Narrow indexing variable when type-testing indexed access expressions

[changwoolab]

Bug Report

🔎 Search Terms

type, inference, object, never

🕗 Version & Regression Information

4.3.5

⏯ Playground Link

Playground link with relevant code

💻 Code

interface UserInformationInput {
    userId: string;
    age: number;
}

function test(info: UserInformationInput) {
    console.log("Before info Iteration:", info);
    let k: keyof typeof info;
    for (k in info) {
        if (typeof info[k] === "string") {
            info[k] = "mamamama"; // <-- Error! 
                             // Type 'string' is not assignable to type 'never'.
        }
        else {
            info[k] = 31; // <-- Error!
                        // Type 'number' is not assignable to type 'never'.
        }
    }
    console.log("After info Iteration:", info);
}

test({
    userId: "asdf",
    age: 11
})

🙁 Actual behavior

"typeof info[k]" is inferred to be "never"
But at the line 10(if typeof info[k] === "string), "typeof info[k]" was string or number.

Furthermore, after making sure the type of "info[k]" by using "if", it still remains to be "never".

And, although compiler throws an Error above, when I executed the code, it runs very well with the result below
[LOG]: "Before info Iteration:", { "userId": "asdf", "age": 11 }
[LOG]: "After info Iteration:", { "userId": "mamamama", "age": 31 }

🙂 Expected behavior

I think "typeof info[k]" should be "string | number", not 'never'

[fatcerberus]

See #30769 for why you’re seeing never rather than string | number.

[RyanCavanaugh]

A potential solution would be to narrow k to "userId" (in the truthy case) here.