intersection type between two template literal type will never be 'never' type

[anatawa12]

🔎 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

https://www.typescriptlang.org/play/?ts=5.8.0-dev.20241106#code/GYVwdgxgLglg9mABAQzATwBQEoBcL2IDeiUAFgE5wDuiYIANvQNyIC+AUOxAgM5QkBTPgFY8AAwC2MZCDIB6ACSE+5GGADmrORFICIAazGIAZIjFKVazXNgSB9NQKMBefJixN2cuYl++AegD8Xj5+ZDA8iDykcAwAJogARgK0AgBuAuRJsohxcEK0cFDsQA

💻 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.

playground

🙂 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 expected

playground

Real world case: https://github.com/misskey-dev/misskey/pull/14885#discussion_r1832084893

[Andarist]

The problem is not exclusive to template literals - string mappings are also affected:

const test6: Uppercase<string> & `a${string}` = {} as any;
const test7: Uppercase<string> & Lowercase<string> = {} as any;

[jcalz]

Relevant https://github.com/microsoft/TypeScript/pull/40598#issuecomment-693736212

What about get${string} & ${string}Prop? Should that reduce to get${string}Prop? This could get complicated.

I wanna say "no" - matching patterns up to patterns, while possible, I don't think is as intuitive as matching literals to patterns - I see leaving it as get${string} & ${string}Prop as fine. Especially since with something like a${string} & ${string}a, there's no simple template it could merge down to and actually be equivalent, without also extracting specific literal cases for common prefixes and postfixes, like "a" | a${string}a. So yeah, while it's possible, I think it's not as high value - there's probably more clarity in retaining the input patterns, anyway.

[RyanCavanaugh]

In general we don't have any static guarantees about any two types always producing never when 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.