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intersection type between two template literal type will never be 'never' type #60446
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Description
🔎 Search Terms
intersection template type never
🕗 Version & Regression Information
- I was unable to test this on prior versions than 4.5 because
// ^?does not work on typescript playground
I think this is bug since template literal type is introduced in 4.1.
⏯ Playground Link
💻 Code
function any(): any { throw null; }
const test5: `miauth/${string}/check` & `${string}/timeline` = any();
// ^?
// this should be never but does not🙁 Actual behavior
`miauth/${string}/check` & `${string}/timeline` will remain `miauth/${string}/check` & `${string}/timeline`
not coming never would cause problem with Indexed Access Types
function any(): any { throw null; }
type Endpoints = {
'test/timeline': 0,
'test2/timeline': 0,
'other endpoint': 2,
[ep: `miauth/${string}/check`]: 1, // no error at test2 if comment
}
type TimelineEndpoints = keyof Endpoints & `${string}/timeline`;
const test2: Endpoints[TimelineEndpoints] = any();
const test3: 0 = test2;
// This assignment should not have type errors,
// but because `miauth/${string}/check` & `${string}/timeline` is not never,
// test2 become 0|1 so we cannot assign test2 to test3.🙂 Expected behavior
`miauth/${string}/check` & `${string}/timeline` should become never and Endpoints[TimelineEndpoints] in snippet above should become 0
Additional information about the issue
Of course, we cannot assign variable typed `miauth/${string}/check` to `${string}/timeline` and vice versa.
function any(): any { throw null; }
const test0: `miauth/${string}/check` = any();
const test1: `${string}/timeline` = test0; // error here, as expectedReal world case: misskey-dev/misskey#14885 (comment)
Activity
Andarist commentedon Nov 7, 2024
The problem is not exclusive to template literals - string mappings are also affected:
jcalz commentedon Nov 7, 2024
Relevant #40598 (comment)
RyanCavanaugh commentedon Nov 7, 2024
In general we don't have any static guarantees about any two types always producing
neverwhen intersected. There is definitely a gray line (literals do, some template literals do, but not all of them), but it is not computable in general to know if two arbitrary types have a nonzero overlap. I don't really want to complicate this logic any further than it already is just to further a problem that won't actually be solved.typescript-bot commentedon Nov 10, 2024
This issue has been marked as "Not a Defect" and has seen no recent activity. It has been automatically closed for house-keeping purposes.