Template literal type reduction is inconsistent (regression in 5.5-beta) #58687
Description
🔎 Search Terms
Template literal type reduction
🕗 Version & Regression Information
This changed in 5.5-beta (as reported in #58685).
⏯ Playground Link
💻 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.
Activity
ahejlsberg commentedon May 29, 2024
Previously,
`${T}`would always strip enum-ness. Following #55371, whenT extends string,`${T}`is simplified to justT, 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 justEwhenEis 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 commentedon May 29, 2024
See my comment here. My recommendation is that we revert #55371.