if (typeof test.someKey === 'object') {
    console.log(test.someKeyThatExistOnlyIfOtherKeyIsNotFalse);
}

Expected behavior: String in someKeyThatExistOnlyIfOtherKeyIsNotFalse should be logged

Actual behavior:

error TS2339: Property 'someKeyThatExistOnlyIfSomeKeyIsNotFalse' does not exist on type 'Test2'.
  Property 'someKeyThatExistOnlyIfSomeKeyIsNotFalse' does not exist on type '{ someKey: false; }'.

29     console.log(test2.someKeyThatExistOnlyIfSomeKeyIsNotFalse); // TS2339: Property 'someKeyThatExistOnlyIfSomeKeyIsNotFalse' does not exist on type '{ someKey: false; }'.

Playground Link: https://www.typescriptlang.org/play/index.html#code/C4TwDgpgBAKhDOwCMUC8UDeAoKurwHsBbCAaQhAC4oAzAQwBt4IsBfKAH0xz0JPKrc8w-MTIUAkgDt4ASwAmEalICuRAEYQATj1ysANLtH8KMABZ1gAUQAesxAHkpDEBJoPgZ7QInwAcgTAAGKMzNSIWrJSAOYA3GzxWIoAxgx0WtDJBDLAUMAIyNRwiEiJsjRQABT5JQB0fOIgAJRCeFkyBAwQtQwE0dUFSPViAuaWtvbATi5uHl5aPv6BIUwQTbFQAPSbUA6kAIRsWFigkLAFAExorbgNAtT0q2ycN8aN1NgieNuwXlDZLn+Umg8nKNG0ECkyWgNC0xDy4GgxWQ+jePhkCkydCkUE0UBUUkUNCiEHkQLRFCMwjukgxigA-Mo1JodMIDEYaSAxtY7I5nK53J5vJIlsFQkp8MBIjF4qxEik0hkoO1EHlLkVLmUKtVEQQKjVgBdhiYQGhUOgAOQEdQAKwgyWAFpanza2UIXR6fQGiCNnO5Ez5M0F80WATFq3WWx2MAAyhcAMzxgCc1AACnDIFpQFALX6LDzJtMBXNha5RStmBaoPICAgoFJAlAILzctkEWcLRgKYJHswNqwLbU2EA

Take a look at type Test1. The only one difference from type Test2 is someKeyInside should be always defined.

Related Issues:

[bterlson]

Just chiming in to say I would love this fixed! Our repro is something like the following:

interface A { x: string, y: string };
interface B { x: number, y: number };
type X = A | B;
 
declare var bar: X;
 
if (typeof bar.x === 'string') {
    let y = bar.y; // string | number, but really should be string.
}

Essentially we have some methods we’d like to return union of object types from. The object types are mostly distinct with a little overlap. Ideally users would just check the field that they want is present, and TS would do the math and present a proper type inside the if block.

We ran in to this because we were using a tagged union pattern which was working fine, but then attempted to generalize it, and found the narrowing stopped working.

[jack-williams]

I’m not sure typeof has ever triggered narrowing by discriminant.