Why is the SupposedId type below not considered a type identity?
I'm getting an error in Typescript saying that
Type 'T' is not assignable to type 'SupposedId<T>'How is it possible that T cannot be assigned to either T or T? What am I missing?
type SupposedId<T> = T extends object ? T : T;
function makeId<T>(test: T): SupposedId<T> {
return test // <- Type 'T' is not assignable to type 'SupposedId<T>'
}
The reason behind this issue relates to the concept of distributed conditional types. The SupposedId type distributes to T extends any | T extends object. Your case is indeed unique as both resolve to T, but the conditional type behavior varies based on whether or not T extends object.
To address this, according to the official documentation, you can resolve it by encapsulating square brackets around each side of the extends keyword:
type NewSupposedId<T> = [T] extends [object] ? T : T;
function generateNewId<T>(input: T): NewSupposedId<T> {
/* `input` now falls into either `T extends object` or `T extends any`,
instead of `T extends object | T extends any` */
return input;
}
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