Type guard does not narrow the same as if-statement

TypeScript Version: 3.5.1

Search Terms:

• Type guard function
• If-statement guard
• Narrow
• Discriminated union

Code

enum E1 {
    element0 = "element0",
    element1 = "element1",
    element2 = "element2",
}
enum E2 {
    element3 = "element3",
    element4 = "element4",
    element5 = "element5",
}

declare const obj : (
    | {
        type : E1,
        data : {
            object : {
                x : "e1"
            }
        }
    }
    | {
        type : E2,
        data : {
            object : {
                x : "e2"
            }
        }
    }
);

if (obj.type == E1.element0) {
    //OK
    const type : "element0" = obj.type;
    //OK
    const x : "e1" = obj.data.object.x;
}

if (
    obj.type == E1.element0 ||
    obj.type == E1.element1 ||
    obj.type == E1.element2
) {
    //OK
    const type : "element0"|"element1"|"element2" = obj.type;
    //OK
    const x : "e1" = obj.data.object.x;
}

declare function isE1 (mixed : unknown) : mixed is E1;

if (isE1(obj.type)) {
    //OK
    const type : "element0"|"element1"|"element2" = obj.type;
    //Expected : OK
    //Actual : Error; Type '"e1" | "e2"' is not assignable to type '"e1"'.
    const x : "e1" = obj.data.object.x;
}

Expected behavior:

2nd and 3rd examples should both work.

Actual behavior:

2nd example works.
3rd example does not work.

Playground Link:

Playground

Related Issues:

Start with the supplied Playground reproduction and compare the direct enum checks with the user-defined isE1 type guard. Investigate TypeScript's control-flow narrowing for discriminated unions and verify the expected result: the third example should narrow obj.data.object.x to "e1". Add or update a regression test if the relevant compiler test location is identified.