Union in template literal simplifying unexpectedly

[rj-lee]

🔎 Search Terms

template literal union distributive conditional type

🕗 Version & Regression Information

5.4.5 and Nightly

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.4.5#code/C4TwDgpgBAYg9nAclAvFABgMwQEgN4DkAjAAwkFQA+UAFOvgHYCuAtgEYQBOAvulAGRQ83AJS8A3ACgA9NKihIsBMjRZceenmbsuvAUO68ZciAA9IAY2AQAJqigAibHFIkHVDM-ybtHHn0FhXiA

💻 Code

type FooN = `foo${'100' | (`${number}` & {})}`;
// type FooN = `foo${`${number}` & {}}`
// expected = "foo100" | `foo${`${number}` & {}}`

🙁 Actual behavior

'100' and `${number}` & {} are being simplified to `${number}` & {}. FooN does not have autocomplete for 'foo100'

🙂 Expected behavior

FooN should be of type "foo100" | `foo${`${number}` & {}} and provide autocomplete for foo100 while allowing foo101 or any other number

Additional information about the issue

No response

[jcalz]

(There are no distributive conditional types in this issue)

#48034 #48044 #49839 #54188 #54648 #56458 #57807 #57918

From previous issues having to do with branded primitives in template literals, it's not clear what the "correct" behavior should be here. Looks like different people have very different expectations.

[rj-lee]

I can see how people might want it differently, but what if we wanted to intentionally distribute it?

type Foo<T extends string> = T extends T ? `foo${T}` : never;
type FooN = Foo<`100` | `${number & {}}`>;
// FooN is still `foo${number & {}}`

[RyanCavanaugh]

I don't know any good way to reason about this. Either foo100 is a foo${number & {}}, in which case reduction is correct, or it isn't, in which case foo${number & {}} has no constituents.

[jcalz]

Possibly if #33471 is ever implemented there would be a way to have this work also. The issue isn’t that the reduction is incorrect, but that it obscures information people want to keep around.

[fatcerberus]

@RyanCavanaugh I assume this to be the template-string analogue of

type T = 1 | 2 | 3 | (number & {});

which is (IIUC, very much intentionally) not reduced today even though 1 | 2 | 3 are all subsumed by number & {}

[typescript-bot]

This issue has been marked as "Not a Defect" and has seen no recent activity. It has been automatically closed for house-keeping purposes.