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Union in template literal simplifying unexpectedly #58340
Description
Activity
(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.
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${number & {}}`
I don't know any good way to reason about this. Either
foo100is afoo${number & {}}, in which case reduction is correct, or it isn't, in which casefoo${number & {}}has no constituents.Possibly if #33471 is ever implemented there would be a way to have this work also. The issue isn’t that the reduction is incorrect, but that it obscures information people want to keep around.
Ryan Cavanaugh (@RyanCavanaugh) I assume this to be the template-string analogue of
type T = 1 | 2 | 3 | (number & {});
which is (IIUC, very much intentionally) not reduced today even though
1 | 2 | 3are all subsumed bynumber & {}This issue has been marked as "Not a Defect" and has seen no recent activity. It has been automatically closed for house-keeping purposes.
- locked as resolved and limited conversation to collaborators
on Oct 22, 2025
🔎 Search Terms
template literal union distributive conditional type
🕗 Version & Regression Information
5.4.5 and Nightly
⏯ Playground Link
https://www.typescriptlang.org/play/?ts=5.4.5#code/C4TwDgpgBAYg9nAclAvFABgMwQEgN4DkAjAAwkFQA+UAFOvgHYCuAtgEYQBOAvulAGRQ83AJS8A3ACgA9NKihIsBMjRZceenmbsuvAUO68ZciAA9IAY2AQAJqigAibHFIkHVDM-ybtHHn0FhXiA
💻 Code
🙁 Actual behavior
'100'and`${number}` & {}are being simplified to`${number}` & {}.FooNdoes not have autocomplete for'foo100'🙂 Expected behavior
FooNshould be of type"foo100" | `foo${`${number}` & {}}and provide autocomplete forfoo100while allowingfoo101or any other numberAdditional information about the issue
No response