Template literal type reduction is inconsistent (regression in 5.5-beta)

[ahejlsberg]

🔎 Search Terms

Template literal type reduction

🕗 Version & Regression Information

This changed in 5.5-beta (as reported in #58685).

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.5.0-beta&ssl=7&ssc=1&pln=1&pc=1#code/MYewdgzgLgBApmArgWxgURgbxgQxgXhgCIciAaGAIwOMqJgF8BuAKBagE8AHOGAZSgAnAJZgA5sIBmHADwAVeAA8oCACYQY0EeIB8NAAYASTHIb7WLADZxYALwCMALhhHMaMzRJEmMX398A9AEwAPIA1lY2MLYATM4C2hLSMmh6hF4+MEHogoIgggCELEA

💻 Code

const enum E { a = "a", b = "b" };

type Stringify<T extends string> = `${T}`;

let z1: `${E}` = "a";        // Ok
let z2: Stringify<E> = "a";  // Error!

🙁 Actual behavior

Error as indicated.

🙂 Expected behavior

No error.

Additional information about the issue

This is a regression from 5.4.5. Error is caused by reduction introduced in #55371.

[ahejlsberg]

Previously, `${T}` would always strip enum-ness. Following #55371, when T extends string, `${T}` is simplified to just T, and enum-ness is no longer stripped. That's technically a breaking change. Furthermore, #55371 is inconsistent in that it doesn't simplify `${E}` to just E when E is an enum type. One way or the other, we should be consistent here. But I'm not really a fan of the change in #55371.

[ahejlsberg]

See my comment here. My recommendation is that we revert #55371.