Cannot use conditional type to strongly type this type hint for callback

[mqudsi]

🔎 Search Terms

"conditional this type"

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about conditional types, infer,

⏯ Playground Link

https://www.typescriptlang.org/play?ts=5.2.2#code/GYVwdgxgLglg9mABAWwIYAcA8AVANAVVwGFEBeRcAEwFNgYxrKA+ACigAsYBnALkQEEAToNQBPHE1yIIqADayARqggBrPm069EJagA8o1MJS4Ujteo0QB+AcLETEfAHIInIeagWzqmIpMQAbnIg1Hx4iPQ0unxgIMgK1IJSqHaiYQDaALoAlGRMiISIHNxCAOZWfETZfPhZiADeAFCILdIIXFCIgtRc7lA1deRZANzNrRDtnRNgBvpkRZpl1jbFXKOtiMBwgogs3p0w8wAMwxGImAvcAHTeYKUcpwDUjzC5TRsb0x1F86vpMJl1h8Wl9OiB5jJ5EpVFdIbIWNNZlApMiIijNNkgcDur1ZFB-pl5iAsYgAL5jFrdKAgQRIHF9Uak0ZAA

💻 Code

function map<T,U,C = undefined>(this: Array<T>, callback: (this: C extends undefined ? Array<T> : NonNullable<C>, value: T, index: number, array: T[]) => U, thisArg?: C): U[] {
    const result: U[] = [];
    const context = thisArg ?? this;
    for (let i = 0; i < this.length; ++i) {
        const t = this[i];
        const u = callback.call(context, t, i, this);
        result[i] = u;
    }
    return result;
};

🙁 Actual behavior

TypeScript seems to be unable to resolve the conditional assignment to context with the conditional type C extends undefined ? Array<T> : NonNullable<C>

🙂 Expected behavior

tsc should be able to deduce that the type of context matches the type of callback's this type hint.

Additional information about the issue

I tried a number of different variations for the conditional assignment of context, trying to see if the problem was with when I was this to context, but all of the ones I tried gave me variations of the same error.

• this ?? thisArg
• typeof thisArg === "undefined" ? this : thisArg
• typeof thisArg === "undefined" ? this : <NonNullable<C>> thisArg

(Obviously using <any> in any number of places in the code would work around the issue.)

[fatcerberus]

Similar to #33912; conditional types aren't related to the results of control-flow analysis so you'll have to cast.

[mqudsi]

Thanks.

Closing as a duplicate of #33912